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