Parallel performance drops off a cliff

[Neil Mitchell]

>> > 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? ...

16 on the best machine we've got. This particular application is a bit
weird, I suspect it will be spending very little time actually
executing Haskell and most of the time waiting for shell commands. As
such, the efficiency of Haskell isn't a problem - parallel garbage
collection shouldn't make any difference.

Thanks

Neil