程式碼1-1 新生代 Minor GC

部落格1

部落格2

一、新生代 Minor GC

public class One {
    private static final int _1MB = 1024 * 1024;

    /**
     * VM引數：-XX:+UseSerialGC -verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8
     */
    public static void testAllocation() {
        byte[] allocation1, allocation2, allocation3, allocation4;
        allocation1 
 
= new byte[2 * _1MB];
        allocation2 = new byte[2 * _1MB];
        allocation3 = new byte[2 * _1MB];
        allocation4 = new byte[4 * _1MB];  // 出現一次Minor GC
    }

    public static void main(String[] args) {
        testAllocation();
    }
}

[GC (Allocation Failure) [DefNew: 6304K->639K(9216K), 0.0028333 secs] 6304K->4735K(19456K), 0.0028708 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap
 def new generation   total 9216K, used 7104K
 
 [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  eden space 8192K,  78% used [0x00000000fec00000, 0x00000000ff250698, 0x00000000ff400000)
  from space 1024K,  62% used [0x00000000ff500000, 0x00000000ff59fd38, 0x00000000ff600000)
  to   space 1024K,   0% used [0x00000000ff400000, 0x00000000ff400000, 0x00000000ff500000)
 tenured generation   total 10240K, used 4096K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
   the space 10240K,  40% used [0x00000000ff600000, 0x00000000ffa00020, 0x00000000ffa00200, 0x0000000100000000)
 Metaspace       used 3486K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 386K, capacity 388K, committed 512K, reserved 1048576K

這裡有點問題感覺，不知道4MB的allocation4為什麼被分配到了老年區。jdk版本是1.8的。

二、大物件直接進入老年代

public class Two {
    private static final int _1MB = 1024 * 1024;
    /*
     * VM引數：  -verbose:gc
                -Xms20M
                -Xmx20M
                -Xmn10M
                -XX:+PrintGCDetails
                -XX:SurvivorRatio=8
                -XX:+UseSerialGC
                -XX:PretenureSizeThreshold=3145728
     */
    public static void testPretenureSizeThreshold(){
        byte[] allocation;
        allocation = new byte[4 * _1MB];
    }

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

Heap
 def new generation   total 9216K, used 2372K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  eden space 8192K,  28% used [0x00000000fec00000, 0x00000000fee51270, 0x00000000ff400000)
  from space 1024K,   0% used [0x00000000ff400000, 0x00000000ff400000, 0x00000000ff500000)
  to   space 1024K,   0% used [0x00000000ff500000, 0x00000000ff500000, 0x00000000ff600000)
 tenured generation   total 10240K, used 4096K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
   the space 10240K,  40% used [0x00000000ff600000, 0x00000000ffa00010, 0x00000000ffa00200, 0x0000000100000000)
 Metaspace       used 3466K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 383K, capacity 388K, committed 512K, reserved 1048576K