Haskell 中的并行映射 [英] Parallel map in haskell

问题描述

是否有一些 map 的替代品来并行评估列表?我不需要它偷懒.

Is there some substitute of map which evaluates the list in parallel? I don't need it to be lazy.

类似:pmap :: (a -> b) ->[a] ->[b] 让我 pmap 昂贵的函数 big_list 并让我的所有内核都达到 100%.

Something like: pmap :: (a -> b) -> [a] -> [b] letting me pmap expensive_function big_list and have all my cores at 100%.

推荐答案

是的，参见 并行包:

ls `using` parList rdeepseq

将通过 rdeepseq 策略并行评估列表中的每个元素.请注意，如果您的元素太便宜而无法获得并行评估每个元素的好处(因为它节省了每个元素的火花)，则使用具有良好块值的 parListChunk 可能会提供更好的性能.

will evaluate each element of the list in parallel via the rdeepseq strategy. Note the use of parListChunk with a good chunk value might give better performance if your elements are too cheap to get a benefit evaluating each one in parallel (because it saves on sparking for each element).

根据您的问题，我觉得我应该解释为什么这是一个答案.那是因为 Haskell 很懒！考虑语句

Based on your question I feel I should explain why this is an answer. It's because Haskell is lazy! Consider the statement

let bs = map expensiveFunction as

未评估任何内容.您刚刚创建了一个映射 expensiveFunction 的 thunk.那么我们如何并行评估它?

Nothing has been evaluated. You've just created a thunk that maps expensiveFunction. So how do we evaluate it in parallel?

let bs = map expensiveFunction as
    cs = bs `using` parList rdeepseq

现在不要在以后的计算中使用 bs 列表，而是使用 cs 列表.IOW，您不需要并行地图，您可以使用常规(懒惰)地图和并行评估策略.

Now don't use the bs list in your future computations, instead use the cs list. IOW, you don't need a parallel map, you can use the regular (lazy) maps and a parallel evaulation strategy.

如果你环顾四周，你会看到 parMap 函数执行我在此处展示的操作，但包装为一个辅助函数.

And if you look around enough you'll see the parMap function that does what I showed here but wrapped into one helper function.

针对您的评论，以下代码对您不起作用吗?它对我有用.

In response to your comment, does the below code not work for you? it works for me.

import Control.Parallel.Strategies

func as =
        let bs = map (+1) as
            cs = bs `using` parList rdeepseq
        in cs

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