Using taskset , powertop and perf with qemu-kvm

Hello ,

I wish to run some cpu intensive tasks on a vm , use only one core specifically #0 of the 6 available ( Xeon 3670 ) and all the while record basic perf events like cpu-cycles , instructions etc.

I am using taskset to define the cpu-affinity of the qemu process :
taskset -p 0x1 pgrep qemu (assuming only 1 vm is running )

I can verify that the vm is now running only on core 0 using top. top also shows that the qemu process is using 99% of it's share of CPU time ( %CPU column in top ) upon running the tasks in vm. However what I fail to see is the increase in time spent of the core 0 in the C0 ( active ) state via powertop. It barely manages to cross 12%. If I run the same tasks ( FFT benchmark ) on the host on a different core ( say #1 ) and monitor it using powertop again , I can see that C0 % for core 1 is above 100% ( 104 ~ 107 ) So why does not the output of top ( qemu is using 99% of it's cpu time , which is spent on core 0 ) co-relate with powertop ( which says core 0 is idle for around 88% of time ) ?
Also , powertop reports that core 2 is being used heavily ( 104 % ) and I suspect this is one which is executing the vm operations. But why ?
There are no other cpu/memory intensive tasks running on the host.

Next , I am using the perf kvm record function to save the stats. If I use all the 7 core counters available along-with -a option , perf generates ~2MB/second . Is there a way to save just the raw count of the events ?
Also , if instead of -a , if I give -C 0 , perf just saves around 0.016MB/s as opposed to 1.894MB/s in case of -a option. If the vm is indeed using core 0 , then perf should have recorded the events.

On a last note , does perf kvm automatically selects the vm to profile ( in case multiple vms are running ) on the basis of the guest.kallsyms and guest.modules files passed to it ?

Thanks ,
Jaspal
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Quoted 9 lines · Jaspal

Hello ,

I wish to run some cpu intensive tasks on a vm , use only one core specifically #0 of the 6 available ( Xeon 3670 ) and all the while record basic perf events like cpu-cycles , instructions etc.

I am using taskset to define the cpu-affinity of the qemu process :
taskset -p 0x1 pgrep qemu (assuming only 1 vm is running )

pgrep tells you only the pid of the io thread. vcpu runs in a separate thread.

Quoted 28 lines

I can verify that the vm is now running only on core 0 using top. top also shows that the qemu process is using 99% of it's share of CPU time ( %CPU column in top ) upon running the tasks in vm. However what I fail to see is the increase in time spent of the core 0 in the C0 ( active ) state via powertop. It barely manages to cross 12%. If I run the same tasks ( FFT benchmark ) on the host on a different core ( say #1 ) and monitor it using powertop again , I can see that C0 % for core 1 is above 100% ( 104 ~ 107 ) So why does not the output of top ( qemu is using 99% of it's cpu time , which is spent on core 0 ) co-relate with powertop ( which says core 0 is idle for around 88% of time ) ?
Also , powertop reports that core 2 is being used heavily ( 104 % ) and I suspect this is one which is executing the vm operations. But why ?
There are no other cpu/memory intensive tasks running on the host.

Next , I am using the perf kvm record function to save the stats. If I use all the 7 core counters available along-with -a option , perf generates ~2MB/second . Is there a way to save just the raw count of the events ?
Also , if instead of -a , if I give -C 0 , perf just saves around 0.016MB/s as opposed to 1.894MB/s in case of -a option. If the vm is indeed using core 0 , then perf should have recorded the events.

On a last note , does perf kvm automatically selects the vm to profile ( in case multiple vms are running ) on the basis of the guest.kallsyms and guest.modules files passed to it ?

Quoted 6 lines

Thanks ,
Jaspal
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Gleb.