Re: [PATCH RFC V2 1/1] x86, perf: Add a freq pmu driver

Andy Lutomirski
2015-07-23T15:53:19Z

On Thu, Jul 23, 2015 at 8:44 AM, Stephane Eranian <eranian@google.com> wrote:
> I understand the value of the tsc and smi events. It is not
> clear to me what aperf/mperf buys you over cycles and ref-cycles:
>
> $ perf stat -a -e msr/aperf/,msr/mperf/,cycles,ref-cycles -C 1 -I 1000 sleep 10
> # time counts unit events
> 2.000361718 14,826,353 msr/aperf/
> 2.000361718 11,865,170 msr/mperf/
> 2.000361718 17,170,101 cycles
> 2.000361718 13,629,675 ref-cycles
>
> Only the ratio aperf/mperf is defined, here 1.25 and the ratio
> cycles/ref-cycles is 1.25 as well. So what is a situation where
> aperf/mperf provides better info than cycles/ref-cycles?
> The SDM also says aperf/mperf only defined when running in C0 mode.

They're free-running and always on, which means that you can never
fail to schedule them.

--Andy