I was writing some parallel code (asynchronous database writes for an event logger, but that's besides the point), and it seemed like the parallelized version (i.e. compiled with -threaded -with-rtsopts=-N2) wasn't running fast enough. I boiled it down to a dead-simple test: import Control.Concurrent import Data.Time.Clock.POSIX import System.Environment main :: IO () main = do n <- getArgs >>= return . read . head t1 <- getPOSIXTime work n t2 <- getPOSIXTime putStrLn $ show $ t2 - t1 putStrLn $ show $ (fromIntegral n :: Double) / (fromRational . toRational $ t2 - t1) work :: Integer -> IO () work n = do forkIO $ putStrLn $ seq (fact n) "Done" putStrLn $ seq (fact n) "Done" fact :: Integer -> Integer fact 1 = 1 fact n = n * fact (n - 1) (I know this is not the best way to time things but I think it suffices for this test.) Compiled with ghc --make -O3 test.hs, ./test 500000 runs for 74 seconds. Compiling with ghc --make -O3 -threaded -with-rtsopts=-N, ./test 500000 runs for 82 seconds (and seems to be using 2 cpu cores instead of just 1, on a 4-core machine). What gives? Mike S Craig