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
as opposed to 1.894MB 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 ?