Hi, reading a previous thread I got interested. I simplified the example pointed by dons in import Control.Parallel main = a `par` b `pseq` print (a + b ) where a = ack 3 11 b = ack 3 11 ack 0 n = n+1 ack m 0 = ack (m-1) 1 ack m n = ack (m-1) (ack m (n-1)) compiled with ghc --make prova -O2 -threaded timings paolino@paolino-casa:~$ time ./prova +RTS -N1 32762 real 0m7.031s user 0m6.304s sys 0m0.004s paolino@paolino-casa:~$ time ./prova +RTS -N2 32762 real 0m6.997s user 0m6.728s sys 0m0.020s paolino@paolino-casa:~$ without optimizations it gets worse paolino@paolino-casa:~$ time ./prova +RTS -N1 32762 real 1m20.706s user 1m18.197s sys 0m0.104s paolino@paolino-casa:~$ time ./prova +RTS -N2 32762 real 1m38.927s user 1m45.039s sys 0m0.536s paolino@paolino-casa:~$ staring at the resource usage graph I can see it does use 2 cores when told to do it, but with -N1 the used cpu goes 100% and with -N2 they both run just over 50% thanks for comments paolino