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

[Iustin Pop]

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