[Haskell-cafe] Haskell performance question
Don Stewart
dons at galois.com
Thu Nov 8 16:00:31 EST 2007
nominolo:
> On Thu, 2007-11-08 at 10:33 -0800, Dan Piponi wrote:
> > I see lots of shootout examples where Haskell programs seem to perform
> > comparably with C programs, but I find it hard to reproduce anything
> > like those figures when testing with my own code. So here's a simple
> > case:
> >
> > I have this C program:
> >
> > #include <stdio.h>
> >
> > #define n 100000000
> >
> > double a[n];
> >
> > int main()
> > {
> > int i;
> > for (i = 0; i<n; ++i)
> > {
> > a[i] = 1.0f;
> > }
> > for (i = 0; i<n-1; ++i)
> > {
> > a[i+1] = a[i]+a[i+1];
> > }
> > printf("%f\n",a[n-1]);
> > }
> >
> > And this Haskell program:
> >
> > import Data.Array.IO
> > import Control.Monad
> >
> > n = 10000000
> >
> > main = do
> > a <- newArray (0,n-1) 1.0 :: IO (IOUArray Int Double)
> > forM_ [0..n-2] $ \i -> do { x <- readArray a i; y <- readArray a
> > (i+1); writeArray a (i+1) (x+y) }
> > x <- readArray a (n-1)
> > print x
> >
> > Even though 'n' is 10 times bigger in the C program it runs much
> > faster than the Haskell program on my MacBook Pro with Haskell 6.6.1.
> > I've tried lots of different combinations of flags that I've found in
> > various postings to haskell-cafe but to no avail.
> >
> > What flags do I need to get at least within a factor of 2 or 3 of C?
> > Am I using the wrong kind of array? Or is Haskell always going to be
> > 15-20 times slower for this kind of numerical work?
>
> Wow. You should *really* try using GHC 6.8.1:
>
> GHC 6.6.1 (-O2)
>
> real 0m7.062s
> user 0m5.464s
> sys 0m0.288s
>
> GHC 6.8.0.20071019
>
> real 0m0.723s
> user 0m0.616s
> sys 0m0.100s
>
> result is "2.0e7" for both
So that looks like a bug in the simplifier in 6.6.1 that has been fixed.
This should go in the testsuite, perhaps, Simon?
-- Don
More information about the Haskell-Cafe
mailing list