Hi Mario, It looks like the parallelize function is getting inlined when it's in the same file, but not when it's in a separate file. Adding a {-# INLINE parallelize #-} pragma to the module with parallelize recovers all the performance for me. You could probably see exactly what's happening in more detail by going through the Core output. John Lato

Message: 23 Date: Thu, 21 May 2009 22:59:51 -0400 From: Mario Bla?evi? Subject: [Haskell-cafe] A problem with par and modules boundaries... To: Message-ID: <.1242961191@magma.ca> Content-Type: text/plain; charset="utf-8"

I'll cut to the chase. The short program below works perfectly: when I compile it with -O2 -threaded and run with +RTS -N2 command-line options, I get a nearly 50% real-time improvement:

$ time ./primes-test +RTS -N2 5001

real    0m9.307s user    0m16.581s sys     0m0.200s

However, if I move the `parallelize' definition into another module and import that module, the performance is completely lost:

$ time ./primes-test +RTS -N2 5001

real    0m15.282s user    0m15.165s sys     0m0.080s

I'm confused. I know that `par` must be able work across modules boundaries, because Control.Parallel.Strategies is a module and presumably it works. What am I doing wrong?

...

module Main where

import Control.Parallel import Data.List (find) import Data.Maybe (maybe)

--import Parallelizable parallelize a b = a `par` (b `pseq` (a, b))

test :: Integer -> Integer -> Integer test n1 n2 = let (p1, p2) = parallelize                                (product $ factors $ product [1..n1])                                (product $ factors $ product [1..n2])              in p2 `div` p1

factors n = maybe [n] (\k-> (k : factors (n `div` k)))                   (find (\k-> n `mod` k == 0) [2 .. n - 1])

main = print (test 5000 5001)