Hi,
I've been trying to use Control.Concurrent.mergeIO to parallelize
computation, and can't make it work. In the sample program below, I expect
the function 'parallelTest' to be almost twice as fast as 'sequentialTest',
and to compute its results in two threads, as implied by the documentation
for mergeIO. This is not what happens. If I link my program with the
option '-threaded', the running process does have three threads. If I run
with the option "+RTS -N2", the process will have 5 threads. In no case
does the process appear to be using more than one CPU, and in fact it is
slower with the threading options turned on.
I'm sure I am doing something obviously (to someone else) wrong. Any ideas?
I am running the latest version of Mac OSX on a core2 duo machine with 2
cores, using ghc version 6.10.4.
Cheers, Brock
My test program follows:
{-# OPTIONS_GHC -fglasgow-exts #-}
module Main where
import Control.Concurrent
import Random
doSum :: RandomGen g => g -> Int -> Integer
doSum g count
= let runner curG sum numDone
| numDone == count = sum
| otherwise
= let (newNum :: Integer, newG) = random curG
newSum = sum + newNum
newNumDone = numDone + 1
in ((runner $! newG) $! newSum) $! newNumDone
in runner g 0 0
sequentialTest
= do let gen = mkStdGen 0
(g0,g1) = split gen
count = 10000000
sum0 = doSum g0 count
sum1 = doSum g1 count
total = sum0 + sum1
putStrLn $ "total: " ++ show total
parallelTest
= do let gen = mkStdGen 0
(g0,g1) = split gen
count = 10000000
sum0 = doSum g0 count
sum1 = doSum g1 count
[res0, res1] <- mergeIO [sum0] [sum1]
let total = res0 + res1
putStrLn $ "total: " ++ show total
main
= parallelTest
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