xen: if on Xen, &#34;flatten&#34; the scheduling domain hierarchy

Quoted 23 lines · Dario Faggioli

Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter
whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.

Quoted 11 lines

Let me state this again (hoping to make myself as clear as possible):
no
matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for
vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).

What about Windows guests?

Quoted 10 lines

For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.

Wow. That is an pretty simple patch!!

Quoted 26 lines

Note that the patch deals directly with scheduling domains, and there is
no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is
the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any
unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.

...

Quoted 10 lines

REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
- whether you think, looking at this preliminary set of numbers, that this is something worth continuing investigating,
- if yes, what other workloads and benchmark it would make sense to throw at it.

The thing that I was worried about is that we would be modifying the generic code, but your changes are all in Xen code!

Woot!

In terms of workloads, I am CCing Herbert who I hope can provide advise on this.

Herbert, the full email is here:
http://lists.xen.org/archives/html/xen-devel/2015-08/msg01691.html

Quoted 2 lines

Thanks and Regards,
Dario

Juergen Groß

maintainer

2015-08-20 18:19 UTC #3 lore raw

In reply to Boris Ostrovsky

Quoted 86 lines · Dario Faggioli

Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.

...

Quoted 4 lines

REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,

Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.

Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours. Correct me if I'm wrong, but I think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.

Quoted 2 lines

- whether you think, looking at this preliminary set of numbers, that this is something worth continuing investigating,

I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?

I still believe making the guest scheduler's decisions independant from cpuid values is the way to go, as this will enable us to support more scenarios (e.g. cpuid based licensing). For HVM guests and old PV guests mangling the cpuid should still be done, though.

Quoted 2 lines

- if yes, what other workloads and benchmark it would make sense to throw at it.

As you already mentioned an overcommitted host should be looked at as well.

Thanks for doing the measurements,

Juergen

Boris Ostrovsky

reviewer

2015-08-31 16:13 UTC #4 lore raw

Quoted 105 lines · Juergen Groß

Quoted 87 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours. Correct me if I'm wrong, but I think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.

Quoted 6 lines

Quoted 2 lines
- whether you think, looking at this preliminary set of numbers, that this is something worth continuing investigating,
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?

If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?

Also, it seems to me that Xen guests would not be the only ones having to deal with topology inconsistencies due to migrating VCPUs. Don't KVM guests, for example, have the same problem? And if yes, perhaps we should try solving it in non-Xen-specific way (especially given that both of those patches look pretty simple and thus are presumably easy to integrate into common code).

And, as George already pointed out, this should be an optional feature --- if a guest spans physical nodes and VCPUs are pinned then we don't always want flat topology/domains.

-boris

Quoted 9 lines

I still believe making the guest scheduler's decisions independant from cpuid values is the way to go, as this will enable us to support more scenarios (e.g. cpuid based licensing). For HVM guests and old PV guests mangling the cpuid should still be done, though.
Quoted 2 lines
- if yes, what other workloads and benchmark it would make sense to throw at it.

Quoted 8 lines

As you already mentioned an overcommitted host should be looked at as well.
Thanks for doing the measurements,
Juergen

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Boris Ostrovsky

reviewer

2015-08-31 16:13 UTC #5 lore raw

In reply to Juergen Groß

Quoted 105 lines · Juergen Groß

Quoted 87 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours. Correct me if I'm wrong, but I think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.

Quoted 6 lines

Quoted 2 lines
- whether you think, looking at this preliminary set of numbers, that this is something worth continuing investigating,
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?

If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?

And, as George already pointed out, this should be an optional feature --- if a guest spans physical nodes and VCPUs are pinned then we don't always want flat topology/domains.

-boris

Quoted 17 lines

I still believe making the guest scheduler's decisions independant from cpuid values is the way to go, as this will enable us to support more scenarios (e.g. cpuid based licensing). For HVM guests and old PV guests mangling the cpuid should still be done, though.
Quoted 2 lines
- if yes, what other workloads and benchmark it would make sense to throw at it.
As you already mentioned an overcommitted host should be looked at as well.
Thanks for doing the measurements,
Juergen

Juergen Gross

maintainer

2015-09-02 11:58 UTC #6 lore raw

Quoted 112 lines · Boris Ostrovsky

Quoted 109 lines · Juergen Groß
Quoted 88 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours. Correct me if I'm wrong, but I think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.
Quoted 2 lines
- whether you think, looking at this preliminary set of numbers, that this is something worth continuing investigating,

Quoted 6 lines

Quoted 3 lines
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?
If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?

Hmm, sort of.

OTOH this would it make hard to make use of some of the topology information in case of e.g. pinned vcpus (as George pointed out).

Quoted 6 lines

Also, it seems to me that Xen guests would not be the only ones having to deal with topology inconsistencies due to migrating VCPUs. Don't KVM guests, for example, have the same problem? And if yes, perhaps we should try solving it in non-Xen-specific way (especially given that both of those patches look pretty simple and thus are presumably easy to integrate into common code).

Indeed. I'll have a try.

Quoted 3 lines

And, as George already pointed out, this should be an optional feature --- if a guest spans physical nodes and VCPUs are pinned then we don't always want flat topology/domains.

Yes, it might be a good idea to be able to keep some of the topology levels. I'll modify my patch to make this command line selectable.

Juergen

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Juergen Gross

maintainer

2015-09-02 11:58 UTC #7 lore raw

Quoted 112 lines · Boris Ostrovsky

Quoted 109 lines · Juergen Groß
Quoted 88 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours. Correct me if I'm wrong, but I think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.
Quoted 2 lines
- whether you think, looking at this preliminary set of numbers, that this is something worth continuing investigating,

Quoted 6 lines

Quoted 3 lines
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?
If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?

Hmm, sort of.

OTOH this would it make hard to make use of some of the topology information in case of e.g. pinned vcpus (as George pointed out).

Quoted 6 lines

Also, it seems to me that Xen guests would not be the only ones having to deal with topology inconsistencies due to migrating VCPUs. Don't KVM guests, for example, have the same problem? And if yes, perhaps we should try solving it in non-Xen-specific way (especially given that both of those patches look pretty simple and thus are presumably easy to integrate into common code).

Indeed. I'll have a try.

Quoted 3 lines

And, as George already pointed out, this should be an optional feature --- if a guest spans physical nodes and VCPUs are pinned then we don't always want flat topology/domains.

Yes, it might be a good idea to be able to keep some of the topology levels. I'll modify my patch to make this command line selectable.

Juergen

Boris Ostrovsky

reviewer

2015-09-02 14:09 UTC #8 lore raw

Quoted 120 lines · Juergen Gross

Quoted 119 lines · Boris Ostrovsky
Quoted 115 lines · Juergen Groß
Quoted 90 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours. Correct me if I'm wrong, but I think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.
Quoted 3 lines
- whether you think, looking at this preliminary set of numbers,
that
this is something worth continuing investigating,
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?

Quoted 7 lines

Quoted 2 lines
If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?
Hmm, sort of.
OTOH this would it make hard to make use of some of the topology information in case of e.g. pinned vcpus (as George pointed out).

I didn't mean to just set has_mp to zero unconditionally (for Xen, or any other, guest). We'd need to have some logic as to when to set it to false.

-boris

Quoted 14 lines

Quoted 7 lines
Also, it seems to me that Xen guests would not be the only ones having to deal with topology inconsistencies due to migrating VCPUs. Don't KVM guests, for example, have the same problem? And if yes, perhaps we should try solving it in non-Xen-specific way (especially given that both of those patches look pretty simple and thus are presumably easy to integrate into common code).
Indeed. I'll have a try.
Quoted 3 lines
And, as George already pointed out, this should be an optional feature --- if a guest spans physical nodes and VCPUs are pinned then we don't always want flat topology/domains.

Quoted 5 lines

Yes, it might be a good idea to be able to keep some of the topology levels. I'll modify my patch to make this command line selectable.
Juergen

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Boris Ostrovsky

reviewer

2015-09-02 14:09 UTC #9 lore raw

In reply to Boris Ostrovsky

Quoted 120 lines · Juergen Gross

Quoted 119 lines · Boris Ostrovsky
Quoted 115 lines · Juergen Groß
Quoted 90 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours. Correct me if I'm wrong, but I think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.
Quoted 3 lines
- whether you think, looking at this preliminary set of numbers,
that
this is something worth continuing investigating,
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?

Quoted 7 lines

Quoted 2 lines
If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?
Hmm, sort of.
OTOH this would it make hard to make use of some of the topology information in case of e.g. pinned vcpus (as George pointed out).

I didn't mean to just set has_mp to zero unconditionally (for Xen, or any other, guest). We'd need to have some logic as to when to set it to false.

-boris

Quoted 19 lines

Quoted 7 lines
Also, it seems to me that Xen guests would not be the only ones having to deal with topology inconsistencies due to migrating VCPUs. Don't KVM guests, for example, have the same problem? And if yes, perhaps we should try solving it in non-Xen-specific way (especially given that both of those patches look pretty simple and thus are presumably easy to integrate into common code).
Indeed. I'll have a try.
Quoted 3 lines
And, as George already pointed out, this should be an optional feature --- if a guest spans physical nodes and VCPUs are pinned then we don't always want flat topology/domains.
Yes, it might be a good idea to be able to keep some of the topology levels. I'll modify my patch to make this command line selectable.
Juergen

Juergen Gross

maintainer

2015-09-02 14:30 UTC #10 lore raw

Quoted 140 lines · Boris Ostrovsky

Quoted 139 lines · Juergen Gross
Quoted 129 lines · Boris Ostrovsky
Quoted 126 lines · Juergen Groß
Quoted 103 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way
of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter
whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does
not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no
matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for
vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two
situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in
there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is
no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is
the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it
comply with whatever logic or requirements we want, that won't have any
unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a
16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without
any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours. Correct me if I'm wrong, but I think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus,
while my approach can easily be extended to do so.
Quoted 3 lines
- whether you think, looking at this preliminary set of numbers,
that
this is something worth continuing investigating,
Quoted 6 lines
Quoted 3 lines
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?
If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?
Hmm, sort of.

Quoted 7 lines

Quoted 2 lines
OTOH this would it make hard to make use of some of the topology information in case of e.g. pinned vcpus (as George pointed out).
I didn't mean to just set has_mp to zero unconditionally (for Xen, or any other, guest). We'd need to have some logic as to when to set it to false.

In case we want to be able to use some of the topology information this would mean we'd have two different mechanisms to either disable all topology usage or only parts of it. I'd rather have a way to specify which levels of the topology information (numa nodes, cache siblings, core siblings) are to be used. Using none is just one possibility with all levels disabled.

Juergen

Quoted 8 lines

Quoted 8 lines
Quoted 7 lines
Also, it seems to me that Xen guests would not be the only ones having to deal with topology inconsistencies due to migrating VCPUs. Don't KVM guests, for example, have the same problem? And if yes, perhaps we should try solving it in non-Xen-specific way (especially given that both of those patches look pretty simple and thus are presumably easy to integrate into common code).

Quoted 8 lines

Quoted 8 lines
Indeed. I'll have a try.
Quoted 3 lines
And, as George already pointed out, this should be an optional feature --- if a guest spans physical nodes and VCPUs are pinned then we don't always want flat topology/domains.
Yes, it might be a good idea to be able to keep some of the topology levels. I'll modify my patch to make this command line selectable.

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Juergen Gross

maintainer

2015-09-02 14:30 UTC #11 lore raw

Quoted 140 lines · Boris Ostrovsky

Quoted 139 lines · Juergen Gross
Quoted 129 lines · Boris Ostrovsky
Quoted 126 lines · Juergen Groß
Quoted 103 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way
of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter
whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does
not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no
matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for
vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two
situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in
there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is
no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is
the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it
comply with whatever logic or requirements we want, that won't have any
unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a
16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without
any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours. Correct me if I'm wrong, but I think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus,
while my approach can easily be extended to do so.
Quoted 3 lines
- whether you think, looking at this preliminary set of numbers,
that
this is something worth continuing investigating,
Quoted 6 lines
Quoted 3 lines
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?
If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?
Hmm, sort of.

Quoted 7 lines

Quoted 2 lines
OTOH this would it make hard to make use of some of the topology information in case of e.g. pinned vcpus (as George pointed out).
I didn't mean to just set has_mp to zero unconditionally (for Xen, or any other, guest). We'd need to have some logic as to when to set it to false.

Juergen

Quoted 16 lines

Quoted 16 lines
Quoted 7 lines
Also, it seems to me that Xen guests would not be the only ones having to deal with topology inconsistencies due to migrating VCPUs. Don't KVM guests, for example, have the same problem? And if yes, perhaps we should try solving it in non-Xen-specific way (especially given that both of those patches look pretty simple and thus are presumably easy to integrate into common code).
Indeed. I'll have a try.
Quoted 3 lines
And, as George already pointed out, this should be an optional feature --- if a guest spans physical nodes and VCPUs are pinned then we don't always want flat topology/domains.
Yes, it might be a good idea to be able to keep some of the topology levels. I'll modify my patch to make this command line selectable.

Dario Faggioli

author

2015-09-15 17:17 UTC #12 lore raw

Quoted 17 lines · Juergen Gross

Quoted 8 lines · Boris Ostrovsky
Quoted 7 lines · Juergen Gross
Quoted 2 lines · Boris Ostrovsky
If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?
Hmm, sort of.
Quoted 7 lines
Quoted 2 lines
OTOH this would it make hard to make use of some of the topology information in case of e.g. pinned vcpus (as George pointed out).
I didn't mean to just set has_mp to zero unconditionally (for Xen, or any other, guest). We'd need to have some logic as to when to set it to false.

Quoted 7 lines

In case we want to be able to use some of the topology information this would mean we'd have two different mechanisms to either disable all topology usage or only parts of it. I'd rather have a way to specify which levels of the topology information (numa nodes, cache siblings, core siblings) are to be used. Using none is just one possibility with all levels disabled.

I agree, indeed, acting on has_mp seems overkill/not ideal to me too (I'm not even sure I fully understand how it's used in set_cpu_sibling_map()... I'll dig more).

However...

Quoted 10 lines

Quoted 10 lines
Quoted 10 lines
Quoted 7 lines
Also, it seems to me that Xen guests would not be the only ones having to deal with topology inconsistencies due to migrating VCPUs. Don't KVM guests, for example, have the same problem? And if yes, perhaps we should try solving it in non-Xen-specific way (especially given that both of those patches look pretty simple and thus are presumably easy to integrate into common code).
Indeed. I'll have a try.

...yes, this is an interesting point, and it's worth try looking at how to implement things that way.

