Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

Peter Zijlstra
2016-08-17T16:26:53Z

On Mon, Aug 15, 2016 at 07:03:00AM +0200, Ingo Molnar wrote:
>
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> > Make sure you actually use "perf record -e cycles:pp" or something
> > that uses PEBS to get real profiles using CPU performance counters.
>
> Btw., 'perf record -e cycles:pp' is the default now for modern versions
> of perf tooling (on most x86 systems) - if you do 'perf record' it will
> just use the most precise profiling mode available on that particular
> CPU model.
>
> If unsure you can check the event that was used, via:
>
> triton:~> perf report --stdio 2>&1 | grep '# Samples'
> # Samples: 27K of event 'cycles:pp'

Problem here is that Dave is using a KVM thingy. Getting hardware
counters in a guest is somewhat tricky but doable, but PEBS does not
virtualize.