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

[Don Stewart]

Answer recorded at:

    http://haskell.org/haskellwiki/Performance/Parallel

daniel.is.fischer:
> 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)
> 
> 
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