OK, I think I figured it out. If I understand correctly, I was just computing the input lists in parallel. The actual values were computed in the main thread lazily, later. This seems unintuitive to me. Shouldn't the merge functions force the evaluation of their arguments? Surely one wouldn't be calling them if they wanted to compute the results lazily. On Sun, Mar 14, 2010 at 6:25 PM, Brock Peabody wrote:

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