I've carefully followed the instructions in Real World Haskell for parallel sorting: In SortMain.hs 1. Set testfunction=sort Then del *.o (force recompilation) ghc --make -O2 sortMain.hs run SortMain and I get 500000 numbers => 3.9735 sec. 2. Set testFunction = parSort2 2 del *.o ghc -threaded --make -O2 sortMain.hs run SortMain +RTS -N2 -RTS, I get => 5.3 sec run SortMain +RTS -N1 -RTS, I get => 4.3 sec With the +RTS -s option I get 3 SPARK3 (3 converted, 0 pruned) All other testFunctions are worse. I have a dual core computer that runs Windows XP. I've experimented a lot with par, pseq ... in other programs. I almost always get a few hundred sparks. Half of them are converted and the other half gets pruned. But I never got a faster program (then sequential). What am I doing wrong, any ideas? Kees