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

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Message-ID: <20120906212557.GD2448@linux.vnet.ibm.com>
Date:	Thu, 6 Sep 2012 14:25:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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, 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.

But ratio is still the method of choice when you are looking at pure
scalability issues, especially when the offending code increased in
overhead by a large factor.

							Thanx, Paul

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