[Haskell-cafe] Re: Parallel Pi

[Simon Marlow]

On 17/03/10 21:30, Daniel Fischer wrote:
> Am Mittwoch 17 März 2010 19:49:57 schrieb Artyom Kazak:
>> Hello!
>> I tried to implement the parallel Monte-Carlo method of computing Pi
>> number, using two cores:
> <move>
>>
>> But it uses only on core:
>>
> <snip>
>>
>> We see that our one spark is pruned. Why?
>>
>
> Well, the problem is that your tasks don't do any real work - yet.
> piMonte returns a thunk pretty immediately, that thunk is then evaluated by
> show, long after your chance for parallelism is gone. You must force the
> work to be done _in_ r1 and r2, then you get parallelism:
>
>    Generation 0:  2627 collections,  2626 parallel,  0.14s,  0.12s elapsed
>    Generation 1:     1 collections,     1 parallel,  0.00s,  0.00s elapsed
>
>    Parallel GC work balance: 1.79 (429262 / 240225, ideal 2)
>
>                          MUT time (elapsed)       GC time  (elapsed)
>    Task  0 (worker) :    0.00s    (  8.22s)       0.00s    (  0.00s)
>    Task  1 (worker) :    8.16s    (  8.22s)       0.01s    (  0.01s)
>    Task  2 (worker) :    8.00s    (  8.22s)       0.13s    (  0.11s)
>    Task  3 (worker) :    0.00s    (  8.22s)       0.00s    (  0.00s)
>
>    SPARKS: 1 (1 converted, 0 pruned)
>
>    INIT  time    0.00s  (  0.00s elapsed)
>    MUT   time   16.14s  (  8.22s elapsed)
>    GC    time    0.14s  (  0.12s elapsed)
>    EXIT  time    0.00s  (  0.00s elapsed)
>    Total time   16.29s  (  8.34s elapsed)
>
>    %GC time       0.9%  (1.4% elapsed)
>
>    Alloc rate    163,684,377 bytes per MUT second
>
>    Productivity  99.1% of total user, 193.5% of total elapsed
>
> But alas, it is slower than the single-threaded calculation :(
>
>    INIT  time    0.00s  (  0.00s elapsed)
>    MUT   time    7.08s  (  7.10s elapsed)
>    GC    time    0.08s  (  0.08s elapsed)
>    EXIT  time    0.00s  (  0.00s elapsed)
>    Total time    7.15s  (  7.18s elapsed)

It works for me (GHC 6.12.1):

   SPARKS: 1 (1 converted, 0 pruned)

   INIT  time    0.00s  (  0.00s elapsed)
   MUT   time    9.05s  (  4.54s elapsed)
   GC    time    0.12s  (  0.09s elapsed)
   EXIT  time    0.00s  (  0.01s elapsed)
   Total time    9.12s  (  4.63s elapsed)

wall-clock speedup of 1.93 on 2 cores.

What hardware are you using there? Have you tried changing any GC settings?

Cheers,
	Simon