From: michael rice, Thursday, May 26, 2011 Subject: Re: [Haskell-cafe] Parallel compilation and execution?
Thank, Daniel
Multiple threads are in evidence in my system monitor, but I wonder why I'm getting two different answers, one twice the other. The first is the parallel solution and the second is the non.
Not exactly twice - (2n-1). What you get from counting all nodes in a binary tree (of function calls), compared to counting just the leaves. A program using only pure code, par, and pseq should be deterministic, so if it gets different value by building with and without threaded, changing the number of threads, or just running again, that's a bug in GHC (If it runs out of memory sometimes and not other times, that's not technically a bug, but still interesting). Replacing par with the second argument should also give an equivalent program. pseq isn't really parallel at all, but if you'd like to remove it, then replacing it with seq or just the second argument might change strictness and evaluation order, but shouldn't change the result. Brandon.