为什么Seq.iter和Seq.map这么慢?

问题描述

在F#中考虑以下代码:

Consider this code in F#:

let n = 10000000
let arr = Array.init n (fun _ -> 0)

let rec buildList n acc i = if i = n then acc else buildList n (0::acc) (i + 1)
let lst = buildList n [] 0

let doNothing _ = ()
let incr x = x + 1

#time

arr |> Array.iter doNothing         // this takes 14ms
arr |> Seq.iter doNothing           // this takes 74ms

lst |> List.iter doNothing          // this takes 19ms
lst |> Seq.iter doNothing           // this takes 88ms

arr |> Array.map incr               // this takes 33ms
arr |> Seq.map incr |> Seq.toArray  // this takes 231ms!

lst |> List.map incr                // this takes 753ms
lst |> Seq.map incr |> Seq.toList   // this takes 2111ms!!!!

为什么Seq模块上的iter和map函数比Array和List模块等效项慢得多?

Why is the iter and map functions on the Seq module so much slower than the Array and List module equivalents?

推荐答案

一旦调用Seq，您将丢失类型信息-移至列表中的下一个元素需要调用IEnumerator.MoveNext.与Array相比，您只增加了一个索引，而对于List，您可以仅取消引用了一个指针.本质上，您将获得列表中每个元素的额外功能调用.

Once you call in to Seq you lose the type information - moving to the next element in the list requires a call to IEnumerator.MoveNext. Compare to for Array you just increment an index and for List you can just dereference a pointer. Essentially, you are getting an extra function call for each element in the list.

出于类似原因，转换回List和Array的速度也会降低代码

The conversions back to List and Array also slow the code down for similar reasons

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