为什么我的Java lambda与虚拟任务比没有它快得多？
2022-09-03 02:47:58
我知道对Java微基准标记做出判断是非常令人担忧的，但是我看到了一些看起来很奇怪的东西，我想得到一些解释。
请注意，我没有为此使用JMH框架。我知道这一点，但我不想为此而达到那么远。
我将提供整个代码示例，但简而言之，当我测试这两种方法的性能时
private FooPrime[] testStreamToArray(ArrayList<Foo> fooList) {
return (FooPrime[]) fooList.stream().
map(it -> {
return new FooPrime().gamma(it.getAlpha() + it.getBeta());
}).
toArray(FooPrime[]::new);
}
private FooPrime[] testStreamToArray2(ArrayList<Foo> fooList) {
return (FooPrime[]) fooList.stream().
map(it -> {
int stuff = it.getAlpha().length();
return new FooPrime().gamma(it.getAlpha() + it.getBeta());
}).
toArray(FooPrime[]::new);
}
我发现非常令人惊讶的结果。在较大的代码示例中，我测量了四种不同的方法，前三种方法的性能非常接近。它们每次迭代都运行大约50k ns。但是，第二个代码示例始终运行不到该总数的一半。没错。它不是更慢，而是快得多。
最后一次运行显示如下数字：
manualcopy:54575 ns
toarray:53617 ns
streamtoarray:52990 ns
streamtoarray2:24217 ns
每次运行都有与此类似的数字。
现在，我将提供整个类和基类。请注意，我确实有一个“预热”阶段，在开始计时之前，我将测试方法执行几千次。另请注意，尽管这最后运行“testStreamToArray2”，但我也尝试将该块移动到第一个测试中，并且数字大致相同。注释掉的行是为了让我相信这些方法实际上正在做一些事情（时间仍然与那些没有被注释掉的行大致相同）。
package timings;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class ListToArrayOfPrimesTiming {
public static void main(String[] args) {
ListToArrayOfPrimesTiming tests = new ListToArrayOfPrimesTiming(args);
tests.go();
}
public ListToArrayOfPrimesTiming(String[] args) { }
private void go() {
final ArrayList<Foo> fooList = new ArrayList<>();
for (int ctr = 0; ctr < 1000; ++ ctr) {
fooList.add(new Foo().alpha("a" + ctr).beta("b" + ctr));
}
for (int ctr = 0; ctr < 20000; ++ ctr) {
testManualCopy(fooList);
testToArray(fooList);
testStreamToArray(fooList);
testStreamToArray2(fooList);
}
int iters = 100000;
// Set<Integer> lengths = new HashSet<>();
// Set<FooPrime> distinctFooPrimes = new HashSet<>();
// lengths.clear();
// distinctFooPrimes.clear();
new TimingContainer(iters, "manualcopy", new TimingTest() {
@Override
public void run() {
FooPrime[] fooPrimeArray = testManualCopy(fooList);
// lengths.add(fooPrimeArray.length);
// distinctFooPrimes.add(fooPrimeArray[0]);
}
}).run();
// System.out.println("lengths[" + lengths + "]");
// lengths.clear();
// System.out.println("distinctFooPrimes[" + distinctFooPrimes + "]");
// distinctFooPrimes.clear();
new TimingContainer(iters, "toarray", new TimingTest() {
@Override
public void run() {
FooPrime[] fooPrimeArray = testManualCopy(fooList);
// lengths.add(fooPrimeArray.length);
// distinctFooPrimes.add(fooPrimeArray[0]);
}
}).run();
// System.out.println("lengths[" + lengths + "]");
// lengths.clear();
// System.out.println("distinctFooPrimes[" + distinctFooPrimes + "]");
// distinctFooPrimes.clear();
new TimingContainer(iters, "streamtoarray", new TimingTest() {
@Override
public void run() {
FooPrime[] fooPrimeArray = testStreamToArray(fooList);
// lengths.add(fooPrimeArray.length);
// distinctFooPrimes.add(fooPrimeArray[0]);
}
}).run();
// System.out.println("lengths[" + lengths + "]");
// lengths.clear();
// System.out.println("distinctFooPrimes[" + distinctFooPrimes + "]");
// distinctFooPrimes.clear();
new TimingContainer(iters, "streamtoarray2", new TimingTest() {
@Override
public void run() {
FooPrime[] fooPrimeArray = testStreamToArray2(fooList);
// lengths.add(fooPrimeArray.length);
// distinctFooPrimes.add(fooPrimeArray[0]);
}
}).run();
// System.out.println("lengths[" + lengths + "]");
// lengths.clear();
// System.out.println("distinctFooPrimes[" + distinctFooPrimes + "]");
// distinctFooPrimes.clear();
}
private FooPrime[] testManualCopy(ArrayList<Foo> fooList) {
FooPrime[] fooPrimeArray = new FooPrime[fooList.size()];
int index = -1;
for (Foo foo: fooList) {
++ index;
fooPrimeArray[index] = new FooPrime().gamma(foo.getAlpha() + foo.getBeta());
}
return fooPrimeArray;
}
private FooPrime[] testToArray(ArrayList<Foo> fooList) {
List<FooPrime> fooPrimeList = new ArrayList<>();
for (Foo foo: fooList) {
fooPrimeList.add(new FooPrime().gamma(foo.getAlpha() + foo.getBeta()));
}
return fooPrimeList.toArray(new FooPrime[fooList.size()]);
}
private FooPrime[] testStreamToArray(ArrayList<Foo> fooList) {
return (FooPrime[]) fooList.stream().
map(it -> {
return new FooPrime().gamma(it.getAlpha() + it.getBeta());
}).
toArray(FooPrime[]::new);
}
private FooPrime[] testStreamToArray2(ArrayList<Foo> fooList) {
return (FooPrime[]) fooList.stream().
map(it -> {
int stuff = it.getAlpha().length();
return new FooPrime().gamma(it.getAlpha() + it.getBeta());
}).
toArray(FooPrime[]::new);
}
public static FooPrime fooToFooPrime(Foo foo) {
return new FooPrime().gamma(foo.getAlpha() + foo.getBeta());
}
public static class Foo {
private String alpha;
private String beta;
public String getAlpha() { return alpha; }
public String getBeta() { return beta; }
public void setAlpha(String alpha) { this.alpha = alpha; }
public void setBeta(String beta) { this.beta = beta; }
public Foo alpha(String alpha) { this.alpha = alpha; return this; }
public Foo beta(String beta) { this.beta = beta; return this; }
}
public static class FooPrime {
private String gamma;
public String getGamma() { return gamma; }
public void setGamma(String gamma) { this.gamma = gamma; }
public FooPrime gamma(String gamma) { this.gamma = gamma; return this; }
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((gamma == null) ? 0 : gamma.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
FooPrime other = (FooPrime) obj;
if (gamma == null) {
if (other.gamma != null)
return false;
} else if (!gamma.equals(other.gamma))
return false;
return true;
}
@Override
public String toString() {
return "FooPrime [gamma=" + gamma + "]";
}
}
}
基类：
package timings;
public class TimingContainer {
private int iterations;
private String label;
private TimingTest timingTest;
public TimingContainer(int iterations, String label, TimingTest timingTest) {
this.iterations = iterations;
this.label = label;
this.timingTest = timingTest;
}
public void run() {
long startTime = System.nanoTime();
for (int ctr = 0; ctr < iterations; ++ ctr) {
timingTest.randomize();
timingTest.run();
}
long endTime = System.nanoTime();
long totalns = (endTime - startTime);
System.out.println(label + ":" + (totalns / iterations) + " ns");
}
}
