Haskell parMap性能如何？

Question

我试图用一个非常简单的例子来测试parMap vs 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没有加速,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

问题是parMap为每个单独的列表元素引发并行计算.根据您的评论,您似乎根本不会对列表进行分块 - 这需要使用parListChunk策略.

因此parMap具有高开销,因此每个火花简单地对一个数字进行平方的事实意味着其成本被该开销所淹没.