paolo.veronelli:
A better test program
import Control.Parallel
main = a `par` b `pseq` print (a + b ) where a = ack 3 11 b = fib 39
ack 0 n = n+1 ack m 0 = ack (m-1) 1 ack m n = ack (m-1) (ack m (n-1))
fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2)
running it , these are the results
From the resource graph and the timings it is clear that the program is not able to use all the 2 cores powers, considering computing 'a' alone is about 7 seconds and 'b' alone 9. What is retaining the cpu's to run full power ?
Some advice on analysing parallel programs: * Read Simon Marlow's excellent paper: "Runtime Support for Multicore Haskell" http://ghcmutterings.wordpress.com/2009/03/03/new-paper-runtime-support-for-... * Install GHC Head from 2009, it includes spark profiling. * Never bother measuring without using -O or -O2 -- unoptimized code is just not useful to think about. * Compile your program: $ ghc-6.11.20090228 -O2 --make -threaded A.hs [1 of 1] Compiling Main ( A.hs, A.o ) Linking A ... * Run it with +RTS -sstderr $ ./A +RTS -N2 -sstderr ./A +RTS -N2 -sstderr 63262367 ... Generation 0: 12021 collections, 0 parallel, 1.33s, 1.42s elapsed Generation 1: 2 collections, 1 parallel, 0.00s, 0.00s elapsed Parallel GC work balance: 1.03 (426 / 414, ideal 2) SPARKS: 1 (1 converted, 0 pruned) Productivity 90.7% of total user, 126.4% of total elapsed And we see 3 things: * It did use the parallel GC : good! * It did convert 1 spark to a thread: good! * Productivity was greater than 100%: good! So this looks fine! So I suspect you're falling into the ghc 6.10.x series bug where < 3 sparks would not be converted. Try with either 3 sparks, or using GHC 6.11.x series. -- Don