Thanks and Regards,
Dario

Signature · 4 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

Dario Faggioli

author

2015-09-15 17:17 UTC #13 lore raw

Quoted 17 lines · Juergen Gross

Quoted 8 lines · Boris Ostrovsky
Quoted 7 lines · Juergen Gross
Quoted 2 lines · Boris Ostrovsky
If set_cpu_sibling_map()'s has_mp is false, wouldn't we effectively have both of your patches?
Hmm, sort of.
Quoted 7 lines
Quoted 2 lines
OTOH this would it make hard to make use of some of the topology information in case of e.g. pinned vcpus (as George pointed out).
I didn't mean to just set has_mp to zero unconditionally (for Xen, or any other, guest). We'd need to have some logic as to when to set it to false.

Quoted 7 lines

In case we want to be able to use some of the topology information this would mean we'd have two different mechanisms to either disable all topology usage or only parts of it. I'd rather have a way to specify which levels of the topology information (numa nodes, cache siblings, core siblings) are to be used. Using none is just one possibility with all levels disabled.

I agree, indeed, acting on has_mp seems overkill/not ideal to me too (I'm not even sure I fully understand how it's used in set_cpu_sibling_map()... I'll dig more).

However...

Quoted 10 lines

Quoted 10 lines
Quoted 10 lines
Quoted 7 lines
Also, it seems to me that Xen guests would not be the only ones having to deal with topology inconsistencies due to migrating VCPUs. Don't KVM guests, for example, have the same problem? And if yes, perhaps we should try solving it in non-Xen-specific way (especially given that both of those patches look pretty simple and thus are presumably easy to integrate into common code).
Indeed. I'll have a try.

...yes, this is an interesting point, and it's worth try looking at how to implement things that way.

Thanks and Regards,
Dario

Signature · 4 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Dario Faggioli

author

2015-09-15 16:51 UTC #14 lore raw

In reply to Juergen Groß

Quoted 87 lines · Juergen Groß

Quoted 81 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Quoted 5 lines
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.

Quoted 9 lines

...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.

Hey,

Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)

Quoted 5 lines

Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours.

Ok. Just let me mention that 'Credit2 + gang scheduling' might not be exactly around the corner (although, we can prioritize working on it if we want).

In principle, I think it's a really nice idea. I still don't have clear in mind how we would handle a couple of situations, but let's leave this aside for now, and stay on-topic.

Quoted 5 lines

Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.

I'm not sure I understand what you mean here. As far as the (Linux) scheduler is concerned, your patch and mine do the exact same thing:
they arrange for the scheduling domains, when they're built, during boot, not to consider hyperthreads or multi-cores.

Mine does it by removing the SMT (and the MC) level from the data structure in the scheduler that is used as a base for configuring the scheduling domains. Yours does it by making the topology bitmaps that are used at each one of those level all look the same. In fact, with your patch applied, I get the exact same situation as with mine, as far as scheduling domains are concerned: there is only one scheduling domain, with a different scheduling group for each vCPU inside it.

In my case, that one scheduling domain is the special one that I define in xen_sched_domain_topology (in arch/x86/xen/smp.c), in my patch (it's called PCPU). In your case, it's the DIE scheduling domain, i.e., the one coming from the last level defined in default_topology (in kernel/sched/core.c). I'd have to recheck, but ISTR that, since you're setting all the bitmaps for all the levels to the same value, previous levels are created, recognised to be all equal, and merged/discarded.

IOW, mine is using a scheduler provided interface explicitly, via set_sched_topology(), i.e., the way an architecture (and in this case the architecture would be 'xen') let the scheduler know about its topology quirks:
http://lxr.free-electrons.com/ident?i=set_sched_topology
Basically, I'm telling the scheduler <<Hey, you're on Xen, don't bother looking for hyperthreads, as they don't make any sense!>>.

Yours is changing the topology bitmaps directly. Basically, you're telling nothing to the scheduler, which then goes down looking for SMTs and MCs, but finds none.

All this being said, the effect is the same, and the reason why the scheduling inside the guest changes --between mainline and both mine or your patch-- is because of scheduling domains, or so it is how I understood it.

Therefore, I don't really understand why you're saying one approach is more easily extensible toward anything... What am I missing?

Quoted 7 lines

Quoted 2 lines
- whether you think, looking at this preliminary set of numbers, that this is something worth continuing investigating,
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?

I'll run some tests, but yes, I 100% expect the numbers to look the same. Actually, I did a very quick check, for a few cases, already, and that is indeed the case, but I'll report back when I'll have the full data set.

Quoted 6 lines

Quoted 2 lines
- if yes, what other workloads and benchmark it would make sense to throw at it.
As you already mentioned an overcommitted host should be looked at as well.

Sure.

Quoted 2 lines

Thanks for doing the measurements,

And more of them will be coming. ISTR you telling me in Seattle that you (or some teammates of yours) were running some benches too... Any output from that yet? :-)

Thanks and Regards,
Dario

Signature · 16 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Dario Faggioli

author

2015-09-15 16:51 UTC #15 lore raw

In reply to Juergen Groß

Quoted 87 lines · Juergen Groß

Quoted 81 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Quoted 5 lines
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.

Quoted 9 lines

...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.

Hey,

Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)

Quoted 5 lines

Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours.

Ok. Just let me mention that 'Credit2 + gang scheduling' might not be exactly around the corner (although, we can prioritize working on it if we want).

In principle, I think it's a really nice idea. I still don't have clear in mind how we would handle a couple of situations, but let's leave this aside for now, and stay on-topic.

Quoted 5 lines

Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.

Yours is changing the topology bitmaps directly. Basically, you're telling nothing to the scheduler, which then goes down looking for SMTs and MCs, but finds none.

Therefore, I don't really understand why you're saying one approach is more easily extensible toward anything... What am I missing?

Quoted 7 lines

Quoted 2 lines
- whether you think, looking at this preliminary set of numbers, that this is something worth continuing investigating,
I believe as both approaches lead to the same topology information used by the scheduler (all vcpus are regarded as being equal) your numbers should apply to my patch as well. Would you mind verifying this?

Quoted 6 lines

Quoted 2 lines
- if yes, what other workloads and benchmark it would make sense to throw at it.
As you already mentioned an overcommitted host should be looked at as well.

Sure.

Quoted 2 lines

Thanks for doing the measurements,

And more of them will be coming. ISTR you telling me in Seattle that you (or some teammates of yours) were running some benches too... Any output from that yet? :-)

Thanks and Regards,
Dario

Signature · 16 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

Juergen Gross

maintainer

2015-09-21 05:49 UTC #16 lore raw

Quoted 125 lines · Dario Faggioli

Quoted 87 lines · Juergen Groß
Quoted 81 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Quoted 5 lines
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
Quoted 9 lines
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Hey,
Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)
Quoted 5 lines
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours.
Ok. Just let me mention that 'Credit2 + gang scheduling' might not be exactly around the corner (although, we can prioritize working on it if we want).
In principle, I think it's a really nice idea. I still don't have clear in mind how we would handle a couple of situations, but let's leave this aside for now, and stay on-topic.
Quoted 5 lines
Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.
I'm not sure I understand what you mean here. As far as the (Linux) scheduler is concerned, your patch and mine do the exact same thing:
they arrange for the scheduling domains, when they're built, during boot, not to consider hyperthreads or multi-cores.

Quoted 7 lines

Mine does it by removing the SMT (and the MC) level from the data structure in the scheduler that is used as a base for configuring the scheduling domains. Yours does it by making the topology bitmaps that are used at each one of those level all look the same. In fact, with your patch applied, I get the exact same situation as with mine, as far as scheduling domains are concerned: there is only one scheduling domain, with a different scheduling group for each vCPU inside it.

Uuh, nearly.

Your case won't deal correctly with NUMA, as the generic NUMA code is using set_sched_topology() as well. One of NUMA and Xen will win and overwrite the other's settings.

To do things correctly you will have to handle NUMA as well.

Juergen

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Juergen Gross

maintainer

2015-09-21 05:49 UTC #17 lore raw

Quoted 125 lines · Dario Faggioli

Quoted 87 lines · Juergen Groß
Quoted 81 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Quoted 5 lines
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
Quoted 9 lines
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Hey,
Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)
Quoted 5 lines
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours.
Ok. Just let me mention that 'Credit2 + gang scheduling' might not be exactly around the corner (although, we can prioritize working on it if we want).
In principle, I think it's a really nice idea. I still don't have clear in mind how we would handle a couple of situations, but let's leave this aside for now, and stay on-topic.
Quoted 5 lines
Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.
I'm not sure I understand what you mean here. As far as the (Linux) scheduler is concerned, your patch and mine do the exact same thing:
they arrange for the scheduling domains, when they're built, during boot, not to consider hyperthreads or multi-cores.

Quoted 7 lines

Mine does it by removing the SMT (and the MC) level from the data structure in the scheduler that is used as a base for configuring the scheduling domains. Yours does it by making the topology bitmaps that are used at each one of those level all look the same. In fact, with your patch applied, I get the exact same situation as with mine, as far as scheduling domains are concerned: there is only one scheduling domain, with a different scheduling group for each vCPU inside it.

Uuh, nearly.

Your case won't deal correctly with NUMA, as the generic NUMA code is using set_sched_topology() as well. One of NUMA and Xen will win and overwrite the other's settings.

To do things correctly you will have to handle NUMA as well.

Juergen

Juergen Gross

maintainer

2015-09-22 04:42 UTC #18 lore raw

Quoted 144 lines · Juergen Gross

Quoted 135 lines · Dario Faggioli
Quoted 97 lines · Juergen Groß
Quoted 90 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter
whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no
matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for
vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is
no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is
the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any
unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Quoted 6 lines
Bear also in mind that these results are gathered from Dom0, and without
any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
Quoted 9 lines
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Hey,
Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)
Quoted 5 lines
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours.
Ok. Just let me mention that 'Credit2 + gang scheduling' might not be exactly around the corner (although, we can prioritize working on it if we want).
In principle, I think it's a really nice idea. I still don't have clear in mind how we would handle a couple of situations, but let's leave this aside for now, and stay on-topic.
Quoted 5 lines
Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.
I'm not sure I understand what you mean here. As far as the (Linux) scheduler is concerned, your patch and mine do the exact same thing:
they arrange for the scheduling domains, when they're built, during boot, not to consider hyperthreads or multi-cores.
Quoted 7 lines
Mine does it by removing the SMT (and the MC) level from the data structure in the scheduler that is used as a base for configuring the scheduling domains. Yours does it by making the topology bitmaps that are used at each one of those level all look the same. In fact, with your patch applied, I get the exact same situation as with mine, as far as scheduling domains are concerned: there is only one scheduling domain, with a different scheduling group for each vCPU inside it.

Quoted 7 lines

Uuh, nearly.
Your case won't deal correctly with NUMA, as the generic NUMA code is using set_sched_topology() as well. One of NUMA and Xen will win and overwrite the other's settings.
To do things correctly you will have to handle NUMA as well.

One other thing I just discovered: there are other consumers of the topology sibling masks (e.g. topology_sibling_cpumask()) as well.

I think we would want to avoid any optimizations based on those in drivers as well, not only in the scheduler.

Juergen

George Dunlap

2015-09-22 16:22 UTC #19 lore raw

Quoted 5 lines · Juergen Gross

One other thing I just discovered: there are other consumers of the topology sibling masks (e.g. topology_sibling_cpumask()) as well.
I think we would want to avoid any optimizations based on those in drivers as well, not only in the scheduler.

I'm beginning to lose the thread of the discussion here a bit.

Juergen / Dario, could one of you summarize your two approaches, and the (alleged) advantages and disadvantages of each one?

Thanks,
-George

Juergen Gross

maintainer

2015-09-23 04:36 UTC #20 lore raw

Quoted 11 lines · George Dunlap

Quoted 5 lines · Juergen Gross
One other thing I just discovered: there are other consumers of the topology sibling masks (e.g. topology_sibling_cpumask()) as well.
I think we would want to avoid any optimizations based on those in drivers as well, not only in the scheduler.
I'm beginning to lose the thread of the discussion here a bit.
Juergen / Dario, could one of you summarize your two approaches, and the (alleged) advantages and disadvantages of each one?

Okay, I'll have a try:

The problem we want to solve:
-----------------------------

The Linux kernel is gathering cpu topology data during boot via the CPUID instruction on each processor coming online. This data is primarily used in the scheduler to decide to which cpu a thread should be migrated when this seems to be necessary. There are other users of the topology information in the kernel (e.g. some drivers try to do optimizations like core-specific queues/lists).

When started in a virtualized environment the obtained data is next to useless or even wrong, as it is reflecting only the status of the time of booting the system. Scheduling of the (v)cpus done by the hypervisor is changing the topology beneath the feet of the Linux kernel without reflecting this in the gathered topology information. So any decisions taken based on that data will be clueless and possibly just wrong.

The minimal solution is to change the topology data in the kernel in a way that all cpus are regarded as equal regarding their relation to each other (e.g. when migrating a thread to another cpu no cpu is preferred as a target).

The topology information of the CPUID instruction is, however, even accessible form user mode and might be used for licensing purposes of any user program (e.g. by limiting the software to run on a specific number of cores or sockets). So just mangling the data returned by CPUID in the hypervisor seems not to be a general solution, while we might want to do it at least optionally in the future.

In the future we might want to support either dynamic topology updates or be able to tell the kernel to use some of the topology data, e.g. when pinning vcpus.

Solution 1 (Dario):
-------------------

Don't use the CPUID derived topology information in the Linux scheduler, but let it use a simple "flat" topology by setting own scheduler domain data under Xen.

Advantages:
+ very clean solution regarding the scheduler interface
+ scheduler decisions are based on a minimal data set
+ small patch

Disadvantages:
- covers the scheduler only, drivers still use the "wrong" data
- a little bit hacky regarding some NUMA architectures (needs either a hook in the code dealing with that architecture or multiple scheduler domain data overwrites)
- future enhancements will make the solution less clean (either need duplicating scheduler domain data or some new hooks in scheduler domain interface)

Solution 2 (Juergen):
---------------------

When booted as a Xen guest modify the topology data built during boot resulting in the same simple "flat" topology as in Dario's solution.

Advantages:
+ the simple topology is seen by all consumers of topology data as the data itself is modified accordingly
+ small patch
+ future enhancements rather easy by selecting which data to modify

Disadvantages:
- interface to scheduler not as clean as in Dario's approach
- scheduler decisions are based on multiple layers of topology data where one layer would be enough to describe the topology

Dario, are you okay with this summary?

Juergen

Juergen Gross

maintainer

2015-09-23 04:36 UTC #21 lore raw

Quoted 11 lines · George Dunlap

Quoted 5 lines · Juergen Gross
One other thing I just discovered: there are other consumers of the topology sibling masks (e.g. topology_sibling_cpumask()) as well.
I think we would want to avoid any optimizations based on those in drivers as well, not only in the scheduler.
I'm beginning to lose the thread of the discussion here a bit.
Juergen / Dario, could one of you summarize your two approaches, and the (alleged) advantages and disadvantages of each one?

