On 06/02/13 16:04, Johan Tibell wrote:
On Wed, Feb 6, 2013 at 2:09 AM, Simon Marlow <marlo...@gmail.com
<mailto:marlo...@gmail.com>> wrote:
This is slightly off topic, but I wanted to plant this thought in
people's brains: we shouldn't place much significance in the average
of a bunch of benchmarks (even the geometric mean), because it
assumes that the benchmarks have a sensible distribution, and we
have no reason to expect that to be the case. For example, in the
results above, we wouldn't expect a 14.7% reduction in runtime to be
seen in a typical program.
Using the median might be slightly more useful, which here would be
something around 0% for runtime, though still technically dodgy.
When I get around to it I'll modify nofib-analyse to report
medians instead of GMs.
Using the geometric mean as a way to summarize the results isn't that
bad. See "How not to lie with statistics: the correct way to summarize
benchmark results"
(http://ece.uprm.edu/~nayda/Courses/Icom6115F06/Papers/paper4.pdf).
Yes - our current usage of GM is because we read that paper :) I've
reported GMs of nofib programs in several papers. I'm not saying the
paper is wrong - the GM is definitely more correct than the AM for
averaging normalised results.
The problem is that we're attributing equal weight to all of our
benchmarks, without any reason to expect that they are representative.
We collect as many benchmarks as we can and hope they are
representative, but in fact it's rarely the case: often a particular
optimisation or regression will hit just one or two benchmarks. So all
I'm saying is that we shouldn't expect the GM to be representative.
Often there's no sensible mean at all - saying "some programs get a lot
better but most don't change" is far more informative than "on average
programs got faster by 1.2%".
That being said, I think the most useful thing to do is to look at the
big losers, as they're often regressions. Making some class of programs
much worse is but improving the geometric mean overall is often worse
than changing nothing at all.
Absolutely.
Cheers,
Simon
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