[Haskell-cafe] forkIO on multicore

[Luke Palmer]

On Fri, Dec 19, 2008 at 9:27 AM, Paul Keir <pkeir at dcs.gla.ac.uk> wrote:

> module Main where
>
> import Control.Concurrent
>
> fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
>
> heavytask m = putMVar m (fibs !! 100000)


Oh, also, heavytask is not very heavy at all.  It just writes the thunk
(fibs !! 100000) into the MVar.  Not a single number is added in this
thread.

You probably meant to have the thread evaluate its argument _before_ writing
it to the variable:

heavytask m = putMVar m $! (fibs !! 100000)

(Or more transparently)

heavytask m = let answer = fibs !! 100000 in answer `seq` putMVar m answer

But as per my other comments, you will not see a speedup (in fact, you will
probably see some slowdown as two threads compete to compute the same
value).

Luke


>
>
> main = do ms <- sequence $ replicate 2 newEmptyMVar
>          mapM_ (forkIO . heavytask) $ tail ms
>          heavytask $ head ms
>          ms' <- mapM takeMVar ms
>          mapM_ print ms'
>
> Regards,
> Paul
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