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

Wishnu Prasetya s.w.b.prasetya at uu.nl
Mon Aug 15 00:10:27 CEST 2011

On 14-8-2011 23:05, Daniel Fischer wrote:
> 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
> information).
Ah ok. Thanks a lot for all the answers and tips.


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