Java 11字符串连接性能与java 8
有人知道为什么我在Java 8和Java 11上运行这段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下的运行速度似乎要慢得多。
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记，但都没有找到与Java 8性能匹配的任何标记。
我只在Windows上进行了测试，所以它在其他操作系统上的表现可能会有所不同。
回答 3
Stack Overflow用户
发布于 2019-03-28 21:03:15
我修改了你的应用程序
Netbeans profiler使用System.nanoTime()代替new Date()以获得更高的精度(有关更多信息，请参见此答案：https://stackoverflow.com/a/1776053/963076).
- Use Netbeans profiler。
- 循环通过10次迭代
将Netbeans 8.2与JDK 8 v181配合使用：
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剖面仪显示了这个遥测结果：
对于使用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
我的结论是: JDK 11正在做更多的工作来提高内存效率。请注意，使用JDK11时，垃圾收集器中的“幸存世代”数量要少得多，内存使用量和易失性也要少得多。权衡似乎是在速度上，但速度的差异小于内存使用的差异。
Stack Overflow用户
发布于 2019-04-01 22:29:38
TL;DR:需要更好的基准测试，更好的设置来控制不同版本之间的差异，等等。大多数基准测试问题都可以通过使用JMH来解决。当前的测试行为似乎可以通过可疑的基准测试方法和默认GC的变化来解释。
请考虑以下内容：
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，它使用了更方便的计时。然后，它测量相同的字节码块，而原始测试预热“初始码”，然后继续测量绝对冷的字节码块。
然后，即时编译将命中该方法，并且您希望再次输入该方法以运行优化的代码，否则您将运行中间的“栈上替换”代码--您可以通过调用test的外部迭代来完成此操作。最重要的是，您希望多次输入以捕获最优化的版本。
而且，由于测试分配了大量内存，因此您需要确定堆的大小。
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在同一字节码上，这里的...and是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这里的...and是带有匹配GC的11.0.2 (9+将缺省值更改为带有JEP 248的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，在每个微小的迭代中都有volatile商店，这花费了相当多的成本，可能会扭曲基准。
还有带有本地化字符串连接(JEP 280)的interaction、线程本地握手(JEP 312)和其他VM修复，但您可能只有在编译过去的target=8时才会看到这一点，这超出了本练习的范围。
Stack Overflow用户
发布于 2019-04-01 17:28:19
这现在已经被确认为JDK中的一个错误：
https://bugs.java.com/bugdatabase/view_bug.do?bug_id=JDK-8221733
https://stackoverflow.com/questions/55395958
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