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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 23 Jul 2015 08:52:50 -0700
Message-ID: <CALCETrUK5ktvpk_831MyGYnxj0kkykSQTHoqWB_7S9j=DKxOug@mail.gmail.com>
Subject: Re: [PATCH RFC V2 1/1] x86, perf: Add a freq pmu driver
To: Stephane Eranian <eranian@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Kan Liang <kan.liang@intel.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        "ak@linux.intel.com" <ak@linux.intel.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        David Ahern <dsahern@gmail.com>, Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@kernel.org>
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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
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