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

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Date:   Wed, 23 Nov 2016 12:25:08 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     kan.liang@...el.com
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, alexander.shishkin@...ux.intel.com,
        tglx@...utronix.de, namhyung@...nel.org, jolsa@...nel.org,
        adrian.hunter@...el.com, wangnan0@...wei.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.
- average perf collection overhead on a CPU. The 10% threshold here
  seems appropiate.

-Andi