是否默认启用G1垃圾收集器的String Deduplication功能？ [英] Is String Deduplication feature of the G1 garbage collector enabled by default?
问题描述
用Java实现的 JEP 192：G1中的字符串重复数据删除 8 Update 20添加了新的String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，以便重复实例字符串是自动连续重复数据删除。
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捆绑在一起的G1垃圾收集器中默认的重复数据删除功能是ON还是OFF使用Java 9？
➠是否有getter方法在运行时验证当前设置？
我不知道在JEP页面之外寻找文档的位置。
至少在 HotSpot - 配备了Java 9的实现， G1垃圾收集器 默认启用。这个事实现在促成了这个问题。有关字符串实习和重复数据删除的详细信息，请参阅 Aleksey Shipilev的2014-10演示文稿在29:00。
默认情况下字符串重复数据删除关闭
对于下面看到的Java 8和Java 9版本， UseStringDeduplication 默认为 false （禁用）。 / p>
验证功能设置的一种方法：列出JVM的所有最终标志然后寻找它。
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 version1.8.0_131
Java（TM）SE运行时环境（版本1.8.0_131-b11）
Java HotSpot（TM） 64位服务器VM（内置25.131-b11，混合模式）
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 version9
Java（TM）SE运行时环境（版本9 + 181）
Java HotSpot（TM）64位服务器VM（内置9 + 181，混合模式）
测试它的另一种方法是使用
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。
$ java -Xmx256m -XX：+ UseG1GC -XX：+ PrintStringDeduplicationStatistics jvm.StringDeDuplicationTester
线程中的异常main java.lang.OutOfMemoryError：Java堆空间
at jvm.StringDeDuplicationTester.main（StringDeDuplicationTester.java:12）
明确将其打开运行。
$ 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 ，空闲：1.7075173秒，阻止：0 / 0.0000000秒]
[检查：130568]
[跳过：0（0.0％）]
[已散列：130450（99.9％）]
[已知：0（0.0％）]
[新： 130568（100.0％）5116.7K]
[重复数据删除：120388（92.2％）4708.0K（92.0％）]
[年轻：0（0.0％）0.0B（0.0％）]
[旧：120388（100.0％）4708.0K（100.0％）]
[总执行：1 / 0.0246084秒，空闲：1 / 1.7075173秒，阻止：0 / 0.0000000秒]
[检查：130568 ]
[跳过：0（0.0％）]
[哈希：130450（99.9％）]
[已知：0（0.0％）]
[新：130568（100.0） ％）5116.7K]
[重复数据删除：120388（92.2％）4708.0K（92.0％）]
[年轻：0（0.0％）0.0B（0.0％）]
[旧： 120388（100.0％）4708.0K（100.0％）]
[表]
[内存使用：264.9K]
[大小：1024，最小值：1024，最大值：16777216]
[参赛作品：10962，载入：1070.5％，缓存：0，已添加：10962，删除：0]
[调整大小计数：0，缩小阈值：682（66 .7％），增长阈值：2048（200.0％）]
[Rehash计数：0，Rehash阈值：120，哈希种子：0x0]
[年龄阈值：3]
[队列]
[丢弃：0]
[GC并发字符串重复数据删除，删除0个条目，0.0000008秒]
...
输出截断
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此特定选项
$ java -Xmx256m -XX：+ UseG1GC -XX：+ PrintStringDeduplicationStatistics -version
无法识别的VM选项'PrintStringDeduplicationStatistics'
错误：无法创建Java虚拟机。
错误：发生致命异常。程序将会退出。
JEP 192: String Deduplication in G1 implemented in Java 8 Update 20 added the new String deduplication feature:
Reduce the Java heap live-data set by enhancing the G1 garbage collector so that duplicate instances of String are automatically and continuously deduplicated.
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.
➠ Is the dedup feature ON or OFF by default in the G1 garbage collector bundled with Java 8 and with Java 9?
➠ Is there a "getter" method to verify the current setting at runtime?
I do not know where to look for documentation beyond the JEP page.
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.
String deduplication off by default
For the versions of Java 8 and Java 9 seen below, UseStringDeduplication is false (disabled) by default.
One way to verify the feature setting: list out all the final flags for JVM and then look for it.
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)
build 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)
Another way to test it is with
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);
}
}
}
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)
Run with explicitly turning it ON.
$ 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
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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