[Haskell-cafe] testing par with simple program

[Don Stewart]

paolo.veronelli:
> A better test program
> 
> import Control.Parallel
>  
> main = a `par` b  `pseq` print (a + b )
>     where
>         a = ack 3 11
>         b = fib 39
>    
> 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)
> 
> running it , these are the results
> 
> From the resource graph and the timings it is clear that the program is not
> able to use all the 2 cores powers, considering computing 'a' alone is about 7
> seconds and 'b' alone 9.
> What is retaining the cpu's to run full power ?
> 

Some advice on analysing parallel programs:

    * Read Simon Marlow's excellent paper: 
        "Runtime Support for Multicore Haskell"
        http://ghcmutterings.wordpress.com/2009/03/03/new-paper-runtime-support-for-multicore-haskell/

    * Install GHC Head from 2009, it includes spark profiling.

    * Never bother measuring without using -O or -O2 -- unoptimized code
      is just not useful to think about.

    * Compile your program:

        $ ghc-6.11.20090228 -O2 --make -threaded A.hs
        [1 of 1] Compiling Main             ( A.hs, A.o )
        Linking A ...

    * Run it with +RTS -sstderr

        $ ./A +RTS -N2 -sstderr
        ./A +RTS -N2 -sstderr 
        63262367

            ...

          Generation 0: 12021 collections,     0 parallel,  1.33s,  1.42s elapsed
          Generation 1:     2 collections,     1 parallel,  0.00s,  0.00s elapsed

          Parallel GC work balance: 1.03 (426 / 414, ideal 2)

          SPARKS: 1 (1 converted, 0 pruned)

          Productivity  90.7% of total user, 126.4% of total elapsed

    And we see 3 things:

        * It did use the parallel GC : good!
        * It did convert 1 spark to a thread: good!
        * Productivity was greater than 100%: good!
    
    So this looks fine!


So I suspect you're falling into the ghc 6.10.x series bug where < 3 sparks would not be converted.
Try with either 3 sparks, or using GHC 6.11.x series.

-- Don