Hello everybody, I am following "A Tutorial on Parallel and Concurrent Programming in Haskell" and I have a problem with making Haskell to use my multi-cores (Core 2 Quad CPU). The Haskel version I used is GHC 6.10.1, for Haskell 98. I compile my below program with command: ghc --make -threaded -debug thread0.hs, and run with: thread0 +RTS -N4 while watching the cpu usage on another terminal (by: mpstat -P ALL 1 100), but the program uses only one core of my Ubuntu Linux. Do any of you know why or has any suggestions? Below is my program: import Control.Concurrent import Control.Concurrent.MVar fib :: Int -> Int fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2) dowork = putStrLn ("fib 35 = " ++ (show (fib 35))) threadA :: MVar Int -> MVar Int -> MVar Int -> IO () threadA valueToSendMVar valueToReadMVar valueToQuit = do -- some work dowork -- perform rendezvous putMVar valueToSendMVar 30 -- send value v <- takeMVar valueToReadMVar putStrLn ("result, fib 30 = " ++ (show v)) dowork -- notify done putMVar valueToQuit 0 -- send value threadB :: MVar Int -> MVar Int -> MVar Int -> IO () threadB valueToReceiveMVar valueToSendMVar valueToQuit = do -- some work dowork -- perform rendezvous by waiting z <- takeMVar valueToReceiveMVar putMVar valueToSendMVar (fib z) -- continue with other work dowork -- notify done putMVar valueToQuit 0 -- send value main :: IO () main = do aQuitA <- newEmptyMVar aQuitB <- newEmptyMVar aMVar <- newEmptyMVar bMVar <- newEmptyMVar forkOS (threadA aMVar bMVar aQuitA ) forkOS (threadB aMVar bMVar aQuitB ) -- wait for threadA and threadB takeMVar aQuitA takeMVar aQuitB return () Thanks a lot, Hoang