聊聊G1 GC的String Deduplication

本文主要研究一下G1 GC的String Deduplication

-XX:+UseStringDeduplication

  • jdk8u20给G1 GC带来了String Deduplication特性来将相同的字符串指向同一份数据,来减少重复字符串的内存开销
  • 该特性默认是关闭的,可以使用-XX:+UseStringDeduplication来开启(前提是使用-XX:+UseG1GC)
  • 具体的实现大致是JVM会记录char[]的weak reference及hash value,当找到一个hash code相同的String时,就会挨个char进行比较,当所有都match,那么其中一个String就会修改指针指向另一个String的char[],这样前者的char[]就可以被回收

实例

实验代码

    @Test
    public void testG1StringDeduplication() throws InterruptedException {
        List 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

  • jdk8u20给G1 GC带来了String Deduplication特性来将相同的字符串指向同一份数据,来减少重复字符串的内存开销
  • 该特性默认是关闭的,可以使用-XX:+UseStringDeduplication来开启(前提是使用-XX:+UseG1GC)
  • 在有大量重复string的前提下,使用G1 GC开启String Deduplication确实能够节省一定的内存,可以节约20%左右的内存,不过这个是理想的情况,因为普通应用里头的string重复的可能不多

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%

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