[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:

> Hi all,
>
> I'm seeing no performance increase with a simple coarse-grained
> 2-thread code using Control.Concurrent. I compile with:
>
> >  hc conc.hs -o conc --make -threaded
>
> and I run with
>
> >  time ./conc +RTS -N2
>
> But using either "-N1" or "-N2", the program runs in about 1.8secs.
> (I'd prefer a longer running thread task, but my fib function
> currently runs out of memory).
>
> Anyway, my program is below, and I'm using GHC version 6.8.2 on
> a 2-core Pentium D. Can anyone help?
>
> module Main where
>
> import Control.Concurrent
>
> fibs = 0 : 1 : zipWith (+) fibs (tail fibs)


This is a serial algorithm.  No matter what the rest of your program is
doing, this calculation is sequential, so you will not see a speedup.

Parallelizing computation of the fibonacci numbers is reasonably tricky.  I
think you might be able to do it using the fib(2n) identity.  But not this
simple algorithm: throwing processors at a problem does not automatically
make it parallel.  :-)


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