[Haskell-cafe] how to user mergeIO

[Brock Peabody]

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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