Okay, I'll have a try:

The problem we want to solve:
-----------------------------

In the future we might want to support either dynamic topology updates or be able to tell the kernel to use some of the topology data, e.g. when pinning vcpus.

Solution 1 (Dario):
-------------------

Don't use the CPUID derived topology information in the Linux scheduler, but let it use a simple "flat" topology by setting own scheduler domain data under Xen.

Advantages:
+ very clean solution regarding the scheduler interface
+ scheduler decisions are based on a minimal data set
+ small patch

Solution 2 (Juergen):
---------------------

When booted as a Xen guest modify the topology data built during boot resulting in the same simple "flat" topology as in Dario's solution.

Dario, are you okay with this summary?

Juergen

Dario Faggioli

author

2015-09-23 08:31 UTC #22 lore raw

Quoted 7 lines · Juergen Gross

Quoted 3 lines · George Dunlap
Juergen / Dario, could one of you summarize your two approaches, and the
(alleged) advantages and disadvantages of each one?
Okay, I'll have a try:

Thanks for this! ;-)

Quoted 11 lines

The problem we want to solve:
-----------------------------
The Linux kernel is gathering cpu topology data during boot via the CPUID instruction on each processor coming online. This data is primarily used in the scheduler to decide to which cpu a thread should
be migrated when this seems to be necessary. There are other users of the topology information in the kernel (e.g. some drivers try to do optimizations like core-specific queues/lists).

Quoted 11 lines

When started in a virtualized environment the obtained data is next to
useless or even wrong, as it is reflecting only the status of the time
of booting the system. Scheduling of the (v)cpus done by the hypervisor
is changing the topology beneath the feet of the Linux kernel without reflecting this in the gathered topology information. So any decisions
taken based on that data will be clueless and possibly just wrong.

Exactly.

Quoted 8 lines

The minimal solution is to change the topology data in the kernel in a
way that all cpus are regarded as equal regarding their relation to each
other (e.g. when migrating a thread to another cpu no cpu is preferred
as a target).

Quoted 7 lines

The topology information of the CPUID instruction is, however, even accessible form user mode and might be used for licensing purposes of any user program (e.g. by limiting the software to run on a specific number of cores or sockets). So just mangling the data returned by CPUID in the hypervisor seems not to be a general solution, while we might want to do it at least optionally in the future.

Yep. It turned out that, although being what started all this, CPUID handling is a somewhat related but mostly independent problem. :-)

Quoted 5 lines

In the future we might want to support either dynamic topology updates
or be able to tell the kernel to use some of the topology data, e.g. when pinning vcpus.

Indeed. At least for the latter. Dynamic looks really difficult to me, but indeed it would be ideal. Let's see.

Quoted 12 lines

Solution 1 (Dario):
-------------------
Don't use the CPUID derived topology information in the Linux scheduler,
but let it use a simple "flat" topology by setting own scheduler domain
data under Xen.
Advantages:
+ very clean solution regarding the scheduler interface

Yes, this is, I think, one of the main advantages of the patch. The scheduler is offering an interface to architectures to define their topology requirements and I'm using it, for specifying our topology
requirements: the tool for the job. :-D

Quoted 6 lines

+ scheduler decisions are based on a minimal data set
+ small patch
Disadvantages:
- covers the scheduler only, drivers still use the "wrong" data

This is a good point. It was the patch's purpose, TBH, but it's certainly true that, if we need something similar elsewhere, we need to do more.

Quoted 6 lines

- a little bit hacky regarding some NUMA architectures (needs either a
hook in the code dealing with that architecture or multiple
scheduler
domain data overwrites)

As I said in my other email, I'll double check (yes, I also think this is about AMD boxes with intra-socket NUMA nodes).

Quoted 4 lines

- future enhancements will make the solution less clean (either need duplicating scheduler domain data or some new hooks in scheduler domain interface)

This one, I'm not sure I understand.

Quoted 11 lines

Solution 2 (Juergen):
---------------------
When booted as a Xen guest modify the topology data built during boot resulting in the same simple "flat" topology as in Dario's solution.
Advantages:
+ the simple topology is seen by all consumers of topology data as the
data itself is modified accordingly

Yep, that's a good point.

Quoted 4 lines

+ small patch
+ future enhancements rather easy by selecting which data to modify

As for the '-' above about this, I'm not really sure what this means.

Quoted 6 lines

Disadvantages:
- interface to scheduler not as clean as in Dario's approach
- scheduler decisions are based on multiple layers of topology data where one layer would be enough to describe the topology

This is not too big of a deal, IMO. Not at runtime, at least, as far as my investigation went for now. Initialization (of scheduling domains) is a bit clumsy in this case, as scheduling domains are created and then destroyed/collapsed, but after they are setup, the net effect is that there's only one scheduling domain with Juergen's patch too, exactly as with mine.

Quoted 2 lines

Dario, are you okay with this summary?

To most of it, yes, and thanks again for it.

Allow me to add a few points, out of the top of my head:

* we need to check whether the two approaches have the same performance. In principle, they really should, and early results seems to confirm that, but I'd like to run the full set of benches (and I'll do that ASAP);
* I think we want to run even more benchmarks, and run them in different (over)load conditions to better assess the effect of the change
* both our patches provides a solution for Xen (for Xen PV guests, at least for now, to be more precise). It is very likely that, e.g., KVM is in a similar situation, hence it may be worth to look for a more general solution, especially if that buys us something (e.g., HVM support made easy?)

Thanks and Regards,
Dario

PS. BTW, Juergen, you're not on IRC, on #xendevel, are you?

Signature · 4 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Dario Faggioli

author

2015-09-23 08:31 UTC #23 lore raw

Quoted 7 lines · Juergen Gross

Quoted 3 lines · George Dunlap
Juergen / Dario, could one of you summarize your two approaches, and the
(alleged) advantages and disadvantages of each one?
Okay, I'll have a try:

Thanks for this! ;-)

Quoted 11 lines

The problem we want to solve:
-----------------------------
The Linux kernel is gathering cpu topology data during boot via the CPUID instruction on each processor coming online. This data is primarily used in the scheduler to decide to which cpu a thread should
be migrated when this seems to be necessary. There are other users of the topology information in the kernel (e.g. some drivers try to do optimizations like core-specific queues/lists).

Quoted 11 lines

When started in a virtualized environment the obtained data is next to
useless or even wrong, as it is reflecting only the status of the time
of booting the system. Scheduling of the (v)cpus done by the hypervisor
is changing the topology beneath the feet of the Linux kernel without reflecting this in the gathered topology information. So any decisions
taken based on that data will be clueless and possibly just wrong.

Exactly.

Quoted 8 lines

The minimal solution is to change the topology data in the kernel in a
way that all cpus are regarded as equal regarding their relation to each
other (e.g. when migrating a thread to another cpu no cpu is preferred
as a target).

Quoted 7 lines

The topology information of the CPUID instruction is, however, even accessible form user mode and might be used for licensing purposes of any user program (e.g. by limiting the software to run on a specific number of cores or sockets). So just mangling the data returned by CPUID in the hypervisor seems not to be a general solution, while we might want to do it at least optionally in the future.

Yep. It turned out that, although being what started all this, CPUID handling is a somewhat related but mostly independent problem. :-)

Quoted 5 lines

In the future we might want to support either dynamic topology updates
or be able to tell the kernel to use some of the topology data, e.g. when pinning vcpus.

Indeed. At least for the latter. Dynamic looks really difficult to me, but indeed it would be ideal. Let's see.

Quoted 12 lines

Solution 1 (Dario):
-------------------
Don't use the CPUID derived topology information in the Linux scheduler,
but let it use a simple "flat" topology by setting own scheduler domain
data under Xen.
Advantages:
+ very clean solution regarding the scheduler interface

Quoted 6 lines

+ scheduler decisions are based on a minimal data set
+ small patch
Disadvantages:
- covers the scheduler only, drivers still use the "wrong" data

This is a good point. It was the patch's purpose, TBH, but it's certainly true that, if we need something similar elsewhere, we need to do more.

Quoted 6 lines

- a little bit hacky regarding some NUMA architectures (needs either a
hook in the code dealing with that architecture or multiple
scheduler
domain data overwrites)

As I said in my other email, I'll double check (yes, I also think this is about AMD boxes with intra-socket NUMA nodes).

Quoted 4 lines

- future enhancements will make the solution less clean (either need duplicating scheduler domain data or some new hooks in scheduler domain interface)

This one, I'm not sure I understand.

Quoted 11 lines

Solution 2 (Juergen):
---------------------
When booted as a Xen guest modify the topology data built during boot resulting in the same simple "flat" topology as in Dario's solution.
Advantages:
+ the simple topology is seen by all consumers of topology data as the
data itself is modified accordingly

Yep, that's a good point.

Quoted 4 lines

+ small patch
+ future enhancements rather easy by selecting which data to modify

As for the '-' above about this, I'm not really sure what this means.

Quoted 6 lines

Disadvantages:
- interface to scheduler not as clean as in Dario's approach
- scheduler decisions are based on multiple layers of topology data where one layer would be enough to describe the topology

Quoted 2 lines

Dario, are you okay with this summary?

To most of it, yes, and thanks again for it.

Allow me to add a few points, out of the top of my head:

Thanks and Regards,
Dario

PS. BTW, Juergen, you're not on IRC, on #xendevel, are you?

Signature · 4 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

Juergen Gross

maintainer

2015-09-23 09:44 UTC #24 lore raw

Quoted 94 lines · Dario Faggioli

Quoted 7 lines · Juergen Gross
Quoted 3 lines · George Dunlap
Juergen / Dario, could one of you summarize your two approaches, and the
(alleged) advantages and disadvantages of each one?
Okay, I'll have a try:
Thanks for this! ;-)
Quoted 11 lines
The problem we want to solve:
-----------------------------
The Linux kernel is gathering cpu topology data during boot via the CPUID instruction on each processor coming online. This data is primarily used in the scheduler to decide to which cpu a thread should
be migrated when this seems to be necessary. There are other users of the topology information in the kernel (e.g. some drivers try to do optimizations like core-specific queues/lists).
Quoted 11 lines
When started in a virtualized environment the obtained data is next to
useless or even wrong, as it is reflecting only the status of the time
of booting the system. Scheduling of the (v)cpus done by the hypervisor
is changing the topology beneath the feet of the Linux kernel without reflecting this in the gathered topology information. So any decisions
taken based on that data will be clueless and possibly just wrong.
Exactly.
Quoted 8 lines
The minimal solution is to change the topology data in the kernel in a
way that all cpus are regarded as equal regarding their relation to each
other (e.g. when migrating a thread to another cpu no cpu is preferred
as a target).
Quoted 7 lines
The topology information of the CPUID instruction is, however, even accessible form user mode and might be used for licensing purposes of any user program (e.g. by limiting the software to run on a specific number of cores or sockets). So just mangling the data returned by CPUID in the hypervisor seems not to be a general solution, while we might want to do it at least optionally in the future.
Yep. It turned out that, although being what started all this, CPUID handling is a somewhat related but mostly independent problem. :-)
Quoted 5 lines
In the future we might want to support either dynamic topology updates
or be able to tell the kernel to use some of the topology data, e.g. when pinning vcpus.
Indeed. At least for the latter. Dynamic looks really difficult to me, but indeed it would be ideal. Let's see.
Quoted 12 lines
Solution 1 (Dario):
-------------------
Don't use the CPUID derived topology information in the Linux scheduler,
but let it use a simple "flat" topology by setting own scheduler domain
data under Xen.
Advantages:
+ very clean solution regarding the scheduler interface
Yes, this is, I think, one of the main advantages of the patch. The scheduler is offering an interface to architectures to define their topology requirements and I'm using it, for specifying our topology
requirements: the tool for the job. :-D
Quoted 6 lines
+ scheduler decisions are based on a minimal data set
+ small patch
Disadvantages:
- covers the scheduler only, drivers still use the "wrong" data
This is a good point. It was the patch's purpose, TBH, but it's certainly true that, if we need something similar elsewhere, we need to do more.
Quoted 6 lines
- a little bit hacky regarding some NUMA architectures (needs either a
hook in the code dealing with that architecture or multiple
scheduler
domain data overwrites)

Quoted 8 lines

As I said in my other email, I'll double check (yes, I also think this is about AMD boxes with intra-socket NUMA nodes).
Quoted 4 lines
- future enhancements will make the solution less clean (either need duplicating scheduler domain data or some new hooks in scheduler domain interface)
This one, I'm not sure I understand.

What would you do for keeping the topology information of one level, e.g. hyperthreads, in case we'd have a gang-scheduler in Xen? Either you would copy the line:

{ cpu_smt_mask, cpu_smt_flags, SD_INIT_NAME(SMT) },

from kernel/sched/core.c into your topology array, or you would add a way in kernel/sched/core.c to remove all but this entry and add your entry on top of it.

Quoted 12 lines

Quoted 4 lines
Solution 2 (Juergen):
---------------------
Quoted 8 lines
When booted as a Xen guest modify the topology data built during boot resulting in the same simple "flat" topology as in Dario's solution.
Advantages:
+ the simple topology is seen by all consumers of topology data as the
data itself is modified accordingly

Quoted 7 lines

Yep, that's a good point.
Quoted 4 lines
+ small patch
+ future enhancements rather easy by selecting which data to modify
As for the '-' above about this, I'm not really sure what this means.

In the case mentioned above I just wouldn't zap the topology_sibling_cpumask in my patch.

Quoted 15 lines

Quoted 6 lines
Disadvantages:
- interface to scheduler not as clean as in Dario's approach
- scheduler decisions are based on multiple layers of topology data where one layer would be enough to describe the topology
This is not too big of a deal, IMO. Not at runtime, at least, as far as my investigation went for now. Initialization (of scheduling domains) is a bit clumsy in this case, as scheduling domains are created and then destroyed/collapsed, but after they are setup, the net effect is that there's only one scheduling domain with Juergen's patch too, exactly as with mine.
Quoted 2 lines
Dario, are you okay with this summary?

Quoted 8 lines

To most of it, yes, and thanks again for it.
Allow me to add a few points, out of the top of my head:
* we need to check whether the two approaches have the same performance. In principle, they really should, and early results seems to confirm that, but I'd like to run the full set of benches (and I'll do that ASAP);

Thanks.

Quoted 8 lines

* I think we want to run even more benchmarks, and run them in different (over)load conditions to better assess the effect of the change
* both our patches provides a solution for Xen (for Xen PV guests, at least for now, to be more precise). It is very likely that, e.g., KVM is in a similar situation, hence it may be worth to look for a more general solution, especially if that buys us something (e.g., HVM support made easy?)

I wanted to look at this as soon as we've decided which way to go.

I had some discussion with a kvm guy last week and he seemed not to be convinced they need something else as mangling CPUID (what they already do).

