[Haskell-cafe] how to read CPU time vs wall time report from GHC?
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%
That makes sense... But are you sure thats how i should read this? I
dont want to jump happy too early.
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