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