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