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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