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

[Peter Zijlstra]

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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