: PV-domain SMP performance
PATCH 1/1 : PV-domain SMP performance
Hi,
I've played a little bit with the xen scheduler to enhance the performance of paravirtualized SMP domains including Dom0.
Under heavy system load a vcpu might be descheduled in a critical section. This in turn leads to even higher system load if other vcpus of the same domain are waiting for the descheduled vcpu to leave the critical section.
I've created a patch for xen and for the linux kernel to show that cooperative scheduling can help to avoid this problem or make it less critical.
A vcpu might set a flag "no_desched" in its vcpu_info structure (I've used an unused hole) which will tell the xen scheduler to keep the vcpu running for some more time. If the vcpu would have been descheduled otherwise, the guest is infomred by another flag in the vcpu_info to voluntarily give up control after leaving the critical section. If the guest is not cooperative it will be descheduled after 1 msec anyway.
I've made some tests in Dom0 with a small benchmark producing high system load using
time -p (dd if=/dev/urandom count=1M | cat >/dev/null)
The system is a 4 processor x86_64 machine running latest XEN-unstable. The tests are performed in dom0. For each test run the sums of the time outputs are printed (2 parallel runs lasting 60 seconds each will print 120 seconds). Multiple tests returned very similar results (deviation about 1-2%).
First configuration: 4 vcpus, no pinning:
-----------------------------------------
1 run: real: 79.92 user: 1.04 sys: 78.83 2 runs: real: 271.28 user: 1.91 sys: 269.35 4 runs: real: 882.32 user: 5.70 sys: 875.50
Second configuration: 4 vcpus, all pinned to cpu 0:
---------------------------------------------------
1 run: real: 400.55 user: 0.10 sys: 380.28 2 runs: real: 1270.68 user: 2.58 sys: 653.28 4 runs: real: 1558.27 user: 20.99 sys: 368.10
The same tests with my patches:
First configuration: 4 vcpus, no pinning:
-----------------------------------------
1 run: real: 81.85 user: 1.00 sys: 81.29 2 runs: real: 229.62 user: 2.07 sys: 191.31 4 runs: real: 878.63 user: 3.61 sys: 873.76
Second configuration: 4 vcpus, all pinned to cpu 0:
---------------------------------------------------
1 run: real: 274.06 user: 0.74 sys: 58.88 2 runs: real: 999.77 user: 1.27 sys: 98.61
4 runs: real: 1251.00 user: 16.58 sys: 291.66
This result was achieved by avoiding descheduling of a vcpu only when irqs are blocked. Even better results might be possible with some fine tuning (e.g. instrumenting bh_enable/bh_disable).
I think system time has dropped remarkably!
Patch 1 is hypervisor support
Patch 2 is my Linux support in irq_enable and irq_disable
Juergen
Signature · 5 lines
Juergen Gross Principal Developer IP SW OS6 Telephone: +49 (0) 89 636 47950 Fujitsu Siemens Computers e-mail: [email protected] Otto-Hahn-Ring 6 Internet: www.fujitsu-siemens.com D-81739 Muenchen Company details: www.fujitsu-siemens.com/imprint.html
Quoted 4 lines · Juergen Gross
This result was achieved by avoiding descheduling of a vcpu only when irqs are blocked. Even better results might be possible with some fine tuning (e.g. instrumenting bh_enable/bh_disable).
I think system time has dropped remarkably!
It's nice, but it'd be more compelling if a win was shown on a real benchmark. Under normal workloads there is actually little lock contention in the Linux kernel, and hence I think scope for wins are limited.
Also, pv_ops Linux already has some extra smartness in its spinlock implementation. A spinner will sleep after some time, making it more likely that the lock holder will run (who then wakes the sleeper when the lock is released). You'd need to compare with that approach (which required no extra hypervisor interfaces).
-- Keir
Quoted 12 lines · Keir Fraser
Quoted 4 lines · Juergen Gross
This result was achieved by avoiding descheduling of a vcpu only when irqs are blocked. Even better results might be possible with some fine tuning (e.g. instrumenting bh_enable/bh_disable).
