Hi, I tried a get into concurrent Haskell using multiple cores. The program below creates 2 task in different threads, executes them, synchronizes the threads using MVar () and calculates the time needed. import System.CPUTime import Control.Concurrent import Control.Concurrent.MVar myTask1 = do return $! fac 60000 print "Task1 done!" where fac 0 = 1 fac n = n * fac (n-1) myTask2 = do return $! fac' 60000 1 1 print "Task2 done!" where fac' n m p = if m>n then p else fac' n (m+1) (m*p) main = do mvar <- newEmptyMVar pico1 <- getCPUTime forkIO (myTask1 >> putMVar mvar ()) myTask2 takeMVar mvar pico2 <- getCPUTime print (pico2 - pico1) I compiled the code using $ ghc FirstFork.hs -threaded and executed it by $ main +RTS -N1 resp. $ main +RTS -N2 I use GHC 6.8.3 on Vista with an Intel Dual Core processor. Instead of getting a speed up when using 2 cores I get as significant slow down, even though there is no sharing in my code above (at least none I am aware of. BTW, that was reason that I use 2 different local factorial functions). On my computer the 1-core version takes about 8.3sec and the 2-core version 12.8sec. When I increase the numbers from 60000 to 100000 the time difference gets even worse (30sec vs 51 sec). Can anybody give me an idea what I am doing wrong? Thanks, Michael