linux-kernel - RE: [PATCH 13/14] perf tools: warn on high overhead

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Date:   Wed, 23 Nov 2016 22:03:24 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Andi Kleen <andi@...stfloor.org>
CC:     "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "Hunter, Adrian" <adrian.hunter@...el.com>,
        "wangnan0@...wei.com" <wangnan0@...wei.com>,
        "mark.rutland@....com" <mark.rutland@....com>
Subject: RE: [PATCH 13/14] perf tools: warn on high overhead



> 
> kan.liang@...el.com writes:
> 
> > From: Kan Liang <kan.liang@...el.com>
> >
> > The rough overhead rate can be caculated by the sum of all kinds of
> > overhead / elapsed time.
> > If the overhead rate is higher than 10%, warning the user.
> 
> Thinking about this more: this is comparing the cost of a single CPU to the
> total wall clock time. This isn't very good and can give confusing results with
> many cores.
> 
> Perhaps we need two separate metrics here:
> 
> - cost of perf record on its CPU (or later on if it gets multi threaded
>   more multiple). Warn if this is >50% or so.

What's the formula for cost of perf record on its CPU?
The cost only includes user space overhead or all overhead?
What is the divisor?


> - average perf collection overhead on a CPU. The 10% threshold here
>   seems appropiate.
For the average, do you mean add all overheads among CPUs together
and divide the CPU#?

To calculate the rate, the divisor is wall clock time, right?

Thanks,
Kan