JDK1.8 大对象直接进入老年代(参考深入理解JAVA虚拟机)

jvm参数-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:PretenureSizeThreshold=3145728代码package com.memory.gc;public class PretenureSizeThreshold {privat...

ciqingloveless

10716人浏览 · 2018-08-20 20:27:47

ciqingloveless · 2018-08-20 20:27:47 发布

jvm参数

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

代码

package com.memory.gc;

public class PretenureSizeThreshold {

    private static final int _1MB = 1024 * 1024;

    public static void testPretenureSizeThreshold(){
        byte[] allocation;
        allocation = new byte[5 * _1MB];
    }

    public static void main(String[] args) {
        PretenureSizeThreshold.testPretenureSizeThreshold();
    }
}

结果

Heap
 PSYoungGen      total 9216K, used 7612K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 92% used [0x00000000ff600000,0x00000000ffd6f080,0x00000000ffe00000)
  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)
 Metaspace       used 3472K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 381K, capacity 388K, committed 512K, reserved 1048576K

明显看出与原书不同，原因是PretenureSizeThreshold参数只对Serial和ParNew两款收集器有效，Parallel Scavenge收集器不认识这个参数，Parallel Scavenge收集器一般并不需要设置。如果遇到必须使用此参数的场合，可以考虑ParNew加CMS的收集器组合。
若想实现书中的效果修改jvm参数

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

代码

package com.memory.gc;

public class PretenureSizeThreshold {

    private static final int _1MB = 1024 * 1024;

    public static void testPretenureSizeThreshold(){
        byte[] allocation;
        allocation = new byte[5 * _1MB];
    }

    public static void main(String[] args) {
        PretenureSizeThreshold.testPretenureSizeThreshold();
    }
}

结果

Heap
 par new generation   total 9216K, used 2492K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  eden space 8192K,  30% used [0x00000000fec00000, 0x00000000fee6f070, 0x00000000ff400000)
  from space 1024K,   0% used [0x00000000ff400000, 0x00000000ff400000, 0x00000000ff500000)
  to   space 1024K,   0% used [0x00000000ff500000, 0x00000000ff500000, 0x00000000ff600000)
 tenured generation   total 10240K, used 5120K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
   the space 10240K,  50% used [0x00000000ff600000, 0x00000000ffb00010, 0x00000000ffb00200, 0x0000000100000000)
 Metaspace       used 3407K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 374K, capacity 388K, committed 512K, reserved 1048576K