聊聊G1 GC的String Deduplication
序
本文主要研究一下G1 GC的String Deduplication
-XX:+UseStringDeduplication
前提是使用-XX:+UseG1GC
实例
实验代码
@Test
public void testG1StringDeduplication() throws InterruptedException {
List<String> data = IntStream.rangeClosed(1,10000000)
.mapToObj(i -> "number is " + ( i % 2 == 0 ? "odd" : "even"))
.collect(Collectors.toList());
System.gc();
long bytes = RamUsageEstimator.sizeOfAll(data);
System.out.println("string list size in MB:" + bytes*1.0/1024/1024);
System.out.println("used heap size in MB:" + ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed()*1.0/1024/1024);
System.out.println("used non heap size in MB:" + ManagementFactory.getMemoryMXBean().getNonHeapMemoryUsage().getUsed()*1.0/1024/1024);
}
关闭StringDeduplication
-XX:+UseG1GC -XX:-UseStringDeduplication
输出如下：
string list size in MB:586.8727111816406 used heap size in MB:831.772346496582 used non heap size in MB:6.448394775390625
- 整个jvm heap占用了约831MB，其中string list占用了约586MB
开启StringDeduplication
-XX:+UseG1GC -XX:+UseStringDeduplication
输出如下：
string list size in MB:296.83294677734375 used heap size in MB:645.0970153808594 used non heap size in MB:6.376350402832031
- 整个jvm heap占用了约645MB，其中string list占用了约296MB
小结
前提是使用-XX:+UseG1GC
doc
- Java 8 String deduplication vs. String.intern()
- JEP 192: String Deduplication in G1
- String Deduplication – A new feature in Java 8 Update 20
- Java 8 Update 20: String Deduplication
- Duplicate Strings: How to Get Rid of Them and Save Memory
- String deduplication feature (from Java 8 update 20)
- G1 GC: Reducing Memory Consumption by 20%
正文到此结束
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