Re: [Haskell-cafe] Haskell performance question

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

#define n 100000000

double a[n];

int main() { int i; for (i = 0; i

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