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

[Jaspal <jaspal.iiith@gmail.com>]

On 10/10/2012 03:38 PM, Gleb Natapov wrote:
> 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.
Thanks. It works using the correct tid.

--
Jaspal