linux-kernel - Re: [PATCH 1/1] Fix number of events displayed in header

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Date:	Tue, 20 Mar 2012 15:04:10 -0500
From:	Ashay Rane <ashay.r@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	David Ahern <dsahern@...il.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-perf-users@...r.kernel.org,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Ashay Rane <ashay.rane@...c.utexas.edu>
Subject: Re: [PATCH 1/1] Fix number of events displayed in header

Oh, okay. Thanks for the clarification!

However, my reasoning behind including the actual count of the events
was that (as far as I can tell) it is not displayed in any of the perf
report outputs. I understand that the count is not precise (because of
sampling errors) but the sampling frequency (-F) can always be
adjusted to get a more accurate number.

With the inclusion of the count, it then becomes possible to compare
multiple different versions of a program. Hence the patch.

Ashay


2012/3/20 Peter Zijlstra <a.p.zijlstra@...llo.nl>:
> On Tue, 2012-03-20 at 14:51 -0500, Ashay Rane wrote:
>> The problem I am seeing is that the `perf record' output does not
>> comform with the output of `perf stat'. For example, for `hackbench 10
>> process 1000', I see 19 x 10^9 cycles reported by `perf stat'.
>> However, `perf report -n' prints "Events: 2K cycles" and `perf report
>> -n --dso hackbench' prints "Events: 47 cycles".
>
> Ah, read it as if perf report is saying: Samples: 47 of event: cycles.
>
> That is, its telling you the cycles event had 47 overflows causing 47
> samples to be taken (IP, etc..) and written out.
>
> The point of reporting this number is that it puts the accuracy of the
> percentages into perspective. The more samples, the greater the coverage
> and more accurate the actual percentage of time spend in the various
> functions.
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