From: Andi Kleen <andi@firstfloor.org>
To: kan.liang@intel.com
Cc: peterz@infradead.org, mingo@redhat.com, acme@kernel.org,
        linux-kernel@vger.kernel.org, alexander.shishkin@linux.intel.com,
        tglx@linutronix.de, namhyung@kernel.org, jolsa@kernel.org,
        adrian.hunter@intel.com, wangnan0@huawei.com, mark.rutland@arm.com
Subject: Re: [PATCH 13/14] perf tools: warn on high overhead
References: <1479894292-16277-1-git-send-email-kan.liang@intel.com>
        <1479894292-16277-14-git-send-email-kan.liang@intel.com>
Date: Wed, 23 Nov 2016 12:25:08 -0800
In-Reply-To: <1479894292-16277-14-git-send-email-kan.liang@intel.com> (kan
        liang's message of "Wed, 23 Nov 2016 04:44:51 -0500")
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kan.liang@intel.com writes:

> From: Kan Liang <kan.liang@intel.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