[LLVMdev] Why is the default LNT aggregation function min instead of mean

David Blaikie dblaikie at gmail.com
Thu Jan 16 17:17:07 PST 2014

Right - you usually won't see a normal distribution in the noise of test
results. You'll see results clustered around the lower bound with a long
tail of slower and slower results. Depending on how many samples you do it
might be appropriate to take the mean of the best 3, for example - but the
general approach of taking the fastest N does have some basis in any case.

Not necessarily the right answer, the only right answer, etc.

On Thu, Jan 16, 2014 at 5:05 PM, Chris Matthews <chris.matthews at apple.com>wrote:

> I think the idea with min is that it would the the ideal fastest run.  The
> other runs were ‘slowed' by system noise or something else.
> On Jan 16, 2014, at 5:03 PM, Tobias Grosser <tobias at grosser.es> wrote:
> > Hi,
> >
> > I am currently investigating how to ensure that LNT only shows relevant
> performance regressions for the -O3 performance tests I am running.
> >
> > One question that came up here is why the default aggregate function for
> LNT is 'min' instead of 'mean'. This looks a little surprising from the
> statistical point, but also from looking at my test results picking 'min'
> seems to be an inferior choice.
> >
> > For all test runs I have looked at, picking mean largely reduces the
> run-over-run changes reported due to noise.
> >
> > See this run e.g:
> >
> > If we use the median, we just get just one change reported:
> >
> >
> http://llvm.org/perf/db_default/v4/nts/20661?num_comparison_runs=10&test_filter=&test_min_value_filter=&aggregation_fn=median&compare_to=20659&submit=Update
> >
> > If you use min, we get eight reports one claiming over 100% performance
> > reduction for a case that is really just pure noise. I am planning to
> look into using better statistical methods. However, as a start, could we
> switch the default to 'mean'?
> >
> > Cheers,
> > Tobias
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