为什么这个Haskell数组填充操作这么慢？

Question

对于特定的任务，我需要在可变数组中进行大量快速、单独的写操作。为了检查性能，我使用了以下测试：

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

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

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

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);

vh:haskell apple1$ time node bench.js
549755289600

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

#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);
};

Answer 1

GHC不会像C那样产生良好的紧密循环。根据我的经验，运行时的因数为3是正常的。

要获得更好的性能，请使用向量库：

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