On Fri, Dec 19, 2008 at 9:27 AM, Paul Keir
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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe