为什么Clojure Reducers r / fold不提供perf perf？ [英] Why does this Clojure Reducers r/fold provide no perf benefit?

问题描述

我想知道为什么下面的代码提供没有加速的情况下r / fold？我是否对reducer有所了解？

I'm wondering why the code below provides no speedup in the case of r/fold? Am I misunderstanding something about reducers?

我使用Ubuntu 12.04开发盒运行速度非常慢（虽然有两个核心），通过emacs和lein run，具有相同的结果。

I'm running it on a pretty slow (although with 2 cores) Ubuntu 12.04 dev box, both through emacs and lein run, each with the same results.

(require '[clojure.core.reducers :as r])
(.. Runtime getRuntime availableProcessors)

;; 2

(let
  [n 80000000
   vs #(range n)]

  (time (reduce + (vs)))
  (time (r/fold + (vs)))

已用时间：26076.434324毫秒

已用时间：25500.234034 msecs

"Elapsed time: 26076.434324 msecs"
"Elapsed time: 25500.234034 msecs"

>

推荐答案

你折叠一个seq。平行折叠只发生在持久性向量和地图上。

You are folding over a seq. Parallel fold only happens on persistent vectors and maps right now.

也有各种各样的原因，为什么这种perf测试不如使用像 Criterium ，但是这可能是一个单独的讨论（一些原因是垃圾回收，JVM预热和内联，在Emacs和lein，boxed和checked数学等上的有趣的默认jvm设置）。

There are also all sorts of reasons why this kind of perf testing is inferior to using something like Criterium, but that's probably a separate discussion. (Some of the reasons are garbage collection, JVM warmup and inlining, funky default jvm settings on both Emacs and lein, boxed and checked math, etc.)

对于上面的许多原因仍然错误，但稍微更有用的比较：

Still wrong for many of the above reasons but slightly more useful comparison:

(require '[clojure.core.reducers :as r])
(def v (vec (range 800000)))
(dotimes [_ 100] (time (reduce + v)))
(dotimes [_ 100] (time (r/fold + v)))

的最后2次跑步。

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