Perf tests which are better than expected on perf builds

[Ian Lynagh]

On Sat, Jul 20, 2013 at 11:26:10AM -0700, Edward Z. Yang wrote:
> These tests have been doing better than expected in the nightlies
> for some while.
> 
> > Unexpected failures:
> >    perf/compiler  T3064 [stat too good] (normal)
> >    perf/compiler  T3294 [stat too good] (normal)
> >    perf/compiler  T5642 [stat too good] (normal)
> >    perf/haddock   haddock.Cabal [stat too good] (normal)
> >    perf/haddock   haddock.base [stat too good] (normal)
> 
> Unfortunately, fixing them is not a simple matter of shifting
> the ranges up, since the tests only exceed expectations on
> a /perf/ build, so on a normal build such as 'quick', these
> tests all pass normally.
> 
> I could bump up the upper bounds so that the builder stops bleating
> about them; perhaps we could do something more complicated where the
> expected performance depends on what level of optimization GHC was built
> with (but I don't know how to implement this.)
> 
> Thoughts?

The problem with just widening the bounds to cover 2 different types of
build is that it increases the chance that performance changes won't
actually be noticed by thge person responsible.

Having different bounds for different build configurations is a pain,
because (a) the testsuite has to work out which set of bounds to use,
and (b) you now have even more wobbly values to keep up-to-date.

I think perhaps the best thing would be to add some sort of (per-test?)
fudge factor for non-validate builds. That way validate will still find
performance regressions, like it does today, but other builds are less
likely to give false positives.


Thanks
Ian
-- 
Ian Lynagh, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/