Quoted 5 lines

Thanks and Regards,
Dario
PS. BTW, Juergen, you're not on IRC, on #xendevel, are you?

I'd like to, but I'd need an invitation. My user name is juergen_gross.

Juergen

Juergen Gross

maintainer

2015-09-23 09:44 UTC #25 lore raw

Quoted 94 lines · Dario Faggioli

Quoted 7 lines · Juergen Gross
Quoted 3 lines · George Dunlap
Juergen / Dario, could one of you summarize your two approaches, and the
(alleged) advantages and disadvantages of each one?
Okay, I'll have a try:
Thanks for this! ;-)
Quoted 11 lines
The problem we want to solve:
-----------------------------
The Linux kernel is gathering cpu topology data during boot via the CPUID instruction on each processor coming online. This data is primarily used in the scheduler to decide to which cpu a thread should
be migrated when this seems to be necessary. There are other users of the topology information in the kernel (e.g. some drivers try to do optimizations like core-specific queues/lists).
Quoted 11 lines
When started in a virtualized environment the obtained data is next to
useless or even wrong, as it is reflecting only the status of the time
of booting the system. Scheduling of the (v)cpus done by the hypervisor
is changing the topology beneath the feet of the Linux kernel without reflecting this in the gathered topology information. So any decisions
taken based on that data will be clueless and possibly just wrong.
Exactly.
Quoted 8 lines
The minimal solution is to change the topology data in the kernel in a
way that all cpus are regarded as equal regarding their relation to each
other (e.g. when migrating a thread to another cpu no cpu is preferred
as a target).
Quoted 7 lines
The topology information of the CPUID instruction is, however, even accessible form user mode and might be used for licensing purposes of any user program (e.g. by limiting the software to run on a specific number of cores or sockets). So just mangling the data returned by CPUID in the hypervisor seems not to be a general solution, while we might want to do it at least optionally in the future.
Yep. It turned out that, although being what started all this, CPUID handling is a somewhat related but mostly independent problem. :-)
Quoted 5 lines
In the future we might want to support either dynamic topology updates
or be able to tell the kernel to use some of the topology data, e.g. when pinning vcpus.
Indeed. At least for the latter. Dynamic looks really difficult to me, but indeed it would be ideal. Let's see.
Quoted 12 lines
Solution 1 (Dario):
-------------------
Don't use the CPUID derived topology information in the Linux scheduler,
but let it use a simple "flat" topology by setting own scheduler domain
data under Xen.
Advantages:
+ very clean solution regarding the scheduler interface
Yes, this is, I think, one of the main advantages of the patch. The scheduler is offering an interface to architectures to define their topology requirements and I'm using it, for specifying our topology
requirements: the tool for the job. :-D
Quoted 6 lines
+ scheduler decisions are based on a minimal data set
+ small patch
Disadvantages:
- covers the scheduler only, drivers still use the "wrong" data
This is a good point. It was the patch's purpose, TBH, but it's certainly true that, if we need something similar elsewhere, we need to do more.
Quoted 6 lines
- a little bit hacky regarding some NUMA architectures (needs either a
hook in the code dealing with that architecture or multiple
scheduler
domain data overwrites)

Quoted 8 lines

As I said in my other email, I'll double check (yes, I also think this is about AMD boxes with intra-socket NUMA nodes).
Quoted 4 lines
- future enhancements will make the solution less clean (either need duplicating scheduler domain data or some new hooks in scheduler domain interface)
This one, I'm not sure I understand.

What would you do for keeping the topology information of one level, e.g. hyperthreads, in case we'd have a gang-scheduler in Xen? Either you would copy the line:

{ cpu_smt_mask, cpu_smt_flags, SD_INIT_NAME(SMT) },

from kernel/sched/core.c into your topology array, or you would add a way in kernel/sched/core.c to remove all but this entry and add your entry on top of it.

Quoted 12 lines

Quoted 4 lines
Solution 2 (Juergen):
---------------------
Quoted 8 lines
When booted as a Xen guest modify the topology data built during boot resulting in the same simple "flat" topology as in Dario's solution.
Advantages:
+ the simple topology is seen by all consumers of topology data as the
data itself is modified accordingly

Quoted 7 lines

Yep, that's a good point.
Quoted 4 lines
+ small patch
+ future enhancements rather easy by selecting which data to modify
As for the '-' above about this, I'm not really sure what this means.

In the case mentioned above I just wouldn't zap the topology_sibling_cpumask in my patch.

Quoted 15 lines

Quoted 6 lines
Disadvantages:
- interface to scheduler not as clean as in Dario's approach
- scheduler decisions are based on multiple layers of topology data where one layer would be enough to describe the topology
This is not too big of a deal, IMO. Not at runtime, at least, as far as my investigation went for now. Initialization (of scheduling domains) is a bit clumsy in this case, as scheduling domains are created and then destroyed/collapsed, but after they are setup, the net effect is that there's only one scheduling domain with Juergen's patch too, exactly as with mine.
Quoted 2 lines
Dario, are you okay with this summary?

Quoted 8 lines

To most of it, yes, and thanks again for it.
Allow me to add a few points, out of the top of my head:
* we need to check whether the two approaches have the same performance. In principle, they really should, and early results seems to confirm that, but I'd like to run the full set of benches (and I'll do that ASAP);

Thanks.

Quoted 8 lines

* I think we want to run even more benchmarks, and run them in different (over)load conditions to better assess the effect of the change
* both our patches provides a solution for Xen (for Xen PV guests, at least for now, to be more precise). It is very likely that, e.g., KVM is in a similar situation, hence it may be worth to look for a more general solution, especially if that buys us something (e.g., HVM support made easy?)

I wanted to look at this as soon as we've decided which way to go.

I had some discussion with a kvm guy last week and he seemed not to be convinced they need something else as mangling CPUID (what they already do).

Quoted 5 lines

Thanks and Regards,
Dario
PS. BTW, Juergen, you're not on IRC, on #xendevel, are you?

I'd like to, but I'd need an invitation. My user name is juergen_gross.

Juergen

George Dunlap

2015-09-23 10:24 UTC #26 lore raw

Quoted 83 lines · Juergen Gross

Quoted 11 lines · George Dunlap
Quoted 5 lines · Juergen Gross
One other thing I just discovered: there are other consumers of the topology sibling masks (e.g. topology_sibling_cpumask()) as well.
I think we would want to avoid any optimizations based on those in drivers as well, not only in the scheduler.
I'm beginning to lose the thread of the discussion here a bit.
Juergen / Dario, could one of you summarize your two approaches, and the (alleged) advantages and disadvantages of each one?
Okay, I'll have a try:
The problem we want to solve:
-----------------------------
The Linux kernel is gathering cpu topology data during boot via the CPUID instruction on each processor coming online. This data is primarily used in the scheduler to decide to which cpu a thread should be migrated when this seems to be necessary. There are other users of the topology information in the kernel (e.g. some drivers try to do optimizations like core-specific queues/lists).
When started in a virtualized environment the obtained data is next to useless or even wrong, as it is reflecting only the status of the time of booting the system. Scheduling of the (v)cpus done by the hypervisor is changing the topology beneath the feet of the Linux kernel without reflecting this in the gathered topology information. So any decisions taken based on that data will be clueless and possibly just wrong.
The minimal solution is to change the topology data in the kernel in a way that all cpus are regarded as equal regarding their relation to each other (e.g. when migrating a thread to another cpu no cpu is preferred as a target).
The topology information of the CPUID instruction is, however, even accessible form user mode and might be used for licensing purposes of any user program (e.g. by limiting the software to run on a specific number of cores or sockets). So just mangling the data returned by CPUID in the hypervisor seems not to be a general solution, while we might want to do it at least optionally in the future.
In the future we might want to support either dynamic topology updates or be able to tell the kernel to use some of the topology data, e.g. when pinning vcpus.
Solution 1 (Dario):
-------------------
Don't use the CPUID derived topology information in the Linux scheduler, but let it use a simple "flat" topology by setting own scheduler domain data under Xen.
Advantages:
+ very clean solution regarding the scheduler interface
+ scheduler decisions are based on a minimal data set
+ small patch
Disadvantages:
- covers the scheduler only, drivers still use the "wrong" data
- a little bit hacky regarding some NUMA architectures (needs either a hook in the code dealing with that architecture or multiple scheduler domain data overwrites)
- future enhancements will make the solution less clean (either need duplicating scheduler domain data or some new hooks in scheduler domain interface)
Solution 2 (Juergen):
---------------------
When booted as a Xen guest modify the topology data built during boot resulting in the same simple "flat" topology as in Dario's solution.
Advantages:
+ the simple topology is seen by all consumers of topology data as the data itself is modified accordingly
+ small patch
+ future enhancements rather easy by selecting which data to modify

Quoted 7 lines

Disadvantages:
- interface to scheduler not as clean as in Dario's approach
- scheduler decisions are based on multiple layers of topology data where one layer would be enough to describe the topology
Dario, are you okay with this summary?

Thanks -- that's very helpful.

-George

George Dunlap

2015-09-23 10:24 UTC #27 lore raw

Quoted 83 lines · Juergen Gross

Quoted 11 lines · George Dunlap
Quoted 5 lines · Juergen Gross
One other thing I just discovered: there are other consumers of the topology sibling masks (e.g. topology_sibling_cpumask()) as well.
I think we would want to avoid any optimizations based on those in drivers as well, not only in the scheduler.
I'm beginning to lose the thread of the discussion here a bit.
Juergen / Dario, could one of you summarize your two approaches, and the (alleged) advantages and disadvantages of each one?
Okay, I'll have a try:
The problem we want to solve:
-----------------------------
The Linux kernel is gathering cpu topology data during boot via the CPUID instruction on each processor coming online. This data is primarily used in the scheduler to decide to which cpu a thread should be migrated when this seems to be necessary. There are other users of the topology information in the kernel (e.g. some drivers try to do optimizations like core-specific queues/lists).
When started in a virtualized environment the obtained data is next to useless or even wrong, as it is reflecting only the status of the time of booting the system. Scheduling of the (v)cpus done by the hypervisor is changing the topology beneath the feet of the Linux kernel without reflecting this in the gathered topology information. So any decisions taken based on that data will be clueless and possibly just wrong.
The minimal solution is to change the topology data in the kernel in a way that all cpus are regarded as equal regarding their relation to each other (e.g. when migrating a thread to another cpu no cpu is preferred as a target).
The topology information of the CPUID instruction is, however, even accessible form user mode and might be used for licensing purposes of any user program (e.g. by limiting the software to run on a specific number of cores or sockets). So just mangling the data returned by CPUID in the hypervisor seems not to be a general solution, while we might want to do it at least optionally in the future.
In the future we might want to support either dynamic topology updates or be able to tell the kernel to use some of the topology data, e.g. when pinning vcpus.
Solution 1 (Dario):
-------------------
Don't use the CPUID derived topology information in the Linux scheduler, but let it use a simple "flat" topology by setting own scheduler domain data under Xen.
Advantages:
+ very clean solution regarding the scheduler interface
+ scheduler decisions are based on a minimal data set
+ small patch
Disadvantages:
- covers the scheduler only, drivers still use the "wrong" data
- a little bit hacky regarding some NUMA architectures (needs either a hook in the code dealing with that architecture or multiple scheduler domain data overwrites)
- future enhancements will make the solution less clean (either need duplicating scheduler domain data or some new hooks in scheduler domain interface)
Solution 2 (Juergen):
---------------------
When booted as a Xen guest modify the topology data built during boot resulting in the same simple "flat" topology as in Dario's solution.
Advantages:
+ the simple topology is seen by all consumers of topology data as the data itself is modified accordingly
+ small patch
+ future enhancements rather easy by selecting which data to modify

Quoted 7 lines

Disadvantages:
- interface to scheduler not as clean as in Dario's approach
- scheduler decisions are based on multiple layers of topology data where one layer would be enough to describe the topology
Dario, are you okay with this summary?

Thanks -- that's very helpful.

-George

George Dunlap

2015-09-22 16:22 UTC #28 lore raw

Quoted 5 lines · Juergen Gross

One other thing I just discovered: there are other consumers of the topology sibling masks (e.g. topology_sibling_cpumask()) as well.
I think we would want to avoid any optimizations based on those in drivers as well, not only in the scheduler.

I'm beginning to lose the thread of the discussion here a bit.

Juergen / Dario, could one of you summarize your two approaches, and the (alleged) advantages and disadvantages of each one?

Thanks,
-George

Juergen Gross

maintainer

2015-09-22 04:42 UTC #29 lore raw

Quoted 144 lines · Juergen Gross

Quoted 135 lines · Dario Faggioli
Quoted 97 lines · Juergen Groß
Quoted 90 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter
whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no
matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for
vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is
no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is
the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any
unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Quoted 6 lines
Bear also in mind that these results are gathered from Dom0, and without
any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.
Quoted 9 lines
...
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.
Hey,
Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)
Quoted 5 lines
Regarding a possible future scenario with credit2 eventually supporting gang scheduling on hyperthreads (which is desirable due to security reasons [side channel attack] and fairness) my patch seems to be more suited for that direction than yours.
Ok. Just let me mention that 'Credit2 + gang scheduling' might not be exactly around the corner (although, we can prioritize working on it if we want).
In principle, I think it's a really nice idea. I still don't have clear in mind how we would handle a couple of situations, but let's leave this aside for now, and stay on-topic.
Quoted 5 lines
Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to migrate threads more easily between hyperthreads opposed to other vcpus, while my approach can easily be extended to do so.
I'm not sure I understand what you mean here. As far as the (Linux) scheduler is concerned, your patch and mine do the exact same thing:
they arrange for the scheduling domains, when they're built, during boot, not to consider hyperthreads or multi-cores.
Quoted 7 lines
Mine does it by removing the SMT (and the MC) level from the data structure in the scheduler that is used as a base for configuring the scheduling domains. Yours does it by making the topology bitmaps that are used at each one of those level all look the same. In fact, with your patch applied, I get the exact same situation as with mine, as far as scheduling domains are concerned: there is only one scheduling domain, with a different scheduling group for each vCPU inside it.

Quoted 7 lines

Uuh, nearly.
Your case won't deal correctly with NUMA, as the generic NUMA code is using set_sched_topology() as well. One of NUMA and Xen will win and overwrite the other's settings.
To do things correctly you will have to handle NUMA as well.

One other thing I just discovered: there are other consumers of the topology sibling masks (e.g. topology_sibling_cpumask()) as well.

I think we would want to avoid any optimizations based on those in drivers as well, not only in the scheduler.

