Re: Using taskset , powertop and perf with qemu-kvm

[Gleb Natapov <gleb@redhat.com>]

On Wed, Oct 10, 2012 at 01:22:22PM +0530, Jaspal wrote:
> 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.

> 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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--
			Gleb.