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)