[Haskell-cafe] Re: Parallel Pi

[Daniel Fischer]

Am Donnerstag 18 März 2010 22:44:55 schrieb 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.

Is that Artyom's original code or with the pseq'ed length?
The original didn't convert any sparks for me (~103% cpu, because of 
parallel GC, but the calculation always used just one thread).
I'm also using 6.12.1.

And, with -N2, I also have a productivity of 193.5%, but the elapsed time 
is larger than the elapsed time for -N1. How long does it take with -N1 for 
you?

It's the same with 6.10.3, no converted sparks for the original code, 
parallelism with the pseq'ed length, but it takes longer than with -N1.

>
> What hardware are you using there?

3.06GHz Pentium 4, 2 cores.
I have mixed results with parallelism, some programmes get a speed-up of 
nearly a factor 2 (wall-clock time), others 1.4, 1.5 or so, yet others take 
about the same wall-clock time as the single threaded programme, some - 
like this - take longer despite using both cores intensively.

> Have you tried changing any GC settings?

I've played around a little with -qg and -qb and -C, but that showed little 
influence. Any tips what else might be worth a try?

>
> Cheers,
> 	Simon