Juergen

Dario Faggioli

author

2015-09-23 07:25 UTC #30 lore raw

Quoted 195 lines · Juergen Gross

Quoted 182 lines · Dario Faggioli
Quoted 134 lines · Juergen Groß
Quoted 132 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by
the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/
msg03241.html
I started looking in more details at scheduling domains in the Linux
kernel. Now, that thread was about CPUID and vNUMA, and their weird way
of interacting, while this thing I'm proposing here is completely
independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter
whether CPUID is reporting accurate, random, meaningful or completely
misleading information, I think that we should do something about how
scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in
Linux, by looking at *any* topology information, because that just does
not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no
matter in how much good shape we put CPUID support, no matter how
beautifully and consistently that will interact with both vNUMA,
licensing requirements and whatever else. It will be always possible for
vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and
on two different NUMA nodes at time t2. Hence, the Linux scheduler
should really not skew his load balancing logic toward any of those two
situations, as neither of them could be considered correct (since
nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any
different, but I haven't looked at how to make the same thing happen in
there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling
domains in such a way that there is only one of them, spanning all the
pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is
no need to alter the masks that will then be used for building and
reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is
the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg
05088.html
This means that when, in future, we will fix CPUID handling and make it
comply with whatever logic or requirements we want, that won't have any
unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed
during boot are available in `dmesg', if the kernel is booted with the
'sched_debug' parameter. It is also possible to look at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called
the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can
tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling
domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a
16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for
now, but I plan to re-run them in DomUs soon (Juergen may be doing
something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched'
and 'vanilla', respectively, in the tables below), and with different
number of build jobs (in case of the Xen build) or of parallel copy of
the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings
benefits, in some cases even huge ones. There are a couple of cases
where we regress, but always only slightly so, especially if comparing
that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without
any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If
we move things in DomU and do overcommit at the Xen scheduler level, I
am expecting even better results.
...
Quoted 9 lines
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and
myself) discussed this topic again.
Hey,
Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)
Quoted 8 lines
Regarding a possible future scenario with credit2 eventually supporting
gang scheduling on hyperthreads (which is desirable due to security
reasons [side channel attack] and fairness) my patch seems to be more
suited for that direction than yours.
Ok. Just let me mention that 'Credit2 + gang scheduling' might not be
exactly around the corner (although, we can prioritize working on it if
we want).
In principle, I think it's a really nice idea. I still don't have clear
in mind how we would handle a couple of situations, but let's leave this
aside for now, and stay on-topic.
Quoted 7 lines
Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to
migrate threads more easily between hyperthreads opposed to other vcpus,
while my approach can easily be extended to do so.
I'm not sure I understand what you mean here. As far as the (Linux) scheduler is concerned, your patch and mine do the exact same thing:
they arrange for the scheduling domains, when they're built, during boot, not to consider hyperthreads or multi-cores.
Quoted 11 lines
Mine does it by removing the SMT (and the MC) level from the data structure in the scheduler that is used as a base for configuring the
scheduling domains. Yours does it by making the topology bitmaps that
are used at each one of those level all look the same. In fact, with
your patch applied, I get the exact same situation as with mine, as far
as scheduling domains are concerned: there is only one scheduling domain, with a different scheduling group for each vCPU inside it.

Quoted 5 lines

Uuh, nearly.
Your case won't deal correctly with NUMA, as the generic NUMA code is using set_sched_topology() as well.

Mmm... have you tried and seen something like this? AFAICT, the NUMA related setup steps of scheduling domains happens after the basic (as in "without taking NUMAness into account") topology has been set already, and builds on top of it.

It uses set_sched_topology() only in a special case which, I'm not sure we'd be hitting.

I'm asking because trying this out, right now, is not straightforward, as PV vNUMA, even with Wei's Linux patches and with either yours or mine one, still incurs in the CPUID issue... I'll try that ASAP, but there are a couple of things I've got to finish for the next few days.

Quoted 3 lines

One of NUMA and Xen will win and
overwrite the other's settings.

Not sure what this means, but as I said, I'll try.

Regards,
Dario

Signature · 4 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Dario Faggioli

author

2015-09-23 07:25 UTC #31 lore raw

[1]: http://developer.amd.com/resources/documentation-articles/articles-whitepapers/introduction-to-magny-cours/

Quoted 195 lines · Juergen Gross

Quoted 182 lines · Dario Faggioli
Quoted 134 lines · Juergen Groß
Quoted 132 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by
the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/
msg03241.html
I started looking in more details at scheduling domains in the Linux
kernel. Now, that thread was about CPUID and vNUMA, and their weird way
of interacting, while this thing I'm proposing here is completely
independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter
whether CPUID is reporting accurate, random, meaningful or completely
misleading information, I think that we should do something about how
scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in
Linux, by looking at *any* topology information, because that just does
not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no
matter in how much good shape we put CPUID support, no matter how
beautifully and consistently that will interact with both vNUMA,
licensing requirements and whatever else. It will be always possible for
vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and
on two different NUMA nodes at time t2. Hence, the Linux scheduler
should really not skew his load balancing logic toward any of those two
situations, as neither of them could be considered correct (since
nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any
different, but I haven't looked at how to make the same thing happen in
there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling
domains in such a way that there is only one of them, spanning all the
pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is
no need to alter the masks that will then be used for building and
reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is
the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg
05088.html
This means that when, in future, we will fix CPUID handling and make it
comply with whatever logic or requirements we want, that won't have any
unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed
during boot are available in `dmesg', if the kernel is booted with the
'sched_debug' parameter. It is also possible to look at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called
the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can
tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling
domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a
16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for
now, but I plan to re-run them in DomUs soon (Juergen may be doing
something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched'
and 'vanilla', respectively, in the tables below), and with different
number of build jobs (in case of the Xen build) or of parallel copy of
the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings
benefits, in some cases even huge ones. There are a couple of cases
where we regress, but always only slightly so, especially if comparing
that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without
any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If
we move things in DomU and do overcommit at the Xen scheduler level, I
am expecting even better results.
...
Quoted 9 lines
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and
myself) discussed this topic again.
Hey,
Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)
Quoted 8 lines
Regarding a possible future scenario with credit2 eventually supporting
gang scheduling on hyperthreads (which is desirable due to security
reasons [side channel attack] and fairness) my patch seems to be more
suited for that direction than yours.
Ok. Just let me mention that 'Credit2 + gang scheduling' might not be
exactly around the corner (although, we can prioritize working on it if
we want).
In principle, I think it's a really nice idea. I still don't have clear
in mind how we would handle a couple of situations, but let's leave this
aside for now, and stay on-topic.
Quoted 7 lines
Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to
migrate threads more easily between hyperthreads opposed to other vcpus,
while my approach can easily be extended to do so.
I'm not sure I understand what you mean here. As far as the (Linux) scheduler is concerned, your patch and mine do the exact same thing:
they arrange for the scheduling domains, when they're built, during boot, not to consider hyperthreads or multi-cores.
Quoted 11 lines
Mine does it by removing the SMT (and the MC) level from the data structure in the scheduler that is used as a base for configuring the
scheduling domains. Yours does it by making the topology bitmaps that
are used at each one of those level all look the same. In fact, with
your patch applied, I get the exact same situation as with mine, as far
as scheduling domains are concerned: there is only one scheduling domain, with a different scheduling group for each vCPU inside it.

Quoted 5 lines

Uuh, nearly.
Your case won't deal correctly with NUMA, as the generic NUMA code is using set_sched_topology() as well.

It uses set_sched_topology() only in a special case which, I'm not sure we'd be hitting.

Quoted 3 lines

One of NUMA and Xen will win and
overwrite the other's settings.

Not sure what this means, but as I said, I'll try.

Regards,
Dario

Signature · 4 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

Juergen Gross

maintainer

2015-09-23 07:35 UTC #32 lore raw

Quoted 201 lines · Dario Faggioli

Quoted 195 lines · Juergen Gross
Quoted 182 lines · Dario Faggioli
Quoted 134 lines · Juergen Groß
Quoted 132 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by
the guest
    http://lists.xenproject.org/archives/html/xen-devel/2015-07/
msg03241.html
I started looking in more details at scheduling domains in the Linux
kernel. Now, that thread was about CPUID and vNUMA, and their weird way
of interacting, while this thing I'm proposing here is completely
independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter
whether CPUID is reporting accurate, random, meaningful or completely
misleading information, I think that we should do something about how
scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in
Linux, by looking at *any* topology information, because that just does
not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no
matter in how much good shape we put CPUID support, no matter how
beautifully and consistently that will interact with both vNUMA,
licensing requirements and whatever else. It will be always possible for
vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and
on two different NUMA nodes at time t2. Hence, the Linux scheduler
should really not skew his load balancing logic toward any of those two
situations, as neither of them could be considered correct (since
nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any
different, but I haven't looked at how to make the same thing happen in
there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling
domains in such a way that there is only one of them, spanning all the
pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is
no need to alter the masks that will then be used for building and
reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is
the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg
05088.html
This means that when, in future, we will fix CPUID handling and make it
comply with whatever logic or requirements we want, that won't have any
unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed
during boot are available in `dmesg', if the kernel is booted with the
'sched_debug' parameter. It is also possible to look at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called
the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can
tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling
domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a
16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for
now, but I plan to re-run them in DomUs soon (Juergen may be doing
something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched'
and 'vanilla', respectively, in the tables below), and with different
number of build jobs (in case of the Xen build) or of parallel copy of
the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings
benefits, in some cases even huge ones. There are a couple of cases
where we regress, but always only slightly so, especially if comparing
that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without
any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If
we move things in DomU and do overcommit at the Xen scheduler level, I
am expecting even better results.
...
Quoted 9 lines
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and
myself) discussed this topic again.
Hey,
Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)
Quoted 8 lines
Regarding a possible future scenario with credit2 eventually supporting
gang scheduling on hyperthreads (which is desirable due to security
reasons [side channel attack] and fairness) my patch seems to be more
suited for that direction than yours.
Ok. Just let me mention that 'Credit2 + gang scheduling' might not be
exactly around the corner (although, we can prioritize working on it if
we want).
In principle, I think it's a really nice idea. I still don't have clear
in mind how we would handle a couple of situations, but let's leave this
aside for now, and stay on-topic.
Quoted 7 lines
Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to
migrate threads more easily between hyperthreads opposed to other vcpus,
while my approach can easily be extended to do so.
I'm not sure I understand what you mean here. As far as the (Linux) scheduler is concerned, your patch and mine do the exact same thing:
they arrange for the scheduling domains, when they're built, during boot, not to consider hyperthreads or multi-cores.
Quoted 11 lines
Mine does it by removing the SMT (and the MC) level from the data structure in the scheduler that is used as a base for configuring the
scheduling domains. Yours does it by making the topology bitmaps that
are used at each one of those level all look the same. In fact, with
your patch applied, I get the exact same situation as with mine, as far
as scheduling domains are concerned: there is only one scheduling domain, with a different scheduling group for each vCPU inside it.
Quoted 5 lines
Uuh, nearly.
Your case won't deal correctly with NUMA, as the generic NUMA code is using set_sched_topology() as well.

Quoted 7 lines

Mmm... have you tried and seen something like this? AFAICT, the NUMA related setup steps of scheduling domains happens after the basic (as in "without taking NUMAness into account") topology has been set already, and builds on top of it.
It uses set_sched_topology() only in a special case which, I'm not sure we'd be hitting.

Depends on the hardware. On some AMD processors one socket covers multiple NUMA nodes. This is the critical case. set_sched_topology() will be called on those machines possibly multiple times when bringing up additional cpus.

Quoted 9 lines

I'm asking because trying this out, right now, is not straightforward, as PV vNUMA, even with Wei's Linux patches and with either yours or mine one, still incurs in the CPUID issue... I'll try that ASAP, but there are a couple of things I've got to finish for the next few days.
Quoted 3 lines
One of NUMA and Xen will win and
overwrite the other's settings.
Not sure what this means, but as I said, I'll try.

Make sure to use the correct hardware (I'm pretty sure this should be the AMD "Magny-Cours" [1]).

Juergen

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

Juergen Gross

maintainer

2015-09-23 07:35 UTC #33 lore raw

[1]: http://developer.amd.com/resources/documentation-articles/articles-whitepapers/introduction-to-magny-cours/

Quoted 201 lines · Dario Faggioli

Quoted 195 lines · Juergen Gross
Quoted 182 lines · Dario Faggioli
Quoted 134 lines · Juergen Groß
Quoted 132 lines · Dario Faggioli
Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by
the guest
    http://lists.xenproject.org/archives/html/xen-devel/2015-07/
msg03241.html
I started looking in more details at scheduling domains in the Linux
kernel. Now, that thread was about CPUID and vNUMA, and their weird way
of interacting, while this thing I'm proposing here is completely
independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter
whether CPUID is reporting accurate, random, meaningful or completely
misleading information, I think that we should do something about how
scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in
Linux, by looking at *any* topology information, because that just does
not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no
matter in how much good shape we put CPUID support, no matter how
beautifully and consistently that will interact with both vNUMA,
licensing requirements and whatever else. It will be always possible for
vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and
on two different NUMA nodes at time t2. Hence, the Linux scheduler
should really not skew his load balancing logic toward any of those two
situations, as neither of them could be considered correct (since
nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any
different, but I haven't looked at how to make the same thing happen in
there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling
domains in such a way that there is only one of them, spanning all the
pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is
no need to alter the masks that will then be used for building and
reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is
the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg
05088.html
This means that when, in future, we will fix CPUID handling and make it
comply with whatever logic or requirements we want, that won't have any
unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed
during boot are available in `dmesg', if the kernel is booted with the
'sched_debug' parameter. It is also possible to look at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called
the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can
tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling
domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a
16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for
now, but I plan to re-run them in DomUs soon (Juergen may be doing
something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched'
and 'vanilla', respectively, in the tables below), and with different
number of build jobs (in case of the Xen build) or of parallel copy of
the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings
benefits, in some cases even huge ones. There are a couple of cases
where we regress, but always only slightly so, especially if comparing
that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without
any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If
we move things in DomU and do overcommit at the Xen scheduler level, I
am expecting even better results.
...
Quoted 9 lines
Quoted 4 lines
REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,
Yesterday at the end of the developer meeting we (Andrew, Elena and
myself) discussed this topic again.
Hey,
Sorry for replying so late, I've been on vacation from right after XenSummit up until yesterday. :-)
Quoted 8 lines
Regarding a possible future scenario with credit2 eventually supporting
gang scheduling on hyperthreads (which is desirable due to security
reasons [side channel attack] and fairness) my patch seems to be more
suited for that direction than yours.
Ok. Just let me mention that 'Credit2 + gang scheduling' might not be
exactly around the corner (although, we can prioritize working on it if
we want).
In principle, I think it's a really nice idea. I still don't have clear
in mind how we would handle a couple of situations, but let's leave this
aside for now, and stay on-topic.
Quoted 7 lines
Correct me if I'm wrong, but I
think scheduling domains won't enable the guest kernel's scheduler to
migrate threads more easily between hyperthreads opposed to other vcpus,
while my approach can easily be extended to do so.
I'm not sure I understand what you mean here. As far as the (Linux) scheduler is concerned, your patch and mine do the exact same thing:
they arrange for the scheduling domains, when they're built, during boot, not to consider hyperthreads or multi-cores.
Quoted 11 lines
Mine does it by removing the SMT (and the MC) level from the data structure in the scheduler that is used as a base for configuring the
scheduling domains. Yours does it by making the topology bitmaps that
are used at each one of those level all look the same. In fact, with
your patch applied, I get the exact same situation as with mine, as far
as scheduling domains are concerned: there is only one scheduling domain, with a different scheduling group for each vCPU inside it.
Quoted 5 lines
Uuh, nearly.
Your case won't deal correctly with NUMA, as the generic NUMA code is using set_sched_topology() as well.

