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

[Edward Z. Yang]

Ah, good catch. :-)

Edward

Excerpts from Iustin Pop's message of Sun Aug 14 14:25:02 -0400 2011:
> 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
>