Parallel performance drops off a cliff

[Neil Mitchell]

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).

Thanks

Neil