Quoted 7 lines

Mmm... have you tried and seen something like this? AFAICT, the NUMA related setup steps of scheduling domains happens after the basic (as in "without taking NUMAness into account") topology has been set already, and builds on top of it.
It uses set_sched_topology() only in a special case which, I'm not sure we'd be hitting.

Quoted 9 lines

I'm asking because trying this out, right now, is not straightforward, as PV vNUMA, even with Wei's Linux patches and with either yours or mine one, still incurs in the CPUID issue... I'll try that ASAP, but there are a couple of things I've got to finish for the next few days.
Quoted 3 lines
One of NUMA and Xen will win and
overwrite the other's settings.
Not sure what this means, but as I said, I'll try.

Make sure to use the correct hardware (I'm pretty sure this should be the AMD "Magny-Cours" [1]).

Juergen

Boris Ostrovsky

reviewer

2015-09-23 12:27 UTC #34 lore raw

Quoted 16 lines · Juergen Gross

Depends on the hardware. On some AMD processors one socket covers multiple NUMA nodes. This is the critical case. set_sched_topology() will be called on those machines possibly multiple times when bringing up additional cpus.
Quoted 9 lines
I'm asking because trying this out, right now, is not straightforward, as PV vNUMA, even with Wei's Linux patches and with either yours or mine one, still incurs in the CPUID issue... I'll try that ASAP, but there are a couple of things I've got to finish for the next few days.
Quoted 3 lines
One of NUMA and Xen will win and
overwrite the other's settings.
Not sure what this means, but as I said, I'll try.

Quoted 8 lines

Make sure to use the correct hardware (I'm pretty sure this should be the AMD "Magny-Cours" [1]).
Juergen
[1]: http://developer.amd.com/resources/documentation-articles/articles-whitepapers/introduction-to-magny-cours/

There are few family 0x10 and 0x15 processors that are like that. You can see whether you have such a system by comparing number of NUMA nodes with number of physical IDs, e.g.:

[root@ovs106 ~]# numactl --hardware |grep available
available: 4 nodes (0-3)
[root@ovs106 ~]# grep "physical id" /proc/cpuinfo | uniq
physical id : 0
physical id : 1
[root@ovs106 ~]#

-boris

Boris Ostrovsky

reviewer

2015-09-23 12:27 UTC #35 lore raw

Quoted 16 lines · Juergen Gross

Depends on the hardware. On some AMD processors one socket covers multiple NUMA nodes. This is the critical case. set_sched_topology() will be called on those machines possibly multiple times when bringing up additional cpus.
Quoted 9 lines
I'm asking because trying this out, right now, is not straightforward, as PV vNUMA, even with Wei's Linux patches and with either yours or mine one, still incurs in the CPUID issue... I'll try that ASAP, but there are a couple of things I've got to finish for the next few days.
Quoted 3 lines
One of NUMA and Xen will win and
overwrite the other's settings.
Not sure what this means, but as I said, I'll try.

Quoted 8 lines

Make sure to use the correct hardware (I'm pretty sure this should be the AMD "Magny-Cours" [1]).
Juergen
[1]: http://developer.amd.com/resources/documentation-articles/articles-whitepapers/introduction-to-magny-cours/

There are few family 0x10 and 0x15 processors that are like that. You can see whether you have such a system by comparing number of NUMA nodes with number of physical IDs, e.g.:

[root@ovs106 ~]# numactl --hardware |grep available
available: 4 nodes (0-3)
[root@ovs106 ~]# grep "physical id" /proc/cpuinfo | uniq
physical id : 0
physical id : 1
[root@ovs106 ~]#

-boris

Juergen Groß

maintainer

2015-08-20 18:19 UTC #36 lore raw

Quoted 86 lines · Dario Faggioli

Hey everyone,
So, as a followup of what we were discussing in this thread:
[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html
I started looking in more details at scheduling domains in the Linux kernel. Now, that thread was about CPUID and vNUMA, and their weird way of interacting, while this thing I'm proposing here is completely independent from them both.
In fact, no matter whether vNUMA is supported and enabled, and no matter whether CPUID is reporting accurate, random, meaningful or completely misleading information, I think that we should do something about how scheduling domains are build.
Fact is, unless we use 1:1, and immutable (across all the guest lifetime) pinning, scheduling domains should not be constructed, in Linux, by looking at *any* topology information, because that just does not make any sense, when vcpus move around.
Let me state this again (hoping to make myself as clear as possible): no matter in how much good shape we put CPUID support, no matter how beautifully and consistently that will interact with both vNUMA, licensing requirements and whatever else. It will be always possible for vCPU #0 and vCPU #3 to be scheduled on two SMT threads at time t1, and on two different NUMA nodes at time t2. Hence, the Linux scheduler should really not skew his load balancing logic toward any of those two situations, as neither of them could be considered correct (since nothing is!).
For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.
OVERALL DESCRIPTION
===================
What this RFC patch does is, in the Xen PV case, configure scheduling domains in such a way that there is only one of them, spanning all the pCPUs of the guest.
Note that the patch deals directly with scheduling domains, and there is no need to alter the masks that will then be used for building and reporting the topology (via CPUID, /proc/cpuinfo, /sysfs, etc.). That is the main difference between it and the patch proposed by Juergen here:
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg05088.html
This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.
Information about how the scheduling domains are being constructed during boot are available in `dmesg', if the kernel is booted with the 'sched_debug' parameter. It is also possible to look
at /proc/sys/kernel/sched_domain/cpu*, and at /proc/schedstat.
With the patch applied, only one scheduling domain is created, called the 'VCPU' domain, spanning all the guest's (or Dom0's) vCPUs. You can tell that from the fact that every cpu* folder
in /proc/sys/kernel/sched_domain/ only have one subdirectory ('domain0'), with all the tweaks and the tunables for our scheduling domain.
EVALUATION
==========
I've tested this with UnixBench, and by looking at Xen build time, on a 16, 24 and 48 pCPUs hosts. I've run the benchmarks in Dom0 only, for now, but I plan to re-run them in DomUs soon (Juergen may be doing something similar to this in DomU already, AFAUI).
I've run the benchmarks with and without the patch applied ('patched' and 'vanilla', respectively, in the tables below), and with different number of build jobs (in case of the Xen build) or of parallel copy of the benchmarks (in the case of UnixBench).
What I get from the numbers is that the patch almost always brings benefits, in some cases even huge ones. There are a couple of cases where we regress, but always only slightly so, especially if comparing that to the magnitude of some of the improvement that we get.
Bear also in mind that these results are gathered from Dom0, and without any overcommitment at the vCPU level (i.e., nr. vCPUs == nr pCPUs). If we move things in DomU and do overcommit at the Xen scheduler level, I am expecting even better results.

...

Quoted 4 lines

REQUEST FOR COMMENTS
====================
Basically, the kind of feedback I'd be really glad to hear is:
- what you guys thing of the approach,

Yesterday at the end of the developer meeting we (Andrew, Elena and myself) discussed this topic again.

Quoted 2 lines

- whether you think, looking at this preliminary set of numbers, that this is something worth continuing investigating,

Quoted 2 lines

- if yes, what other workloads and benchmark it would make sense to throw at it.

As you already mentioned an overcommitted host should be looked at as well.

Thanks for doing the measurements,

Juergen

George Dunlap

2015-08-27 10:24 UTC #37 lore raw

Quoted 213 lines · Dario Faggioli

Hey everyone,

So, as a followup of what we were discussing in this thread:

[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html

For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.

This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.

RESULTS
=======
To have a quick idea of how a benchmark went, look at the '% improvement' row of each table.

I'll put these results online, in a googledoc spreadsheet or something like that, to make them easier to read, as soon as possible.

*** Intel(R) Xeon(R) E5620 @ 2.40GHz

*** pCPUs      16        DOM0 vCPUS  16
*** RAM        12285 MB  DOM0 Memory 9955 MB

# of build jobs                     -j1                   -j6                   -j8                   -j16**                -j24                
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              153.72     152.41      35.33      34.93       30.7      30.33      26.79      25.97      26.88      26.21
                              153.81     152.76      35.37      34.99      30.81      30.36      26.83      26.08         27      26.24
                              153.93     152.79      35.37      35.25      30.92      30.39      26.83      26.13      27.01      26.28
                              153.94     152.94      35.39      35.28      31.05      30.43       26.9      26.14      27.01      26.44
                              153.98     153.06      35.45      35.31      31.17       30.5      26.95      26.18      27.02      26.55
                              154.01     153.23       35.5      35.35       31.2      30.59      26.98       26.2      27.05      26.61
                              154.04     153.34      35.56      35.42      31.45      30.76      27.12      26.21      27.06      26.78
                              154.16      153.5      37.79      35.58      31.68      30.83      27.16      26.23      27.16      26.78
                              154.18     153.71      37.98      35.61      33.73       30.9      27.49      26.32      27.16       26.8
                              154.9      154.67      38.03      37.64      34.69      31.69      29.82      26.38       27.2      28.63
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                        154.067    153.241     36.177     35.536      31.74     30.678     27.287     26.184     27.055     26.732
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     0.325      0.631      1.215      0.771      1.352      0.410      0.914      0.116      0.095      0.704
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                            0.536                 1.772                 3.346                 4.042                 1.194
========================================================================================================================================
====================================================================================================================================================
UNIXBENCH
====================================================================================================================================================
# parallel copies                            1 parallel            6 parrallel           8 parallel            16 parallel**         24 parallel
vanilla/patched                          vanilla    patched    vanilla    pached     vanilla    patched    vanilla    patched    vanilla    patched
----------------------------------------------------------------------------------------------------------------------------------------------------
Dhrystone 2 using register variables       2302.2     2302.1    13157.8    12262.4    15691.5    15860.1    18927.7    19078.5    18654.3    18855.6
Double-Precision Whetstone                  620.2      620.2     3481.2     3566.9     4669.2     4551.5     7610.1     7614.3    11558.9    11561.3
Execl Throughput                            184.3      186.7      884.6      905.3     1168.4     1213.6     2134.6     2210.2     2250.9       2265
File Copy 1024 bufsize 2000 maxblocks       780.8      783.3     1243.7     1255.5     1250.6     1215.7     1080.9     1094.2     1069.8     1062.5
File Copy 256 bufsize 500 maxblocks         479.8      482.8      781.8      803.6      806.4        781      682.9      707.7      698.2      694.6
File Copy 4096 bufsize 8000 maxblocks      1617.6     1593.5     2739.7     2943.4     2818.3     2957.8     2389.6     2412.6     2371.6     2423.8
Pipe Throughput                             363.9      361.6     2068.6     2065.6       2622     2633.5     4053.3     4085.9     4064.7     4076.7
Pipe-based Context Switching                 70.6      207.2      369.1     1126.8      623.9     1431.3     1970.4     2082.9     1963.8       2077
Process Creation                            103.1        135        503      677.6      618.7      855.4       1138     1113.7     1195.6       1199
Shell Scripts (1 concurrent)                723.2      765.3     4406.4     4334.4     5045.4     5002.5     5861.9     5844.2     5958.8     5916.1
Shell Scripts (8 concurrent)               2243.7     2715.3     5694.7     5663.6     5694.7     5657.8     5637.1     5600.5     5582.9     5543.6
System Call Overhead                          330      330.1     1669.2     1672.4     2028.6     1996.6     2920.5     2947.1     2923.9     2952.5
System Benchmarks Index Score               496.8      567.5     1861.9       2106     2220.3     2441.3     2972.5     3007.9     3103.4     3125.3
----------------------------------------------------------------------------------------------------------------------------------------------------
% increase (of the Index Score)                       14.231                13.110                 9.954                 1.191                 0.706
====================================================================================================================================================

*** Intel(R) Xeon(R) X5650 @ 2.67GHz

*** pCPUs      24        DOM0 vCPUS  16
*** RAM        36851 MB  DOM0 Memory 9955 MB

# of build jobs                     -j1                   -j8                   -j12                   -j24**               -j32
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              119.49     119.47      23.37      23.29      20.12      19.85      17.99       17.9      17.82       17.8
                              119.59     119.64      23.52      23.31      20.16      19.99      18.19      18.05      18.23      17.89
                              119.59     119.65      23.53      23.35      20.19      20.08      18.26      18.09      18.35      17.91
                              119.72     119.75      23.63      23.41       20.2      20.14      18.54       18.1       18.4      17.95
                              119.95     119.86      23.68      23.42      20.24      20.19      18.57      18.15      18.44      18.03
                              119.97      119.9      23.72      23.51      20.38      20.31      18.61      18.21      18.49      18.03
                              119.97     119.91      25.03      23.53      20.38      20.42      18.75      18.28      18.51      18.08
                              120.01     119.98      25.05      23.93      20.39      21.69      19.99      18.49      18.52       18.6
                              120.24     119.99      25.12      24.19      21.67      21.76      20.08      19.74      19.73      19.62
                              120.66     121.22      25.16      25.36      21.94      21.85      20.26       20.3      19.92      19.81
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                        119.919    119.937     24.181      23.73     20.567     20.628     18.924     18.531     18.641     18.372
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     0.351      0.481      0.789      0.642      0.663      0.802      0.851      0.811      0.658      0.741
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                           -0.015                 1.865                -0.297                 2.077                 1.443
========================================================================================================================================
====================================================================================================================================================
UNIXBENCH
====================================================================================================================================================
# parallel copies                            1 parallel            8 parrallel            12 parallel           24 parallel**         32 parallel
vanilla/patched                          vanilla     patched   vanilla     pached     vanilla    patched    vanilla    patched    vanilla    patched
----------------------------------------------------------------------------------------------------------------------------------------------------
Dhrystone 2 using register variables       2650.1     2664.6    18967.8    19060.4    27534.1    27046.8    30077.9    30110.6    30542.1    30358.7
Double-Precision Whetstone                  713.7      713.5     5463.6     5455.1     7863.9     7923.8    12725.1    12727.8    17474.3    17463.3
Execl Throughput                            280.9      283.8     1724.4     1866.5     2029.5     2367.6       2370     2521.3       2453     2506.8
File Copy 1024 bufsize 2000 maxblocks       891.1      894.2       1423     1457.7     1385.6     1482.2     1226.1     1224.2     1235.9     1265.5
File Copy 256 bufsize 500 maxblocks         546.9      555.4        949      972.1      882.8      878.6      821.9      817.7      784.7      810.8
File Copy 4096 bufsize 8000 maxblocks      1743.4     1722.8     3406.5     3438.9     3314.3     3265.9     2801.9     2788.3     2695.2     2781.5
Pipe Throughput                             426.8      423.4     3207.9       3234     4635.1     4708.9       7326     7335.3     7327.2     7319.7
Pipe-based Context Switching                110.2      223.5      680.8     1602.2      998.6     2324.6     3122.1     3252.7     3128.6     3337.2
Process Creation                            130.7      224.4     1001.3     1043.6       1209     1248.2     1337.9     1380.4     1338.6     1280.1
Shell Scripts (1 concurrent)               1140.5     1257.5     5462.8     6146.4     6435.3     7206.1     7425.2     7636.2     7566.1     7636.6
Shell Scripts (8 concurrent)                 3492     3586.7     7144.9       7307       7258     7320.2     7295.1     7296.7     7248.6     7252.2
System Call Overhead                        387.7      387.5     2398.4       2367     2793.8     2752.7     3735.7     3694.2     3752.1     3709.4
System Benchmarks Index Score               634.8      712.6     2725.8     3005.7     3232.4     3569.7     3981.3     4028.8     4085.2     4126.3
----------------------------------------------------------------------------------------------------------------------------------------------------
% increase (of the Index Score)                       12.256                10.269                10.435                 1.193                 1.006
====================================================================================================================================================