I think system time has dropped remarkably!It's nice, but it'd be more compelling if a win was shown on a real benchmark. Under normal workloads there is actually little lock contention in the Linux kernel, and hence I think scope for wins are limited.
Quoted 5 lines
Also,
pv_opsLinux already has some extra smartness in its spinlock implementation. A spinner will sleep after some time, making it more likely that the lock holder will run (who then wakes the sleeper when the lock is released). You'd need to compare with that approach (which required no extra hypervisor interfaces).
Sure, my benchmark is a very special case :-)
The advantage of my solution is a general mechanism to avoid preemption of a vcpu in critical sections instead of dealing with it after it has occured. Is the pv_ops Linux capable to deal with held locks in interrupt handling? What about other code paths which should be completed in short time?
About real world applications:
Again 4 vcpus pinned to one physical cpu, 3 files copied via scp to the test machine at the same time, each file about 50 MB.
Linux-xen from xensource: about 1:50 elapsed time for each job My modified Linux: about 0:50 elapsed time
Juergen
Signature · 5 lines
Juergen Gross Principal Developer IP SW OS6 Telephone: +49 (0) 89 636 47950 Fujitsu Siemens Computers e-mail: [email protected] Otto-Hahn-Ring 6 Internet: www.fujitsu-siemens.com D-81739 Muenchen Company details: www.fujitsu-siemens.com/imprint.html
Quoted 4 lines · Juergen Gross
The advantage of my solution is a general mechanism to avoid preemption of a vcpu in critical sections instead of dealing with it after it has occured. Is the
pv_opsLinux capable to deal with held locks in interrupt handling? What about other code paths which should be completed in short time?
Yes the approach is the other way round to yours. It handles irq-safe locks just fine; no reason for it not to.
Quoted 6 lines
About real world applications:
Again 4 vcpus pinned to one physical cpu, 3 files copied via scp to the test machine at the same time, each file about 50 MB.Linux-xen from xensource: about 1:50 elapsed time for each job My modified Linux: about 0:50 elapsed time
So this provides great wins for those who run multi-vcpu VMs on a single physical CPU? ;-) Actually getting a speedup on this benchmark even in that configuration is a surprise I will admit -- I'd expect most time to be spent in sshd in user space. By 0:50 for each job you mean 0:50 for 50MB? That's 10Mbps and I wouldn't even expect a single CPU working alone to be breaking a sweat. Weird...
-- Keir
Quoted 11 lines · Keir Fraser
Quoted 6 lines · Juergen Gross
About real world applications:
Again 4 vcpus pinned to one physical cpu, 3 files copied via scp to the test machine at the same time, each file about 50 MB.Linux-xen from xensource: about 1:50 elapsed time for each job My modified Linux: about 0:50 elapsed timeSo this provides great wins for those who run multi-vcpu VMs on a single physical CPU? ;-)
Absolutely! :-)
I thought this would be an easy way to force vcpu scheduling without having to use multiple domains.
The picture would be quite similar (may be not so extreme) on a machine with several multi-vcpu domains under heavy load.
Quoted 5 lines
Actually getting a speedup on this benchmark even in that configuration is a surprise I will admit -- I'd expect most time to be spent in sshd in user space. By 0:50 for each job you mean 0:50 for 50MB? That's 10Mbps and I wouldn't even expect a single CPU working alone to be breaking a sweat. Weird...
I think the problem here are the network interrupts which will occur in a round robin fashion on all vcpus. And those are serialized by the scheduler...
Juergen
Signature · 5 lines
Juergen Gross Principal Developer IP SW OS6 Telephone: +49 (0) 89 636 47950 Fujitsu Siemens Computers e-mail: [email protected] Otto-Hahn-Ring 6 Internet: www.fujitsu-siemens.com D-81739 Muenchen Company details: www.fujitsu-siemens.com/imprint.html