[Haskell-cafe] Threading and Mullticore Computation

[Don Stewart]

mwinter:
> Hi,
> 
> I tried a get into concurrent Haskell using multiple cores. The program below
> creates 2 task in different threads, executes them, synchronizes the threads
> using MVar () and calculates the time needed. 
> 
> import System.CPUTime
> import Control.Concurrent
> import Control.Concurrent.MVar
> 
> myTask1 = do
>             return $! fac 60000
>             print "Task1 done!"
>               where fac 0 = 1
>                     fac n = n * fac (n-1)
>                  
> myTask2 = do
>             return $! fac' 60000 1 1
>             print "Task2 done!"
>               where fac' n m p = if  m>n then p else fac'  n (m+1) (m*p)
> 
> main = do
>              mvar <- newEmptyMVar
>              pico1 <- getCPUTime
>              forkIO (myTask1 >> putMVar mvar ())
>              myTask2
>              takeMVar mvar
>              pico2 <- getCPUTime
>              print (pico2 - pico1)
>             
> 
> I compiled the code using
> $ ghc FirstFork.hs -threaded
> and executed it by
> $ main +RTS -N1   resp.   $ main +RTS -N2
> I use GHC 6.8.3 on Vista with an Intel Dual Core processor. Instead of getting
> a speed up when using 2 cores I get as significant slow down, even though there
> is no sharing in my code above (at least none I am aware of. BTW, that was
> reason
> that I use 2 different local factorial functions). On my computer the 1-core
> version
> takes about 8.3sec and the 2-core version 12.8sec. When I increase the numbers
> from 60000 to 100000 the time difference gets even worse (30sec vs 51 sec). Can
> anybody give me an idea what I am doing wrong?


If you just want to check that your machine can do multicore, here's the
"hello world" I've been using:

    import Control.Parallel

    main = a `par` b `par` c `pseq` print (a + b + c)
        where
            a = ack 3 10
            b = fac 42
            c = fib 34

    fac 0 = 1
    fac n = n * fac (n-1)

    ack 0 n = n+1
    ack m 0 = ack (m-1) 1
    ack m n = ack (m-1) (ack m (n-1))

    fib 0 = 0
    fib 1 = 1
    fib n = fib (n-1) + fib (n-2)

To be run as:

    $ ghc -O2 -threaded --make hello.hs 
    [1 of 1] Compiling Main             ( hello.hs, hello.o )
    Linking hello ...

    $ time ./hello +RTS -N2         
    1405006117752879898543142606244511569936384005711076
    ./hello +RTS -N2  2.29s user 0.01s system 152% cpu 1.505 total
                                              ^^^^

-- Don