# of build jobs                     -j1                   -j20                   -j24                  -j48**               -j62
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              267.78     233.25      36.53      35.53      35.98      34.99      33.46      32.13      33.57      32.54
                              268.42     233.92      36.82      35.56      36.12       35.2      34.24      32.24      33.64      32.56
                              268.85     234.39      36.92      35.75      36.15      35.35      34.48      32.86      33.67      32.74
                              268.98     235.11      36.96      36.01      36.25      35.46      34.73      32.89      33.97      32.83
                              269.03     236.48      37.04      36.16      36.45      35.63      34.77      32.97      34.12      33.01
                              269.54     237.05      40.33      36.59      36.57      36.15      34.97      33.09      34.18      33.52
                              269.99     238.24      40.45      36.78      36.58      36.22      34.99      33.69      34.28      33.63
                              270.11     238.48      41.13      39.98      40.22      36.24         38      33.92      34.35      33.87
                              270.96     239.07      41.66      40.81      40.59      36.35      38.99      34.19      34.49      37.24

Quoted 8 lines

                              271.84     240.89      42.07      41.24      40.63      40.06      39.07      36.04      34.69      37.59
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                         269.55    236.688     38.991     37.441     37.554     36.165      35.77     33.402     34.096     33.953
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     1.213      2.503      2.312      2.288      2.031      1.452      2.079      1.142      0.379      1.882
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                           12.191                 3.975                 3.699                 6.620                 0.419
========================================================================================================================================

I'm a bit confused here as to why, if dom0 has 16 vcpus in all of your tests, you change the -j number (apparently) based on the number of pcpus available to Xen. Wouldn't it make more sense to stick with 1/6/8/16/24? That would allow us to have actually comparable numbers.

But in any case, it seems to me that the numbers do show a uniform improvement and no regressions -- I think this approach looks really good, particularly as it is so small and well-contained.

-George

George Dunlap

2015-08-27 17:05 UTC #38 lore raw

Quoted 223 lines · George Dunlap

Quoted 213 lines · Dario Faggioli

Hey everyone,

So, as a followup of what we were discussing in this thread:

[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html

For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.

This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.

RESULTS
=======
To have a quick idea of how a benchmark went, look at the '% improvement' row of each table.

I'll put these results online, in a googledoc spreadsheet or something like that, to make them easier to read, as soon as possible.

*** Intel(R) Xeon(R) E5620 @ 2.40GHz

*** pCPUs      16        DOM0 vCPUS  16
*** RAM        12285 MB  DOM0 Memory 9955 MB

# of build jobs                     -j1                   -j6                   -j8                   -j16**                -j24
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              153.72     152.41      35.33      34.93       30.7      30.33      26.79      25.97      26.88      26.21
                              153.81     152.76      35.37      34.99      30.81      30.36      26.83      26.08         27      26.24
                              153.93     152.79      35.37      35.25      30.92      30.39      26.83      26.13      27.01      26.28
                              153.94     152.94      35.39      35.28      31.05      30.43       26.9      26.14      27.01      26.44
                              153.98     153.06      35.45      35.31      31.17       30.5      26.95      26.18      27.02      26.55
                              154.01     153.23       35.5      35.35       31.2      30.59      26.98       26.2      27.05      26.61
                              154.04     153.34      35.56      35.42      31.45      30.76      27.12      26.21      27.06      26.78
                              154.16      153.5      37.79      35.58      31.68      30.83      27.16      26.23      27.16      26.78
                              154.18     153.71      37.98      35.61      33.73       30.9      27.49      26.32      27.16       26.8
                              154.9      154.67      38.03      37.64      34.69      31.69      29.82      26.38       27.2      28.63
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                        154.067    153.241     36.177     35.536      31.74     30.678     27.287     26.184     27.055     26.732
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     0.325      0.631      1.215      0.771      1.352      0.410      0.914      0.116      0.095      0.704
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                            0.536                 1.772                 3.346                 4.042                 1.194
========================================================================================================================================
====================================================================================================================================================
UNIXBENCH
====================================================================================================================================================
# parallel copies                            1 parallel            6 parrallel           8 parallel            16 parallel**         24 parallel
vanilla/patched                          vanilla    patched    vanilla    pached     vanilla    patched    vanilla    patched    vanilla    patched
----------------------------------------------------------------------------------------------------------------------------------------------------
Dhrystone 2 using register variables       2302.2     2302.1    13157.8    12262.4    15691.5    15860.1    18927.7    19078.5    18654.3    18855.6
Double-Precision Whetstone                  620.2      620.2     3481.2     3566.9     4669.2     4551.5     7610.1     7614.3    11558.9    11561.3
Execl Throughput                            184.3      186.7      884.6      905.3     1168.4     1213.6     2134.6     2210.2     2250.9       2265
File Copy 1024 bufsize 2000 maxblocks       780.8      783.3     1243.7     1255.5     1250.6     1215.7     1080.9     1094.2     1069.8     1062.5
File Copy 256 bufsize 500 maxblocks         479.8      482.8      781.8      803.6      806.4        781      682.9      707.7      698.2      694.6
File Copy 4096 bufsize 8000 maxblocks      1617.6     1593.5     2739.7     2943.4     2818.3     2957.8     2389.6     2412.6     2371.6     2423.8
Pipe Throughput                             363.9      361.6     2068.6     2065.6       2622     2633.5     4053.3     4085.9     4064.7     4076.7
Pipe-based Context Switching                 70.6      207.2      369.1     1126.8      623.9     1431.3     1970.4     2082.9     1963.8       2077
Process Creation                            103.1        135        503      677.6      618.7      855.4       1138     1113.7     1195.6       1199
Shell Scripts (1 concurrent)                723.2      765.3     4406.4     4334.4     5045.4     5002.5     5861.9     5844.2     5958.8     5916.1
Shell Scripts (8 concurrent)               2243.7     2715.3     5694.7     5663.6     5694.7     5657.8     5637.1     5600.5     5582.9     5543.6
System Call Overhead                          330      330.1     1669.2     1672.4     2028.6     1996.6     2920.5     2947.1     2923.9     2952.5
System Benchmarks Index Score               496.8      567.5     1861.9       2106     2220.3     2441.3     2972.5     3007.9     3103.4     3125.3
----------------------------------------------------------------------------------------------------------------------------------------------------
% increase (of the Index Score)                       14.231                13.110                 9.954                 1.191                 0.706
====================================================================================================================================================

*** Intel(R) Xeon(R) X5650 @ 2.67GHz

*** pCPUs      24        DOM0 vCPUS  16
*** RAM        36851 MB  DOM0 Memory 9955 MB

# of build jobs                     -j1                   -j8                   -j12                   -j24**               -j32
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              119.49     119.47      23.37      23.29      20.12      19.85      17.99       17.9      17.82       17.8
                              119.59     119.64      23.52      23.31      20.16      19.99      18.19      18.05      18.23      17.89
                              119.59     119.65      23.53      23.35      20.19      20.08      18.26      18.09      18.35      17.91
                              119.72     119.75      23.63      23.41       20.2      20.14      18.54       18.1       18.4      17.95
                              119.95     119.86      23.68      23.42      20.24      20.19      18.57      18.15      18.44      18.03
                              119.97      119.9      23.72      23.51      20.38      20.31      18.61      18.21      18.49      18.03
                              119.97     119.91      25.03      23.53      20.38      20.42      18.75      18.28      18.51      18.08
                              120.01     119.98      25.05      23.93      20.39      21.69      19.99      18.49      18.52       18.6
                              120.24     119.99      25.12      24.19      21.67      21.76      20.08      19.74      19.73      19.62
                              120.66     121.22      25.16      25.36      21.94      21.85      20.26       20.3      19.92      19.81
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                        119.919    119.937     24.181      23.73     20.567     20.628     18.924     18.531     18.641     18.372
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     0.351      0.481      0.789      0.642      0.663      0.802      0.851      0.811      0.658      0.741
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                           -0.015                 1.865                -0.297                 2.077                 1.443
========================================================================================================================================
====================================================================================================================================================
UNIXBENCH
====================================================================================================================================================
# parallel copies                            1 parallel            8 parrallel            12 parallel           24 parallel**         32 parallel
vanilla/patched                          vanilla     patched   vanilla     pached     vanilla    patched    vanilla    patched    vanilla    patched
----------------------------------------------------------------------------------------------------------------------------------------------------
Dhrystone 2 using register variables       2650.1     2664.6    18967.8    19060.4    27534.1    27046.8    30077.9    30110.6    30542.1    30358.7
Double-Precision Whetstone                  713.7      713.5     5463.6     5455.1     7863.9     7923.8    12725.1    12727.8    17474.3    17463.3
Execl Throughput                            280.9      283.8     1724.4     1866.5     2029.5     2367.6       2370     2521.3       2453     2506.8
File Copy 1024 bufsize 2000 maxblocks       891.1      894.2       1423     1457.7     1385.6     1482.2     1226.1     1224.2     1235.9     1265.5
File Copy 256 bufsize 500 maxblocks         546.9      555.4        949      972.1      882.8      878.6      821.9      817.7      784.7      810.8
File Copy 4096 bufsize 8000 maxblocks      1743.4     1722.8     3406.5     3438.9     3314.3     3265.9     2801.9     2788.3     2695.2     2781.5
Pipe Throughput                             426.8      423.4     3207.9       3234     4635.1     4708.9       7326     7335.3     7327.2     7319.7
Pipe-based Context Switching                110.2      223.5      680.8     1602.2      998.6     2324.6     3122.1     3252.7     3128.6     3337.2
Process Creation                            130.7      224.4     1001.3     1043.6       1209     1248.2     1337.9     1380.4     1338.6     1280.1
Shell Scripts (1 concurrent)               1140.5     1257.5     5462.8     6146.4     6435.3     7206.1     7425.2     7636.2     7566.1     7636.6
Shell Scripts (8 concurrent)                 3492     3586.7     7144.9       7307       7258     7320.2     7295.1     7296.7     7248.6     7252.2
System Call Overhead                        387.7      387.5     2398.4       2367     2793.8     2752.7     3735.7     3694.2     3752.1     3709.4
System Benchmarks Index Score               634.8      712.6     2725.8     3005.7     3232.4     3569.7     3981.3     4028.8     4085.2     4126.3
----------------------------------------------------------------------------------------------------------------------------------------------------
% increase (of the Index Score)                       12.256                10.269                10.435                 1.193                 1.006
====================================================================================================================================================

# of build jobs                     -j1                   -j20                   -j24                  -j48**               -j62
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              267.78     233.25      36.53      35.53      35.98      34.99      33.46      32.13      33.57      32.54
                              268.42     233.92      36.82      35.56      36.12       35.2      34.24      32.24      33.64      32.56
                              268.85     234.39      36.92      35.75      36.15      35.35      34.48      32.86      33.67      32.74
                              268.98     235.11      36.96      36.01      36.25      35.46      34.73      32.89      33.97      32.83
                              269.03     236.48      37.04      36.16      36.45      35.63      34.77      32.97      34.12      33.01
                              269.54     237.05      40.33      36.59      36.57      36.15      34.97      33.09      34.18      33.52
                              269.99     238.24      40.45      36.78      36.58      36.22      34.99      33.69      34.28      33.63
                              270.11     238.48      41.13      39.98      40.22      36.24         38      33.92      34.35      33.87
                              270.96     239.07      41.66      40.81      40.59      36.35      38.99      34.19      34.49      37.24

Quoted 8 lines

                              271.84     240.89      42.07      41.24      40.63      40.06      39.07      36.04      34.69      37.59
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                         269.55    236.688     38.991     37.441     37.554     36.165      35.77     33.402     34.096     33.953
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     1.213      2.503      2.312      2.288      2.031      1.452      2.079      1.142      0.379      1.882
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                           12.191                 3.975                 3.699                 6.620                 0.419
========================================================================================================================================

Quoted 8 lines

I'm a bit confused here as to why, if dom0 has 16 vcpus in all of your tests, you change the -j number (apparently) based on the number of pcpus available to Xen. Wouldn't it make more sense to stick with 1/6/8/16/24? That would allow us to have actually comparable numbers.
But in any case, it seems to me that the numbers do show a uniform improvement and no regressions -- I think this approach looks really good, particularly as it is so small and well-contained.

That said, it's probably a good idea to make this optional somehow, so that if people do decide to do a pinning / partitioning approach, the guest scheduler actually can take advantage of topological information.

-George

George Dunlap

2015-08-27 17:05 UTC #39 lore raw

Quoted 223 lines · George Dunlap

Quoted 213 lines · Dario Faggioli

Hey everyone,

So, as a followup of what we were discussing in this thread:

[Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest
http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg03241.html

For now, this only covers the PV case. HVM case shouldn't be any different, but I haven't looked at how to make the same thing happen in there as well.

This means that when, in future, we will fix CPUID handling and make it comply with whatever logic or requirements we want, that won't have any unexpected side effects on scheduling domains.

RESULTS
=======
To have a quick idea of how a benchmark went, look at the '% improvement' row of each table.

I'll put these results online, in a googledoc spreadsheet or something like that, to make them easier to read, as soon as possible.

