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