[Haskell-cafe] testing par with simple program

[Don Stewart]

paolo.veronelli:
> Hi, reading a previous thread I got interested.
> I simplified the example pointed by dons in
> 
> import Control.Parallel
>  
> main = a `par` b  `pseq` print (a + b )
>     where
>         a = ack 3 11
>         b = ack 3 11
>    
> ack 0 n = n+1
> ack m 0 = ack (m-1) 1
> ack m n = ack (m-1) (ack m (n-1))
> 
> compiled with
> ghc --make prova  -O2 -threaded
> 
> timings
> paolino at paolino-casa:~$ time ./prova +RTS -N1
> 32762
> 
> real    0m7.031s
> user    0m6.304s
> sys    0m0.004s
> paolino at paolino-casa:~$ time ./prova +RTS -N2
> 32762
> 
> real    0m6.997s
> user    0m6.728s
> sys    0m0.020s
> paolino at paolino-casa:~$
> 
> without optimizations it gets worse
> 
> paolino at paolino-casa:~$ time ./prova +RTS -N1
> 32762
> 
> real    1m20.706s
> user    1m18.197s
> sys    0m0.104s
> paolino at paolino-casa:~$ time ./prova +RTS -N2
> 32762
> 
> real    1m38.927s
> user    1m45.039s
> sys    0m0.536s
> paolino at paolino-casa:~$
> 
> staring at the resource usage graph I can see it does use 2 cores when told to
> do it, but with -N1 the used cpu goes 100% and with -N2 they both run just over
> 50%
> 
> thanks for comments


Firstly, a and b are identical, so GHC commons them up. The compiler
transforms it into:

    a `par` a `seq` print (a + a)

So you essentially fork a spark to evaluate 'a', and then have the main
thread also evaluate 'a' again. One of them wins, then you add the
result to itself. The runtime may choose not to convert your first spark
into a thread.

Running with a 2009 GHC head snapshot, we can see with +RTS -sstderr

      SPARKS: 1 (0 converted, 0 pruned)

That indeed, it doesn't convert your `par` into a real thread.

While, for example, the helloworld on the wiki:

    http://haskell.org/haskellwiki/Haskell_in_5_steps

Converts 2 sparks to 2 theads:

    SPARKS: 2 (2 converted, 0 pruned)
    ./B +RTS -threaded -N2 -sstderr  2.13s user 0.04s system 137% cpu 1.570 total 

-- Don