*** Intel(R) Xeon(R) E5620 @ 2.40GHz

*** pCPUs      16        DOM0 vCPUS  16
*** RAM        12285 MB  DOM0 Memory 9955 MB

# of build jobs                     -j1                   -j6                   -j8                   -j16**                -j24
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              153.72     152.41      35.33      34.93       30.7      30.33      26.79      25.97      26.88      26.21
                              153.81     152.76      35.37      34.99      30.81      30.36      26.83      26.08         27      26.24
                              153.93     152.79      35.37      35.25      30.92      30.39      26.83      26.13      27.01      26.28
                              153.94     152.94      35.39      35.28      31.05      30.43       26.9      26.14      27.01      26.44
                              153.98     153.06      35.45      35.31      31.17       30.5      26.95      26.18      27.02      26.55
                              154.01     153.23       35.5      35.35       31.2      30.59      26.98       26.2      27.05      26.61
                              154.04     153.34      35.56      35.42      31.45      30.76      27.12      26.21      27.06      26.78
                              154.16      153.5      37.79      35.58      31.68      30.83      27.16      26.23      27.16      26.78
                              154.18     153.71      37.98      35.61      33.73       30.9      27.49      26.32      27.16       26.8
                              154.9      154.67      38.03      37.64      34.69      31.69      29.82      26.38       27.2      28.63
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                        154.067    153.241     36.177     35.536      31.74     30.678     27.287     26.184     27.055     26.732
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     0.325      0.631      1.215      0.771      1.352      0.410      0.914      0.116      0.095      0.704
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                            0.536                 1.772                 3.346                 4.042                 1.194
========================================================================================================================================
====================================================================================================================================================
UNIXBENCH
====================================================================================================================================================
# parallel copies                            1 parallel            6 parrallel           8 parallel            16 parallel**         24 parallel
vanilla/patched                          vanilla    patched    vanilla    pached     vanilla    patched    vanilla    patched    vanilla    patched
----------------------------------------------------------------------------------------------------------------------------------------------------
Dhrystone 2 using register variables       2302.2     2302.1    13157.8    12262.4    15691.5    15860.1    18927.7    19078.5    18654.3    18855.6
Double-Precision Whetstone                  620.2      620.2     3481.2     3566.9     4669.2     4551.5     7610.1     7614.3    11558.9    11561.3
Execl Throughput                            184.3      186.7      884.6      905.3     1168.4     1213.6     2134.6     2210.2     2250.9       2265
File Copy 1024 bufsize 2000 maxblocks       780.8      783.3     1243.7     1255.5     1250.6     1215.7     1080.9     1094.2     1069.8     1062.5
File Copy 256 bufsize 500 maxblocks         479.8      482.8      781.8      803.6      806.4        781      682.9      707.7      698.2      694.6
File Copy 4096 bufsize 8000 maxblocks      1617.6     1593.5     2739.7     2943.4     2818.3     2957.8     2389.6     2412.6     2371.6     2423.8
Pipe Throughput                             363.9      361.6     2068.6     2065.6       2622     2633.5     4053.3     4085.9     4064.7     4076.7
Pipe-based Context Switching                 70.6      207.2      369.1     1126.8      623.9     1431.3     1970.4     2082.9     1963.8       2077
Process Creation                            103.1        135        503      677.6      618.7      855.4       1138     1113.7     1195.6       1199
Shell Scripts (1 concurrent)                723.2      765.3     4406.4     4334.4     5045.4     5002.5     5861.9     5844.2     5958.8     5916.1
Shell Scripts (8 concurrent)               2243.7     2715.3     5694.7     5663.6     5694.7     5657.8     5637.1     5600.5     5582.9     5543.6
System Call Overhead                          330      330.1     1669.2     1672.4     2028.6     1996.6     2920.5     2947.1     2923.9     2952.5
System Benchmarks Index Score               496.8      567.5     1861.9       2106     2220.3     2441.3     2972.5     3007.9     3103.4     3125.3
----------------------------------------------------------------------------------------------------------------------------------------------------
% increase (of the Index Score)                       14.231                13.110                 9.954                 1.191                 0.706
====================================================================================================================================================

*** Intel(R) Xeon(R) X5650 @ 2.67GHz

*** pCPUs      24        DOM0 vCPUS  16
*** RAM        36851 MB  DOM0 Memory 9955 MB

# of build jobs                     -j1                   -j8                   -j12                   -j24**               -j32
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              119.49     119.47      23.37      23.29      20.12      19.85      17.99       17.9      17.82       17.8
                              119.59     119.64      23.52      23.31      20.16      19.99      18.19      18.05      18.23      17.89
                              119.59     119.65      23.53      23.35      20.19      20.08      18.26      18.09      18.35      17.91
                              119.72     119.75      23.63      23.41       20.2      20.14      18.54       18.1       18.4      17.95
                              119.95     119.86      23.68      23.42      20.24      20.19      18.57      18.15      18.44      18.03
                              119.97      119.9      23.72      23.51      20.38      20.31      18.61      18.21      18.49      18.03
                              119.97     119.91      25.03      23.53      20.38      20.42      18.75      18.28      18.51      18.08
                              120.01     119.98      25.05      23.93      20.39      21.69      19.99      18.49      18.52       18.6
                              120.24     119.99      25.12      24.19      21.67      21.76      20.08      19.74      19.73      19.62
                              120.66     121.22      25.16      25.36      21.94      21.85      20.26       20.3      19.92      19.81
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                        119.919    119.937     24.181      23.73     20.567     20.628     18.924     18.531     18.641     18.372
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     0.351      0.481      0.789      0.642      0.663      0.802      0.851      0.811      0.658      0.741
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                           -0.015                 1.865                -0.297                 2.077                 1.443
========================================================================================================================================
====================================================================================================================================================
UNIXBENCH
====================================================================================================================================================
# parallel copies                            1 parallel            8 parrallel            12 parallel           24 parallel**         32 parallel
vanilla/patched                          vanilla     patched   vanilla     pached     vanilla    patched    vanilla    patched    vanilla    patched
----------------------------------------------------------------------------------------------------------------------------------------------------
Dhrystone 2 using register variables       2650.1     2664.6    18967.8    19060.4    27534.1    27046.8    30077.9    30110.6    30542.1    30358.7
Double-Precision Whetstone                  713.7      713.5     5463.6     5455.1     7863.9     7923.8    12725.1    12727.8    17474.3    17463.3
Execl Throughput                            280.9      283.8     1724.4     1866.5     2029.5     2367.6       2370     2521.3       2453     2506.8
File Copy 1024 bufsize 2000 maxblocks       891.1      894.2       1423     1457.7     1385.6     1482.2     1226.1     1224.2     1235.9     1265.5
File Copy 256 bufsize 500 maxblocks         546.9      555.4        949      972.1      882.8      878.6      821.9      817.7      784.7      810.8
File Copy 4096 bufsize 8000 maxblocks      1743.4     1722.8     3406.5     3438.9     3314.3     3265.9     2801.9     2788.3     2695.2     2781.5
Pipe Throughput                             426.8      423.4     3207.9       3234     4635.1     4708.9       7326     7335.3     7327.2     7319.7
Pipe-based Context Switching                110.2      223.5      680.8     1602.2      998.6     2324.6     3122.1     3252.7     3128.6     3337.2
Process Creation                            130.7      224.4     1001.3     1043.6       1209     1248.2     1337.9     1380.4     1338.6     1280.1
Shell Scripts (1 concurrent)               1140.5     1257.5     5462.8     6146.4     6435.3     7206.1     7425.2     7636.2     7566.1     7636.6
Shell Scripts (8 concurrent)                 3492     3586.7     7144.9       7307       7258     7320.2     7295.1     7296.7     7248.6     7252.2
System Call Overhead                        387.7      387.5     2398.4       2367     2793.8     2752.7     3735.7     3694.2     3752.1     3709.4
System Benchmarks Index Score               634.8      712.6     2725.8     3005.7     3232.4     3569.7     3981.3     4028.8     4085.2     4126.3
----------------------------------------------------------------------------------------------------------------------------------------------------
% increase (of the Index Score)                       12.256                10.269                10.435                 1.193                 1.006
====================================================================================================================================================

# of build jobs                     -j1                   -j20                   -j24                  -j48**               -j62
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              267.78     233.25      36.53      35.53      35.98      34.99      33.46      32.13      33.57      32.54
                              268.42     233.92      36.82      35.56      36.12       35.2      34.24      32.24      33.64      32.56
                              268.85     234.39      36.92      35.75      36.15      35.35      34.48      32.86      33.67      32.74
                              268.98     235.11      36.96      36.01      36.25      35.46      34.73      32.89      33.97      32.83
                              269.03     236.48      37.04      36.16      36.45      35.63      34.77      32.97      34.12      33.01
                              269.54     237.05      40.33      36.59      36.57      36.15      34.97      33.09      34.18      33.52
                              269.99     238.24      40.45      36.78      36.58      36.22      34.99      33.69      34.28      33.63
                              270.11     238.48      41.13      39.98      40.22      36.24         38      33.92      34.35      33.87
                              270.96     239.07      41.66      40.81      40.59      36.35      38.99      34.19      34.49      37.24

Quoted 8 lines

                              271.84     240.89      42.07      41.24      40.63      40.06      39.07      36.04      34.69      37.59
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                         269.55    236.688     38.991     37.441     37.554     36.165      35.77     33.402     34.096     33.953
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     1.213      2.503      2.312      2.288      2.031      1.452      2.079      1.142      0.379      1.882
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                           12.191                 3.975                 3.699                 6.620                 0.419
========================================================================================================================================

Quoted 8 lines

I'm a bit confused here as to why, if dom0 has 16 vcpus in all of your tests, you change the -j number (apparently) based on the number of pcpus available to Xen. Wouldn't it make more sense to stick with 1/6/8/16/24? That would allow us to have actually comparable numbers.
But in any case, it seems to me that the numbers do show a uniform improvement and no regressions -- I think this approach looks really good, particularly as it is so small and well-contained.

-George

Dario Faggioli

author

2015-09-15 14:32 UTC #40 lore raw

Quoted 30 lines · George Dunlap

Quoted 19 lines · Dario Faggioli

# of build jobs                     -j1                   -j20                   -j24                  -j48**               -j62
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              267.78     233.25      36.53      35.53      35.98      34.99      33.46      32.13      33.57      32.54
                              268.42     233.92      36.82      35.56      36.12       35.2      34.24      32.24      33.64      32.56
                              268.85     234.39      36.92      35.75      36.15      35.35      34.48      32.86      33.67      32.74
                              268.98     235.11      36.96      36.01      36.25      35.46      34.73      32.89      33.97      32.83
                              269.03     236.48      37.04      36.16      36.45      35.63      34.77      32.97      34.12      33.01
                              269.54     237.05      40.33      36.59      36.57      36.15      34.97      33.09      34.18      33.52
                              269.99     238.24      40.45      36.78      36.58      36.22      34.99      33.69      34.28      33.63
                              270.11     238.48      41.13      39.98      40.22      36.24         38      33.92      34.35      33.87
                              270.96     239.07      41.66      40.81      40.59      36.35      38.99      34.19      34.49      37.24

Quoted 8 lines

                              271.84     240.89      42.07      41.24      40.63      40.06      39.07      36.04      34.69      37.59
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                         269.55    236.688     38.991     37.441     37.554     36.165      35.77     33.402     34.096     33.953
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     1.213      2.503      2.312      2.288      2.031      1.452      2.079      1.142      0.379      1.882
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                           12.191                 3.975                 3.699                 6.620                 0.419
========================================================================================================================================

Quoted 5 lines

I'm a bit confused here as to why, if dom0 has 16 vcpus in all of your tests, you change the -j number (apparently) based on the number of pcpus available to Xen. Wouldn't it make more sense to stick with 1/6/8/16/24? That would allow us to have actually comparable numbers.

Bah, no, sorry, that was a mistake I made when I cut-&-past'ed the tables in the email... Dom0 always have as much vCPUs as the host has pCPUs. I know this is a rather critical piece of information, so sorry for messing it up! :-/

Quoted 4 lines

But in any case, it seems to me that the numbers do show a uniform improvement and no regressions -- I think this approach looks really good, particularly as it is so small and well-contained.

Yeah, that seems the case... But I really would like to try more configurations and more workloads. I'll do that ASAP.

Thanks and Regards,
Dario

Signature · 4 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

Dario Faggioli

author

2015-09-15 14:32 UTC #41 lore raw

Quoted 30 lines · George Dunlap

Quoted 19 lines · Dario Faggioli

# of build jobs                     -j1                   -j20                   -j24                  -j48**               -j62
vanilla/patched              vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched    vanilla    patched
---------------------------------------------------------------------------------------------------------------------------------------
                              267.78     233.25      36.53      35.53      35.98      34.99      33.46      32.13      33.57      32.54
                              268.42     233.92      36.82      35.56      36.12       35.2      34.24      32.24      33.64      32.56
                              268.85     234.39      36.92      35.75      36.15      35.35      34.48      32.86      33.67      32.74
                              268.98     235.11      36.96      36.01      36.25      35.46      34.73      32.89      33.97      32.83
                              269.03     236.48      37.04      36.16      36.45      35.63      34.77      32.97      34.12      33.01
                              269.54     237.05      40.33      36.59      36.57      36.15      34.97      33.09      34.18      33.52
                              269.99     238.24      40.45      36.78      36.58      36.22      34.99      33.69      34.28      33.63
                              270.11     238.48      41.13      39.98      40.22      36.24         38      33.92      34.35      33.87
                              270.96     239.07      41.66      40.81      40.59      36.35      38.99      34.19      34.49      37.24

Quoted 8 lines

                              271.84     240.89      42.07      41.24      40.63      40.06      39.07      36.04      34.69      37.59
---------------------------------------------------------------------------------------------------------------------------------------
 Avg.                         269.55    236.688     38.991     37.441     37.554     36.165      35.77     33.402     34.096     33.953
---------------------------------------------------------------------------------------------------------------------------------------
 Std. Dev.                     1.213      2.503      2.312      2.288      2.031      1.452      2.079      1.142      0.379      1.882
---------------------------------------------------------------------------------------------------------------------------------------
 % improvement                           12.191                 3.975                 3.699                 6.620                 0.419
========================================================================================================================================

Quoted 5 lines

I'm a bit confused here as to why, if dom0 has 16 vcpus in all of your tests, you change the -j number (apparently) based on the number of pcpus available to Xen. Wouldn't it make more sense to stick with 1/6/8/16/24? That would allow us to have actually comparable numbers.

Quoted 4 lines

But in any case, it seems to me that the numbers do show a uniform improvement and no regressions -- I think this approach looks really good, particularly as it is so small and well-contained.

Yeah, that seems the case... But I really would like to try more configurations and more workloads. I'll do that ASAP.

Thanks and Regards,
Dario

Signature · 4 lines

<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

_______________________________________________
Xen-devel mailing list
[email protected]
http://lists.xen.org/xen-devel

George Dunlap

2015-08-27 10:24 UTC #42 lore raw