[Haskell-cafe] how to read CPU time vs wall time report from GHC?

Daniel Fischer 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 
fruit there.
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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