Clojure的：为什么皮亚杰这么慢？ [英] Clojure: why is aget so slow?
问题描述
在我的思想，Clojure的载体相比，Java数组有轻微的性能损失。因此，我认为传统智慧是为你的code的那些性能关键部分，你会更好使用Java数组。
In my thinking, clojure vectors have a slight performance hit compared to java arrays. As a result I thought that "conventional wisdom" was that for those performance-critical parts of your code, you'd be better off using java arrays.
我的测试，但认为这是不正确的：
My tests however suggest that this is not true:
Clojure 1.3.0
user=> (def x (vec (range 100000)))
#'user/x
user=> (def xa (int-array x))
#'user/xa
user=> (time (loop [i 0 s 0] (if (< i 100000) (recur (inc i) (+ s (nth x i))) s)))
"Elapsed time: 16.551 msecs"
4999950000
user=> (time (loop [i 0 s 0] (if (< i 100000) (recur (inc i) (+ s (aget xa i))) s)))
"Elapsed time: 1271.804 msecs"
4999950000
正如你所看到的，皮亚杰增加了约800％的时间这个加法。这两种方法仍比原生的Java方法要慢，但：
As you can see, the aget adds about 800% time to this addition. Both methods still are way slower than native java though:
public class Test {
public static void main (String[] args) {
int[] x = new int[100000];
for (int i=0;i<100000;i++) {
x[i]=i;
}
long s=0;
long end, start = System.nanoTime();
for (int i=0;i<100000;i++) {
s+= x[i];
}
end = System.nanoTime();
System.out.println((end-start)/1000000.0+" ms");
System.out.println(s);
}
}
> java Test
1.884 ms
4999950000
所以，应我的结论是，皮亚杰比第n个慢80倍，慢约800倍[] -access在java中？
So, should my conclusion be that aget is 80 times slower than nth, and about 800 times slower than []-access in java?
推荐答案
我怀疑这是下降到了皮亚杰功能反射和原始类型的自动装箱......
I suspect this is down to reflection and autoboxing of the primitive types by the aget function....
幸运的是皮亚杰/ ASET必须为避免反射，只是做了直接序列基本数组高性能重载[I]访问（见的这里和<一个href=\"https://github.com/clojure/clojure/blob/clojure-1.3.0/src/jvm/clojure/lang/RT.java#L2067\">here).
Luckily aget/aset have performant overloads for primitive arrays that avoid the reflection and just do a direct array[i] access (See here and here).
您只需要通过一种暗示拿起右边的功能。
You just need to pass a type hint to pick up the right function.
(type xa)
[I ; indicates array of primitive ints
; with type hint on array
;
(time (loop [i 0 s 0]
(if (< i 100000) (recur (inc i)
(+ s (aget ^ints xa i))) s)))
"Elapsed time: 6.79 msecs"
4999950000
; without type hinting
;
(time (loop [i 0 s 0]
(if (< i 100000) (recur (inc i)
(+ s (aget xa i))) s)))
"Elapsed time: 1135.097 msecs"
4999950000
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