【发布时间】：2012-08-03 05:16:04
【问题描述】：
我在 Clojure 中实现了一些基本的复数运算，并注意到它比大致等效的 Java 代码慢了大约 10 倍，即使有类型提示也是如此。
比较：
(defn plus [[^double x1 ^double y1] [^double x2 ^double y2]]
[(+ x1 x2) (+ y1 y2)])
(defn times [[^double x1 ^double y1] [^double x2 ^double y2]]
[(- (* x1 x2) (* y1 y2)) (+ (* x1 y2) (* y1 x2))])
(time (dorun (repeatedly 100000 #(plus [1 0] [0 1]))))
(time (dorun (repeatedly 100000 #(times [1 0] [0 1]))))
输出：
"Elapsed time: 69.429796 msecs"
"Elapsed time: 72.232479 msecs"
与：
public static void main( String[] args ) {
double[] z1 = new double[] { 1, 0 };
double[] z2 = new double[] { 0, 1 };
double[] z3 = null;
long l_StartTimeMillis = System.currentTimeMillis();
for ( int i = 0; i < 100000; i++ ) {
z3 = plus( z1, z2 ); // assign result to dummy var to stop compiler from optimising the loop away
}
long l_EndTimeMillis = System.currentTimeMillis();
long l_TimeTakenMillis = l_EndTimeMillis - l_StartTimeMillis;
System.out.format( "Time taken: %d millis\n", l_TimeTakenMillis );
l_StartTimeMillis = System.currentTimeMillis();
for ( int i = 0; i < 100000; i++ ) {
z3 = times( z1, z2 );
}
l_EndTimeMillis = System.currentTimeMillis();
l_TimeTakenMillis = l_EndTimeMillis - l_StartTimeMillis;
System.out.format( "Time taken: %d millis\n", l_TimeTakenMillis );
doNothing( z3 );
}
private static void doNothing( double[] z ) {
}
public static double[] plus (double[] z1, double[] z2) {
return new double[] { z1[0] + z2[0], z1[1] + z2[1] };
}
public static double[] times (double[] z1, double[] z2) {
return new double[] { z1[0]*z2[0] - z1[1]*z2[1], z1[0]*z2[1] + z1[1]*z2[0] };
}
输出：
Time taken: 6 millis
Time taken: 6 millis
事实上，类型提示似乎没有什么区别：如果我删除它们，我会得到大致相同的结果。真正奇怪的是，如果我在没有 REPL 的情况下运行 Clojure 脚本，我会得到较慢的结果：
"Elapsed time: 137.337782 msecs"
"Elapsed time: 214.213993 msecs"
所以我的问题是：如何才能接近 Java 代码的性能？为什么在没有 REPL 的情况下运行 clojure 时，表达式的计算时间会更长？
更新 ==============
太好了，在deftype 和defns 中使用带有类型提示的deftype，使用dotimes 而不是repeatedly 提供的性能与Java 版本一样好或更好。谢谢你们俩。
(deftype complex [^double real ^double imag])
(defn plus [^complex z1 ^complex z2]
(let [x1 (double (.real z1))
y1 (double (.imag z1))
x2 (double (.real z2))
y2 (double (.imag z2))]
(complex. (+ x1 x2) (+ y1 y2))))
(defn times [^complex z1 ^complex z2]
(let [x1 (double (.real z1))
y1 (double (.imag z1))
x2 (double (.real z2))
y2 (double (.imag z2))]
(complex. (- (* x1 x2) (* y1 y2)) (+ (* x1 y2) (* y1 x2)))))
(println "Warm up")
(time (dorun (repeatedly 100000 #(plus (complex. 1 0) (complex. 0 1)))))
(time (dorun (repeatedly 100000 #(times (complex. 1 0) (complex. 0 1)))))
(time (dorun (repeatedly 100000 #(plus (complex. 1 0) (complex. 0 1)))))
(time (dorun (repeatedly 100000 #(times (complex. 1 0) (complex. 0 1)))))
(time (dorun (repeatedly 100000 #(plus (complex. 1 0) (complex. 0 1)))))
(time (dorun (repeatedly 100000 #(times (complex. 1 0) (complex. 0 1)))))
(println "Try with dorun")
(time (dorun (repeatedly 100000 #(plus (complex. 1 0) (complex. 0 1)))))
(time (dorun (repeatedly 100000 #(times (complex. 1 0) (complex. 0 1)))))
(println "Try with dotimes")
(time (dotimes [_ 100000]
(plus (complex. 1 0) (complex. 0 1))))
(time (dotimes [_ 100000]
(times (complex. 1 0) (complex. 0 1))))
输出：
Warm up
"Elapsed time: 92.805664 msecs"
"Elapsed time: 164.929421 msecs"
"Elapsed time: 23.799012 msecs"
"Elapsed time: 32.841624 msecs"
"Elapsed time: 20.886101 msecs"
"Elapsed time: 18.872783 msecs"
Try with dorun
"Elapsed time: 19.238403 msecs"
"Elapsed time: 17.856938 msecs"
Try with dotimes
"Elapsed time: 5.165658 msecs"
"Elapsed time: 5.209027 msecs"
【问题讨论】：
-
你试过设置
*warn-on-reflection*看是否有任何反射潜入？ -
@DaoWen：不，我从未使用过该设置。我刚刚再次运行脚本并在其顶部使用
(set! *warn-on-reflection* true)，并且没有打印到标准输出的警告，这意味着没有使用反射，对吧？只是想确保我正确使用它。
标签： optimization clojure type-hinting numerical-computing