linux-kernel - Re: [RFC 00/12] perf diff: Factor diff command

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Date:	Fri, 07 Sep 2012 09:05:06 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	David Ahern <dsahern@...il.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [RFC 00/12] perf diff: Factor diff command

On Thu, 2012-09-06 at 14:25 -0700, Paul E. McKenney wrote:
> On Thu, Sep 06, 2012 at 08:41:09PM +0200, Peter Zijlstra wrote:
> > On Thu, 2012-09-06 at 17:46 +0200, Jiri Olsa wrote:
> > > The 'perf diff' and 'std/hist' code is now changed to allow computations
> > > mentioned in the paper. Two of them are implemented within this patchset:
> > >   1) ratio differential profiling
> > >   2) weighted differential profiling
> > 
> > Seems like a useful thing indeed, the explanation of the weighted diff
> > method doesn't seem to contain a why. I know I could go read the paper
> > but... :-)
> 
> Or you could ask the author.  ;-)
> 
> Ratio can be fooled by statistical variations on profiling buckets with
> few counts.  So if you are looking for a 10% difference in execution
> overhead somewhere in a large program, ratio will unhelpfully sort a
> bunch of statistical 2x or 3x noise to the top of the list.
> 
> So you could use the difference in buckets instead of the ratio, but this
> has problems in the case where the two runs being compared got different
> amounts of work done, as is usually the case for timed benchmark runs
> or throughput-based benchmark runs.  In these cases, you use the work
> done (or the measured throughput, as the case may be) as weights.
> The weighted difference will then pinpoint the code that suffered the
> greatest per-unit-work increase in overhead between the two runs.

Ah, ok, I guess that makes sense. Thanks!
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