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