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