[Haskell-beginners] Parallelism?

[Michael Craig]

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
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