On Sunday 14 August 2011, 21:53:21, Iustin Pop wrote:
On Sun, Aug 14, 2011 at 08:32:36PM +0200, Wishnu Prasetya wrote:
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% efficiency).
regards, iustin
That makes sense... But are you sure thats how i should read this?
As far as I know, this is correct.
It is indeed. CPU time is the sum of CPU time for all threads, which is typically larger than elapsed time when several threads run in parallel.
I dont want to jump happy too early.
Well, you algorithm does work in parallel, but if you look at the GC/MUT time, ~60% of the total runtime is spent in GC, so you have a space leak or an otherwise inefficient algorithm.
Not enough data to make more than guesses concerning the cause, but 60% GC definitely indicates a problem with the algorithm (resp. its implementation),
The final speedup is just 3.46s/2.36s, i.e. 1.46x instead of ~4x, so you still have some work to do to make this better.
We don't know the times for a non-threaded run (or an -N1 run), so it could be anything from a slowdown to a > 4× speedup (but it's likely to be a speedup by a factor < 4×).
At least, this is how I read those numbers.
regards, iustin