Dear all, I've thought the following three (dummy) programs would run some of their parts in parallel (on dual core) if compiled with option threaded (smp). The truth is that only the first one exploits multicore CPU. Why? Moreover, using RTS option -sstderr makes runtime not to evaluate in parallel even for the first program. Why? Thanks for tips Dusan My arch: Linux pcx 2.6.24-ARCH #1 SMP PREEMPT Sun Feb 10 15:44:59 CET 2008 x86_64 Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz GenuineIntel GNU/Linux My ghc: The Glorious Glasgow Haskell Compilation System, version 6.8.2 /64bit, binary distro for FC/ ---------------------------------------------------- Prog 1: module Main() where import Control.Parallel import Control.Parallel.Strategies fibs :: Integer -> Integer fibs n | n > 1 = fibs (n-1) + fibs (n-2) | n == 1 = 1 | True = 0 fib n = if n<0 then error "Negative input to fib!" else f1+f2 where [f1,f2] = parMap rnf fibs [(n-1),(n-2)] main = do putStrLn "Starting..." putStrLn $ "Fib 43: " ++ show (fib 43) putStrLn "Done!" ---------------------------------------------------- Prog 2: module Main() where import Control.Concurrent import Control.Concurrent.MVar fibs :: Integer -> Integer fibs n | n > 1 = fibs (n-1) + fibs (n-2) | n == 1 = 1 | True = 0 fib n = if n<0 then error "Negative input to fib!" else do v1 <- newEmptyMVar v2 <- newEmptyMVar h1 <- forkIO $ putMVar v1 $ fibs (n-1) h2 <- forkIO $ putMVar v2 $ fibs (n-2) f1 <- takeMVar v1 f2 <- takeMVar v2 killThread h1 killThread h2 return (f1+f2) main = do putStrLn "Starting..." f <- fib 43 putStrLn $ "Fib 43: " ++ show f putStrLn "Done!" ---------------------------------------------------- Prog 3: module Main() where import Control.Concurrent import Control.Concurrent.MVar fibs :: Integer -> Integer fibs n | n > 1 = fibs (n-1) + fibs (n-2) | n == 1 = 1 | True = 0 fib n = if n<0 then error "Negative input to fib!" else do v1 <- newEmptyMVar v2 <- newEmptyMVar h1 <- forkOS $ putMVar v1 $ fibs (n-1) h2 <- forkOS $ putMVar v2 $ fibs (n-2) f1 <- takeMVar v1 f2 <- takeMVar v2 killThread h1 killThread h2 return (f1+f2) main = do putStrLn "Starting..." f <- fib 43 putStrLn $ "Fib 43: " ++ show f putStrLn "Done!"