Clojure的pmap函数会为URL提取操作生成多少个线程？ [英] How many threads does Clojure's pmap function spawn for URL-fetching operations?

问题描述

pmap 函数的文档让我想知道，像通过Web提取XML提要的集合那样的效率如何。我不知道pmap会产生多少个并发的取指操作，最大值是多少。

The documentation on the pmap function leaves me wondering how efficient it would be for something like fetching a collection of XML feeds over the web. I have no idea how many concurrent fetch operations pmap would spawn and what the maximum would be.

推荐答案

如果检查源，参见：

> (use 'clojure.repl)
> (source pmap)
(defn pmap
  "Like map, except f is applied in parallel. Semi-lazy in that the
  parallel computation stays ahead of the consumption, but doesn't
  realize the entire result unless required. Only useful for
  computationally intensive functions where the time of f dominates
  the coordination overhead."
  {:added "1.0"}
  ([f coll]
   (let [n (+ 2 (.. Runtime getRuntime availableProcessors))
         rets (map #(future (f %)) coll)
         step (fn step [[x & xs :as vs] fs]
                (lazy-seq
                 (if-let [s (seq fs)]
                   (cons (deref x) (step xs (rest s)))
                   (map deref vs))))]
     (step rets (drop n rets))))
  ([f coll & colls]
   (let [step (fn step [cs]
                (lazy-seq
                 (let [ss (map seq cs)]
                   (when (every? identity ss)
                     (cons (map first ss) (step (map rest ss)))))))]
     (pmap #(apply f %) (step (cons coll colls))))))

（+ 2（.. Runtime getRuntime availableProcessors））是那里的主要线索。 pmap将抢先完成（+ 2个处理器）的工作，并通过 future 异步运行它们。因此，如果您有2个核心，那么它将一次启动4个工作，并试图保持领先地位，但最大数量应为2 + n。

The (+ 2 (.. Runtime getRuntime availableProcessors)) is a big clue there. pmap will grab the first (+ 2 processors) pieces of work and run them asynchronously via future. So if you have 2 cores, it's going to launch 4 pieces of work at a time, trying to keep a bit ahead of you but the max should be 2+n.

未来最终使用支持无限线程数量的代理I / O线程池。它会随着工作量的增长而增长，如果未使用线程，则会收缩。

future ultimately uses the agent I/O thread pool which supports an unbounded number of threads. It will grow as work is thrown at it and shrink if threads are unused.

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