Haskell parMap 性能？

Question

我试图用一个非常简单的例子来比较 parMap 与 map:

import Control.Parallel.Strategies
import Criterion.Main

sq x = x^2

a = whnf sum $ map sq [1..1000000]
b = whnf sum $ parMap rseq sq [1..1000000]

main = defaultMain [
    bench "1" a,
    bench "2" b
  ]

我的结果似乎表明 parMap 的加速为零，我想知道为什么会这样？

benchmarking 1
Warning: Couldn't open /dev/urandom
Warning: using system clock for seed instead (quality will be lower)
time                 177.7 ms   (165.5 ms .. 186.1 ms)
                     0.997 R²   (0.992 R² .. 1.000 R²)
mean                 185.1 ms   (179.9 ms .. 194.1 ms)
std dev              8.265 ms   (602.3 us .. 10.57 ms)
variance introduced by outliers: 14% (moderately inflated)

benchmarking 2
time                 182.7 ms   (165.4 ms .. 199.5 ms)
                     0.993 R²   (0.976 R² .. 1.000 R²)
mean                 189.4 ms   (181.1 ms .. 195.3 ms)
std dev              8.242 ms   (5.896 ms .. 10.16 ms)
variance introduced by outliers: 14% (moderately inflated)

Answer 1

问题在于 parMap 为每个单独的列表元素引发并行计算。它根本不会像您从评论中想到的那样对列表进行分块，这需要使用 parListChunk策略。

因此，parMap 的开销很高，因此每个 Spark 仅对一个数字进行平方这一事实意味着其成本被该开销所淹没。