实战JVM参数配置

本案例是想通过JVM参数配置了解到Java虚拟机内存默认参数的设置

代码如下

package com.bjsxt.base001;

import java.text.DecimalFormat;
import java.util.HashMap;
import java.util.Map;

public class Test01 {

    public static void main(String[] args) {

        //-Xms5m -Xmx20m -XX:+PrintGCDetails -XX:+UseSerialGC -XX:+PrintCommandLineFlags
        DecimalFormat df = new DecimalFormat("#,###");
        //查看GC信息
//      System.out.println("max memory:" + df.format(Runtime.getRuntime().maxMemory()));
//      System.out.println("free memory:" + df.format(Runtime.getRuntime().freeMemory()));
//      System.out.println("total memory:" + df.format(Runtime.getRuntime().totalMemory()));
//      
//      byte[] b1 = new byte[1*1024*1024];
//      System.out.println("分配了1M");
//      System.out.println("max memory:" + df.format(Runtime.getRuntime().maxMemory()));
//      System.out.println("free memory:" + df.format(Runtime.getRuntime().freeMemory()));
//      System.out.println("total memory:" + df.format(Runtime.getRuntime().totalMemory()));
        
        Map<Integer,Object> map=new HashMap();
        for (int i = 0; i < 10; i++) {
            byte[] b2 = new byte[1*1024*1024];
            System.out.println("max memory:" + df.format(Runtime.getRuntime().maxMemory()));
            System.out.println("free memory:" + df.format(Runtime.getRuntime().freeMemory()));
            System.out.println("total memory:" + df.format(Runtime.getRuntime().totalMemory()));
//          map.put(i, b2);
        }
        System.out.println("分配了4M");
        System.out.println("max memory:" + df.format(Runtime.getRuntime().maxMemory()));
        System.out.println("free memory:" + df.format(Runtime.getRuntime().freeMemory()));
        System.out.println("total memory:" + df.format(Runtime.getRuntime().totalMemory()));
        
    }
    
}
 

测试过程

启动时配置jvm参数

 -Xms30m -Xmx30m -XX:+PrintGCDetails -XX:+UseSerialGC -XX:+PrintCommandLineFlags

运行发现会进行一次[GC (Allocation Failure)，然后最后分配如下

Heap
 def new generation   total 9216K, used 5031K [0x00000000fe200000, 0x00000000fec00000, 0x00000000fec00000)
  eden space 8192K,  52% used [0x00000000fe200000, 0x00000000fe63c3a8, 0x00000000fea00000)
  from space 1024K,  67% used [0x00000000feb00000, 0x00000000febada58, 0x00000000fec00000)
  to   space 1024K,   0% used [0x00000000fea00000, 0x00000000fea00000, 0x00000000feb00000)
 tenured generation   total 20480K, used 0K [0x00000000fec00000, 0x0000000100000000, 0x0000000100000000)
   the space 20480K,   0% used [0x00000000fec00000, 0x00000000fec00000, 0x00000000fec00200, 0x0000000100000000)
 Metaspace       used 3387K, capacity 4560K, committed 4864K, reserved 1056768K
  class space    used 372K, capacity 388K, committed 512K, reserved 1048576K

很显然只会在年轻代进行处理
接下来我们修改jvm参数

-Xms3m -Xmx3m -XX:+PrintGCDetails -XX:+UseSerialGC -XX:+PrintCommandLineFlags

发现每次循环都会进行一次[GC (Allocation Failure)，然后最后分配如下

Heap
 def new generation   total 1216K, used 1070K [0x00000000ffc00000, 0x00000000ffd50000, 0x00000000ffd50000)
  eden space 1088K,  98% used [0x00000000ffc00000, 0x00000000ffd0ae00, 0x00000000ffd10000)
  from space 128K,   2% used [0x00000000ffd10000, 0x00000000ffd10d48, 0x00000000ffd30000)
  to   space 128K,   0% used [0x00000000ffd30000, 0x00000000ffd30000, 0x00000000ffd50000)
 tenured generation   total 2752K, used 691K [0x00000000ffd50000, 0x0000000100000000, 0x0000000100000000)
   the space 2752K,  25% used [0x00000000ffd50000, 0x00000000ffdfcd50, 0x00000000ffdfce00, 0x0000000100000000)
 Metaspace       used 3386K, capacity 4560K, committed 4864K, reserved 1056768K
  class space    used 372K, capacity 388K, committed 512K, reserved 1048576K

测试了几次，基本都是上面的结果

总结

虚拟机内存分配：年轻代1/3，老年代2/3.
Eden区8/10,from区1/10，to区1/10
但是每次第二次试验老年代基本都为691k很奇怪，记录一下