Hello haskellers, in past few days, a lot of stuff on concurrency went through the conference. I'm trying to use posted things and my own. They work, even in a context switching regime, but I can't exploit all the CPU's on my computer. Always is "active" just one thread and, thus, the computation is even slower than having a sequential version. Below, you can find my code - it computes nothing useful, it's been simplified to test parallelism, nothing else. Where's my error? Regards Dusan import Control.Concurrent -- computes nothing too much useful, but takes a long time ;-) sumAllSums [] = 0 sumAllSums l@(_:xs) = sumlist 0 l + sumAllSums xs where sumlist res [] = res sumlist sr (v:vs) = sumlist (sr+v) vs main = do putStrLn "Starting..." mv1 <- newEmptyMVar mv2 <- newEmptyMVar t1 <- forkOS $ mkSum1 mv1 t2 <- forkOS $ mkSum2 mv2 tt mv1 mv2 forkOS $ do killThread t1 killThread t2 putStrLn "Done!" where mkSum1 mv = do let res = sumAllSums [1..10000] let ms1 = "Sum1: " ++ show res seq (length ms1) (putMVar mv ms1) mkSum2 mv = do let res = sumAllSums [1..10001] let ms2 = "Sum2: " ++ show res seq (length ms2) (putMVar mv ms2) tt mv1 mv2 = do yield mr1 <- tryTakeMVar mv1 case mr1 of Just r1 -> do yield putStrLn r1 yield r2 <- takeMVar mv2 putStrLn r2 Nothing -> do mr2 <- tryTakeMVar mv2 case mr2 of Just r2 -> do yield putStrLn r2 yield r1 <- takeMVar mv1 putStrLn r1 Nothing -> tt mv1 mv2