Parallel performance drops off a cliff

Don Stewart dons at
Tue Apr 21 04:55:25 EDT 2009

> Hi
> >> This is using GHC 6.10.2 on Windows XP, 2 processors. Is this a known
> >> bug, or should I try and replicate it? (benchmark is fairly big and
> >> very dependent on internal things, but I suspect the dramatic
> >> performance slowdown is unlikely to be related to these bits).
> >
> > Yes, what's happening is this: GHC 6.10.2 contains some slightly bogus
> > heuristics about when to turn on the parallel GC, and it just so
> > happens that 8 processors tips it over the point where the parallel GC
> > is enabled for young-generation collections.  In 6.10.2 the parallel
> > GC really didn't help most of the time, but it has undergone a lot of
> > tuning since then, and in the HEAD things are much better (see the
> > results from our ICFP submission).
> Fantastic! I'll disable parallel garbage collection (I'm fairly
> certain it won't have any effect for this application).

How many cores are you planning to use? ...

-- Don

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