JDK1.8 大对象直接进入老年代(参考深入理解JAVA虚拟机)

 2019-12-22 11:21  阅读(3967)
文章分类:JVM

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
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