G1垃圾回收器的String Deduplication功能是否默认开启?
JEP 192:G1 中的字符串重复数据删除 在 Java 8 中实现更新 20 添加了新的字符串重复数据删除功能:
JEP 192: String Deduplication in G1 implemented in Java 8 Update 20 added the new String deduplication feature:
通过增强 G1 垃圾收集器减少 Java 堆实时数据集，以便自动连续删除重复的 String 实例.
Reduce the Java heap live-data set by enhancing the G1 garbage collector so that duplicate instances of String are automatically and continuously deduplicated.
JEP 页面提到命令行选项 UseStringDeduplication (bool) 允许启用或禁用去重功能.但是 JEP 页面并没有指示默认值.
The JEP page mentions that a command-line option UseStringDeduplication (bool) allows the dedup feature to be enabled or disabled. But the JEP page does not go so far as to indicate the default.
➠ 在与 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默认开启还是关闭?
➠ Is the dedup feature ON or OFF by default in the G1 garbage collector bundled with Java 8 and with Java 9?
➠ 是否有getter"方法在运行时验证当前设置?
➠ Is there a "getter" method to verify the current setting at runtime?
我不知道在哪里可以找到 JEP 页面之外的文档.
I do not know where to look for documentation beyond the JEP page.
至少在配备 HotSpot 的 Java 9 实现中，G1 垃圾收集器 是 默认启用.这个事实现在提示了这个问题.有关字符串实习和重复数据删除的更多信息，请参阅 Aleksey Shipilev 的此 2014-10 演示文稿29:00.
In at least the HotSpot-equipped implementations of Java 9, the G1 garbage collector is enabled by default. That fact prompted this Question now. For more info on String interning and deduplication, see this 2014-10 presentation by Aleksey Shipilev at 29:00.
推荐答案
字符串去重关闭默认
对于下面看到的 Java 8 和 Java 9 版本，UseStringDeduplication 默认为 false(禁用).
String deduplication off by default
For the versions of Java 8 and Java 9 seen below, UseStringDeduplication is false (disabled) by default.
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.
One way to verify the feature setting: list out all the final flags for JVM and then look for it.
构建 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)
构建 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)
另一种测试方法是使用
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);
}
}
}
在没有明确指定的情况下运行.
run without explicitly specifying it.
$ 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)
明确打开它运行.
$ 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
<子>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?否.统一日志记录杀死了这个特定的选项.
Note: this output is from build 1.8.0_131-b11. Looks like Java 9 has no option to print String de-duplication statistics. Potential bug ?
No. Unified logging killed this specific option.
$ 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.
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