G1 垃圾收集器是否默认启用了字符串去重功能？
Is String Deduplication feature of the G1 garbage collector enabled by default?
JEP 192: String Deduplication in G1 在 Java 中实现 8 Update 20 添加了新的字符串重复数据删除功能：
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) 允许启用或禁用重复数据删除功能。但是 JEP 页面并没有显示默认值。
➠ 与 Java 8 和 Java 9 捆绑在一起的 G1 垃圾收集器中的重复数据删除功能是默认打开还是关闭？
➠ 是否有“getter”方法在运行时验证当前设置？
除了 JEP 页面之外，我不知道去哪里寻找文档。
至少在 29:00 的 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。
字符串重复数据删除默认关闭
对于下面看到的 Java 8 和 Java 9 的版本，UseStringDeduplication 默认为 false（禁用）。
验证功能设置的一种方法：然后查找。
build 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
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 没有明确指定。
$ 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 明确将其打开。
$ 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？
没有。统一日志记录终止了这个特定选项。
$ 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.
虽然 已经精确地提供了了解 JVM 标志和统计信息的方法，但 link 一些解决这部分问题的有用文档：
I do not know where to look for documentation beyond the JEP page.
- Java9 Release Note describes an implementation of JEP 248:Make G1 the Default Garbage Collector 与行 -
In JDK 9, the default garbage collector is G1 when a garbage collector
is not explicitly specified.
java tool里面详细介绍了flag的用法
-XX:+UseStringDeduplication
Enables string deduplication. By default, this option is disabled. To
use this option, you must enable the garbage-first (G1) garbage
collector.
String deduplication reduces the memory footprint of String objects on
the Java heap by taking advantage of the fact that many String objects
are identical. Instead of each String object pointing to its own
character array, identical String objects can point to and share the
same character array.
如果
也解决那里的开放问题
Java 9 has no option to print String de-duplication statistics.
与JEP 158:Unified JVM Logging implementation in Java9, the garbage collector flags are marked as legacy and alternate way of tracing them is using -Xlog feature. A detailed list of the replacement for converting GC Logging Flags to Xlog is listed here。其中之一建议更换
PrintStringDeduplicationStatistics => -Xlog:stringdedup*=debug
JEP 192: String Deduplication in G1 在 Java 中实现 8 Update 20 添加了新的字符串重复数据删除功能：
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) 允许启用或禁用重复数据删除功能。但是 JEP 页面并没有显示默认值。
➠ 与 Java 8 和 Java 9 捆绑在一起的 G1 垃圾收集器中的重复数据删除功能是默认打开还是关闭？
➠ 是否有“getter”方法在运行时验证当前设置？
除了 JEP 页面之外，我不知道去哪里寻找文档。
至少在 29:00 的 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。
字符串重复数据删除默认关闭
对于下面看到的 Java 8 和 Java 9 的版本，UseStringDeduplication 默认为 false（禁用）。
验证功能设置的一种方法：
build 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
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 没有明确指定。
$ 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 明确将其打开。
$ 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？
没有。统一日志记录终止了这个特定选项。
$ 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.
虽然
I do not know where to look for documentation beyond the JEP page.
- Java9 Release Note describes an implementation of JEP 248:Make G1 the Default Garbage Collector 与行 -
In JDK 9, the default garbage collector is G1 when a garbage collector is not explicitly specified.
java tool里面详细介绍了flag的用法
-XX:+UseStringDeduplication
Enables string deduplication. By default, this option is disabled. To use this option, you must enable the garbage-first (G1) garbage collector.
String deduplication reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that many String objects are identical. Instead of each String object pointing to its own character array, identical String objects can point to and share the same character array.
如果
也解决那里的开放问题Java 9 has no option to print String de-duplication statistics.
与JEP 158:Unified JVM Logging implementation in Java9, the garbage collector flags are marked as legacy and alternate way of tracing them is using -Xlog feature. A detailed list of the replacement for converting GC Logging Flags to Xlog is listed here。其中之一建议更换
PrintStringDeduplicationStatistics => -Xlog:stringdedup*=debug