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

Edward Z. Yang ezyang at MIT.EDU
Sun Aug 14 20:23:55 CEST 2011

Hello Wishnu,

That is slightly odd. What CPU and operating system are you running on?
Include Kernel versions if Linux.


Excerpts from Wishnu Prasetya's message of Sun Aug 14 14:11:36 -0400 2011:
> 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?
> Any help?
> --Wish.

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