PretenureSizeThreshol参数不识别，1.7默认垃圾收集器

最新推荐文章于 2025-05-27 16:41:43 发布

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本文探讨了Java中PretenureSizeThreshold参数的作用机制，该参数决定对象何时直接分配到老年代。通过对比不同垃圾收集器（如ParallelScavenge与Serial收集器）的行为，展示了如何利用此参数优化内存分配。

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PretenureSizeThreshol 设置对象超过多大时内存直接分配到老年代

1.7jdk默认垃圾收集器为 Parallel Scavenge（新生代）+Parallel Old（老年代），PretenureSizeThreshol在jdk1.7未指定垃圾收集器时未生效

    private static final int _1MB = 1024 * 1024;

    /**
     * VM参数：-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+PrintCommandLineFlags -XX:PretenureSizeThreshold=3145728-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+PrintCommandLineFlags -XX:PretenureSizeThreshold=3145728 -XX:+UseSerialGC

*/ public static void testAllocation() { byte[] allocation1 = new byte[4 * _1MB]; }

gc日志显示数组并没有直接分配到老年代

-XX:InitialHeapSize=20971520 -XX:MaxHeapSize=20971520 -XX:MaxNewSize=10485760 -XX:NewSize=10485760 -XX:PretenureSizeThreshold=3145728 -XX:+PrintCommandLineFlags -XX:+PrintGC -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+UseCompressedOops -XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC 
Heap
 PSYoungGen      total 9216K, used 5097K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 62% used [0x00000000ff600000,0x00000000ffafa458,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 10240K, used 0K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  object space 10240K, 0% used [0x00000000fec00000,0x00000000fec00000,0x00000000ff600000)
 PSPermGen       total 21248K, used 2691K [0x00000000f9a00000, 0x00000000faec0000, 0x00000000fec00000)
  object space 21248K, 12% used [0x00000000f9a00000,0x00000000f9ca0c80,0x00000000faec0000)

指定Serial收集器

-verbose:gc-Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8-XX:+PrintCommandLineFlags -XX:PretenureSizeThreshold=3145728 -XX:+UseSerialGC

gc日志显示对象直接分配到老年代

-XX:InitialHeapSize=20971520 -XX:MaxHeapSize=20971520 -XX:MaxNewSize=10485760 -XX:NewSize=10485760 -XX:PretenureSizeThreshold=3145728 -XX:+PrintCommandLineFlags -XX:+PrintGC -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+UseCompressedOops -XX:-UseLargePagesIndividualAllocation -XX:+UseSerialGC 
Heap
 def new generation   total 9216K, used 1001K [0x00000000f9a00000, 0x00000000fa400000, 0x00000000fa400000)
  eden space 8192K,  12% used [0x00000000f9a00000, 0x00000000f9afa448, 0x00000000fa200000)
  from space 1024K,   0% used [0x00000000fa200000, 0x00000000fa200000, 0x00000000fa300000)
  to   space 1024K,   0% used [0x00000000fa300000, 0x00000000fa300000, 0x00000000fa400000)
 tenured generation   total 10240K, used 4096K [0x00000000fa400000, 0x00000000fae00000, 0x00000000fae00000)
   the space 10240K,  40% used [0x00000000fa400000, 0x00000000fa800010, 0x00000000fa800200, 0x00000000fae00000)
 compacting perm gen  total 21248K, used 2691K [0x00000000fae00000, 0x00000000fc2c0000, 0x0000000100000000)
   the space 21248K,  12% used [0x00000000fae00000, 0x00000000fb0a0c80, 0x00000000fb0a0e00, 0x00000000fc2c0000)
No shared spaces configured.