Java 11字符串连接性能与Java 8

Question

有人知道为什么在Java 8和Java 11上运行此代码时会得到如此不同的性能吗？

在不使用任何运行时标志的情况下，与Java 8相比，此代码在Java 11下的运行速度明显慢。

import java.util.Date;

public class PerformanceExperiment {
    public static volatile String s = "";

    public static void main(String[] args)
    {                         
        System.out.println("Starting performance test");
        String s1 = "STRING ONE";
        String s2 = "STRING TWO";
        long now1 = (new Date()).getTime();
        for (long i = 0; i < 1_000_000_00; i++)
        {
            s = "abc " + s1 + " def " + s2;
        }
        long now2 = (new Date()).getTime();
        System.out.println("initial block took " + (now2 - now1) + "ms");
        for (long i = 0; i < 4_000_000_00; i++)
        {
            s = "abc " + s1 + " def " + s2;
        }
        long now3 = (new Date()).getTime();
        System.out.println("Main block took " + (now3 - now2) + "ms");
    }
}

我尝试了许多命令行标志，但没有找到与Java 8性能匹配的任何东西。

我只在Windows上进行过测试，因此它在其他操作系统上的表现可能会有所不同。

Answer 1

我将您的应用修改为

1. 使用System.nanoTime()代替new Date()可以提高精度（有关更多信息，请参见此答案：https://stackoverflow.com/a/1776053/963076）。
2. 使用Netbeans分析器。
3. 循环10次迭代

在JDK 8 v181中使用Netbeans 8.2：

Starting performance test 0
initial block took 3147ms
Main block took 9469ms
Starting performance test 1
initial block took 2398ms
Main block took 9601ms
Starting performance test 2
initial block took 2463ms
Main block took 9671ms
Starting performance test 3
initial block took 2464ms
Main block took 9565ms
Starting performance test 4
initial block took 2410ms
Main block took 9672ms
Starting performance test 5
initial block took 2418ms
Main block took 9598ms
Starting performance test 6
initial block took 2384ms
Main block took 9733ms
Starting performance test 7
initial block took 2402ms
Main block took 9610ms
Starting performance test 8
initial block took 2509ms
Main block took 11222ms
Starting performance test 9
initial block took 2455ms
Main block took 10661ms

分析器显示了此遥测：

使用JDK 11.0.2的Netbeans 10.0：

Starting performance test 0
initial block took 3760ms
Main block took 15056ms
Starting performance test 1
initial block took 3734ms
Main block took 14602ms
Starting performance test 2
initial block took 3615ms
Main block took 14762ms
Starting performance test 3
initial block took 3748ms
Main block took 14534ms
Starting performance test 4
initial block took 3628ms
Main block took 14759ms
Starting performance test 5
initial block took 3625ms
Main block took 14959ms
Starting performance test 6
initial block took 3987ms
Main block took 14967ms
Starting performance test 7
initial block took 3803ms
Main block took 14701ms
Starting performance test 8
initial block took 3599ms
Main block took 14762ms
Starting performance test 9
initial block took 3627ms
Main block took 14434ms

我的结论：为了提高内存效率，JDK 11正在做更多的工作。注意，使用JDK11时，垃圾收集器中“存活的世代”的数量减少了（方式），并且在使用率和易变性方面，内存使用量也显着减少。权衡似乎在速度上，但是速度的差异小于内存使用的差异。

Answer 2

TL; DR：需要更好的基准测试，更好的设置以控制版本之间的差异等。使用JMH可以轻松解决大多数基准测试问题。当前的测试行为似乎是通过可疑的基准测试方法和默认GC的更改来解释的。

考虑一下：

public class PerformanceExperiment {
    public static volatile String s = "";

    public static void main(String[] args) {
        for (int c = 0; c < 5; c++) {
            test();
        }
    }

    public static void test() {
        String s1 = "STRING ONE";
        String s2 = "STRING TWO";
        long time1 = System.currentTimeMillis();
        for (long i = 0; i < 4_000_000_00; i++) {
            s = "abc " + s1 + " def " + s2;
        }
        long time2 = System.currentTimeMillis();
        System.out.println("Main block took " + (time2 - time1) + "ms");
    }
}

首先，它使用更方便的计时。然后，它测量相同的字节码块，而原始测试会预热“初始1”，然后继续测量绝对冷的那个。

然后，JIT编译将调用该方法，并且您想再次重新输入该方法以使优化的代码运行，否则，您将运行中间的“ on-stack-replacement”代码-您可以使用外部调用test的迭代。最重要的是，您想要多次输入以捕获最优化的版本。

而且，由于测试分配了很多内存，因此您希望确定堆大小。

所以，这里：

$ ~/Install/jdk8u191-rh/bin/javac PerformanceExperiment.java
$ ~/Install/jdk8u191-rh/bin/java -Xms2g -Xmx2g PerformanceExperiment
Main block took 10024ms
Main block took 9768ms
Main block took 7249ms
Main block took 7235ms
Main block took 7205ms

...这是同一字节码上的11.0.2：

$ ~/Install/jdk11.0.2/bin/java -Xms2g -Xmx2g PerformanceExperiment
Main block took 9775ms
Main block took 10825ms
Main block took 8635ms
Main block took 8616ms
Main block took 8622ms

...这是具有匹配GC的11.0.2（9+使用JEP 248将默认值更改为G1）：

$ ~/Install/jdk11.0.2/bin/java -Xms2g -Xmx2g -XX:+UseParallelGC PerformanceExperiment
Main block took 9281ms
Main block took 9129ms
Main block took 6725ms
Main block took 6688ms
Main block took 6684ms

最重要的是，每个小迭代中都有volatile个存储，这会花费很多，并且可能会扭曲基准。

还有interaction个带有字符串concat (JEP 280)，线程本地握手(JEP 312)和其他VM修复程序，但您可能只有在编译超过target = 8时才能看到，这超出了本练习的范围。

Answer 3

现在已确认这是JDK中的错误：

https://bugs.java.com/bugdatabase/view_bug.do?bug_id=JDK-8221733