[Haskell-cafe] Haskell performance question

[Thomas Schilling]

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