Why is this Haskell array filling operation so slow?

Question

For a specific task, I need a lot of fast, individual writes in a mutable array. In order to check the performance, I've used the following test:

size :: Int
size = 256*256*16

arr :: UArray Int Int
arr = runST $ do
    arr <- newArray (0,size) 0 :: ST s (STUArray s Int Int)
    forM_ [0..size] $ \i -> do
        writeArray arr i i 
    unsafeFreeze arr

arr_sum = foldl' (\ sum i -> sum + (arr ! i)) 0 [0..size-1]

main = print arr_sum

Here is the result:

vh:haskell apple1$ ghc -O3 bench.hs -o bench; time ./bench
Linking bench ...
549755289600

real    0m0.748s
user    0m0.697s
sys 0m0.048s

I suspected it shouldn't take 0.7s to fill a 256*256*16 array on memory, so I tested an equivalent program in JavaScript:

size = 256*256*16;
x = new Array(size);
s = 0;
for (var i=0; i<size; ++i)
    x[i] = i;
for (var i=0; i<size; ++i)
    s += x[i];
console.log(s);

And the result is:

vh:haskell apple1$ time node bench.js
549755289600

real    0m0.175s
user    0m0.150s
sys 0m0.024s

On C, the time was 0.012s, which is a good lower bound.

#include <stdio.h>

#define SIZE (256*256*16)
double x[SIZE];

int main(){
    int i;
    double s = 0;
    for (i = 0; i<SIZE; ++i)
        x[i] = i;
    for (i = 0; i<SIZE; ++i)
        s += x[i];
    printf("%f",s);
};

So that pretty much confirms my hypothesis that my Haskell program is doing something else other than just filling the array and summing it afterwards. There is probably a hidden stack somewhere, but I can not identify it since I used foldl' and forM_, which I believed were compiled to stack-free code. So, what is the source of inefficiency here?

Answer 1

GHC does not produce nice tight loops like what you accomplish with C. A factor of 3 in run times is about par for the course based on my experience.

To get better performance use the Vector library:

import qualified Data.Vector.Unboxed as V

size = 256*256*16 :: Int

doit = V.foldl' (+) 0 vec
  where vec = V.generate size id 

main = print doit