Here is some more data. It seems the behaviour depends on 32/64 bit arch? ####################################################### waldmann@master:~/tmp$ uname -a Linux master 2.6.18-6-amd64 #1 SMP Fri Dec 12 05:49:32 UTC 2008 x86_64 GNU/Linux waldmann@master:~/tmp$ time ./Par +RTS -N1 496165411 496165411 real 0m22.580s user 0m22.541s sys 0m0.040s waldmann@master:~/tmp$ time ./Par +RTS -N2 496165411 496165411 real 0m21.259s user 0m26.678s sys 0m0.164s ######################################################## waldmann@box:~/tmp> uname -a Linux box 2.6.27.21-0.1-pae #1 SMP 2009-03-31 14:50:44 +0200 i686 i686 i386 GNU/Linux waldmann@box:~/tmp> time ./Par +RTS -N1 496165411 496165411 real 0m29.802s user 0m29.670s sys 0m0.028s waldmann@box:~/tmp> time ./Par +RTS -N2 496165411 496165411 real 0m11.219s user 0m14.917s sys 0m0.164s ################################################### I don't understand this. The fun thing is that the first machine is Intel(R) Xeon(R) CPU X5365 @ 3.00GHz while the second is a much cheaper model name : Genuine Intel(R) CPU T2050 @ 1.60GHz -- View this message in context: http://www.nabble.com/Threads-and-memory-management-tp23221239p23231596.html Sent from the Haskell - Glasgow-haskell-users mailing list archive at Nabble.com.