[Haskell-cafe] A problem with par and modules boundaries...

[Mario Blažević]

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)