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

Wishnu Prasetya s.w.b.prasetya at uu.nl
Sun Aug 14 20:32:36 CEST 2011

On 14-8-2011 20:25, Iustin Pop wrote:
> On Sun, Aug 14, 2011 at 08:11:36PM +0200, Wishnu Prasetya wrote:
>> Hi guys,
>> I'm new in parallel programming with Haskell. I made a simple test
>> program using that par combinator etc, and was a bit unhappy that it
>> turns out to be  slower than its sequential version. But firstly, I
>> dont fully understand how to read the runtime report produced by GHC
>> with -s option:
>>    SPARKS: 5 (5 converted, 0 pruned)
>>    INIT  time    0.02s  (  0.01s elapsed)
>>    MUT   time    3.46s  (  0.89s elapsed)
>>    GC    time    5.49s  (  1.46s elapsed)
>>    EXIT  time    0.00s  (  0.00s elapsed)
>>    Total time    8.97s  (  2.36s elapsed)
>> As I understand it from the documentation, the left time-column is
>> the CPU time, whereas the right one is elapses wall time. But how
>> come that the wall time is less than the CPU time? Isn't wall time =
>> user's perspective of time; so that is CPU time + IO + etc?
> Yes, but if you have multiple CPUs, then CPU time "accumulates" faster
> than wall-clock time.
> Based on the above example, I guess you have or you run the program on 4
> cores (2.36 * 4 = 9.44, which means you got a very nice ~95%
> efficiency).
> regards,
> iustin
That makes sense... But are you sure thats how i should read this?   I 
dont want to jump happy too early.


More information about the Haskell-Cafe mailing list