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

Daniel Fischer daniel.is.fischer at web.de
Fri May 22 08:04:58 EDT 2009


Am Freitag 22 Mai 2009 04:59:51 schrieb 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?
>

You forgot

{-# INLINE parallelize #-}

For me, that works.

> > 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)




More information about the Haskell-Cafe mailing list