[Haskell-cafe] how to read CPU time vs wall time report from GHC?
daniel.is.fischer at googlemail.com
Sun Aug 14 23:05:04 CEST 2011
On Sunday 14 August 2011, 22:42:13, Wishnu Prasetya wrote:
> On 14-8-2011 22:17, Daniel Fischer wrote:
> > We don't know the times for a non-threaded run (or an -N1 run), so it
> > could be anything from a slowdown to a> 4× speedup (but it's likely
> > to be a speedup by a factor< 4×).
> Well, the -N1 is below. The sequential version of the program has almost
> the same profile:
> SPARKS: 5 (1 converted, 4 pruned)
> INIT time 0.00s ( 0.00s elapsed)
> MUT time 2.78s ( 2.99s elapsed)
> GC time 4.35s ( 4.15s elapsed)
> EXIT time 0.00s ( 0.00s elapsed)
> Total time 7.13s ( 7.14s elapsed)
> Am I correct then to say that the speed up with respect to sequential is
> equal to: tot-elapse-time-N1 / tot-elapse-N4 ? So I have 7.14 / 2.36 =
> 3.0 speed up, and not 1.46 as Iustin said?
Yes (with respect to wall-clock time of course).
That's not too bad, though it should be possible to increase that factor.
> I'll probably have to do something with that GC :)
But that should be the first target, there's probably some low-hanging
Maybe increasing the size of the allocation area (+RTS -Ax) or the heap
(+RTS -Hx) would already do some good.
Also do heap profiling to find out what most likely takes so much GC time
(before compiling for profiling, running with +RTS -hT could produce useful
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