是否默认启用G1垃圾收集器的String Deduplication功能？

Question

JEP 192:在Java 8 Update 20中实现的 G1中的字符串重复数据删除添加了新的字符串重复数据删除功能:

通过增强G1垃圾收集器来减少Java堆实时数据集,以便自动且连续地对重复的String实例进行重复数据删除.

JEP页面提到命令行选项UseStringDeduplication (bool)允许启用或禁用重复数据删除功能.但是JEP页面并没有指出默认值.

➠与Java 8和Java 9捆绑在一起的G1垃圾收集器中,重复数据删除功能是否默认为ON或OFF？

➠是否有"getter"方法在运行时验证当前设置？

我不知道在JEP页面之外查找文档的位置.

至少在热点 -equipped的Java 9的实施中,G1垃圾收集器是默认启用.这个事实现在促成了这个问题.有关String interning和重复数据删除的更多信息,请参阅Aleksey Shipilev在29:00的2014-10演示文稿.

Answer 1

字符串重复数据删除默认情况下关闭

对于下面看到的Java 8和Java 9版本,默认情况下UseStringDeduplication是false(禁用).

验证功能设置的一种方法:列出JVM的所有最终标志,然后查找它.

建立1.8.0_131-b11

    $ java -XX:+UseG1GC  -XX:+UnlockDiagnosticVMOptions -XX:+PrintFlagsFinal -version | grep -i 'duplicat'
     bool PrintStringDeduplicationStatistics        = false                               {product}
    uintx StringDeduplicationAgeThreshold           = 3                                   {product}
     bool StringDeduplicationRehashALot             = false                               {diagnostic}
     bool StringDeduplicationResizeALot             = false                               {diagnostic}
     bool UseStringDeduplication                    = false                               {product}
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)

建立9 + 18

    $ java -XX:+UseG1GC  -XX:+UnlockDiagnosticVMOptions -XX:+PrintFlagsFinal -version | grep -i 'duplicat'
    uintx StringDeduplicationAgeThreshold          = 3                                        {product} {default}
     bool StringDeduplicationRehashALot            = false                                 {diagnostic} {default}
     bool StringDeduplicationResizeALot            = false                                 {diagnostic} {default}
     bool UseStringDeduplication                   = false                                    {product} {default}
java version "9"
Java(TM) SE Runtime Environment (build 9+181)
Java HotSpot(TM) 64-Bit Server VM (build 9+181, mixed mode)

测试它的另一种方法是

package jvm;

import java.util.ArrayList;
import java.util.List;

public class StringDeDuplicationTester {

    public static void main(String[] args) throws Exception {
        List<String> strings = new ArrayList<>();
        while (true) {
            for (int i = 0; i < 100_00; i++) {
                strings.add(new String("String " + i));
            }
            Thread.sleep(100);
        }
    }
}

运行时没有明确指定它.

$ java  -Xmx256m -XX:+UseG1GC -XX:+PrintStringDeduplicationStatistics jvm.StringDeDuplicationTester
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at jvm.StringDeDuplicationTester.main(StringDeDuplicationTester.java:12)

运行时明确打开它.

$ java  -Xmx256m -XX:+UseG1GC -XX:+UseStringDeduplication -XX:+PrintStringDeduplicationStatistics jvm.StringDeDuplicationTester
[GC concurrent-string-deduplication, 5116.7K->408.7K(4708.0K), avg 92.0%, 0.0246084 secs]
   [Last Exec: 0.0246084 secs, Idle: 1.7075173 secs, Blocked: 0/0.0000000 secs]
      [Inspected:          130568]
         [Skipped:              0(  0.0%)]
         [Hashed:          130450( 99.9%)]
         [Known:                0(  0.0%)]
         [New:             130568(100.0%)   5116.7K]
      [Deduplicated:       120388( 92.2%)   4708.0K( 92.0%)]
         [Young:                0(  0.0%)      0.0B(  0.0%)]
         [Old:             120388(100.0%)   4708.0K(100.0%)]
   [Total Exec: 1/0.0246084 secs, Idle: 1/1.7075173 secs, Blocked: 0/0.0000000 secs]
      [Inspected:          130568]
         [Skipped:              0(  0.0%)]
         [Hashed:          130450( 99.9%)]
         [Known:                0(  0.0%)]
         [New:             130568(100.0%)   5116.7K]
      [Deduplicated:       120388( 92.2%)   4708.0K( 92.0%)]
         [Young:                0(  0.0%)      0.0B(  0.0%)]
         [Old:             120388(100.0%)   4708.0K(100.0%)]
   [Table]
      [Memory Usage: 264.9K]
      [Size: 1024, Min: 1024, Max: 16777216]
      [Entries: 10962, Load: 1070.5%, Cached: 0, Added: 10962, Removed: 0]
      [Resize Count: 0, Shrink Threshold: 682(66.7%), Grow Threshold: 2048(200.0%)]
      [Rehash Count: 0, Rehash Threshold: 120, Hash Seed: 0x0]
      [Age Threshold: 3]
   [Queue]
      [Dropped: 0]
[GC concurrent-string-deduplication, deleted 0 entries, 0.0000008 secs]
...
output truncated

_{注意:此输出来自build 1.8.0_131-b11.看起来Java 9没有打印字符串重复数据删除统计信息的选项.潜在的错误？ 不.统一日志记录杀死了此特定选项}

$ java  -Xmx256m -XX:+UseG1GC -XX:+PrintStringDeduplicationStatistics -version
Unrecognized VM option 'PrintStringDeduplicationStatistics'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

Answer 2

虽然Jigar已经精确地提供了了解JVM标志和统计数据的方法,但是仍然链接到解决这部分问题的一些有用文档:

我不知道在JEP页面之外查找文档的位置.

• 该Java9发布说明描述的实现JEP 248:让G1默认的垃圾收集与线-

在JDK 9中,当未明确指定垃圾收集器时,默认垃圾收集器为G1.

• 在Java的工具,里面详细介绍了标志的使用

  -XX:+UseStringDeduplication
  

启用字符串重复数据删除.默认情况下,禁用此选项.要使用此选项,必须启用垃圾优先(G1)垃圾收集器.

字符串重复数据删除通过利用许多String对象相同的事实来减少Java堆上的String对象的内存占用.而不是每个String对象指向它自己的字符数组,相同的String对象可以指向并共享相同的字符数组.

如果还有解决那里的公开问题

Java 9没有打印字符串重复数据删除统计信息的选项.

使用JEP 158: Java9中的统一JVM日志记录实现,垃圾收集器标记被标记为遗留,并且跟踪它们的替代方法是使用-Xlog特征.此处列出了将GC Logging Flags转换为Xlog的替换的详细列表.其中一个建议更换

PrintStringDeduplicationStatistics  =>   -Xlog:stringdedup*=debug