[Haskell-cafe] A problem with par and modules boundaries...
Mario Blažević
mblazevic at stilo.com
Thu May 21 22:59:51 EDT 2009
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)
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