java 如何打印垃圾回收花费的时间？

[eightqueen]

package com.v2ex;

public class Test {

    private static final int MB = 1024 * 1024;

    public static void main(String[] args) {
        byte[] bytes1, bytes2, bytes3, bytes4;
        bytes1 = new byte[2 * MB];
        bytes2 = new byte[2 * MB];
        bytes3 = new byte[2 * MB];
        bytes4 = new byte[4 * MB];
    }
}

我的执行命令是 java -verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 com/v2ex/Test
打印的数据是：

Heap
 PSYoungGen      total 9216K, used 6799K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 83% used [0x00000000ff600000,0x00000000ffca3f28,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 10240K, used 4096K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  object space 10240K, 40% used [0x00000000fec00000,0x00000000ff000010,0x00000000ff600000)
 PSPermGen       total 21504K, used 2751K [0x00000000f4600000, 0x00000000f5b00000, 0x00000000fec00000)
  object space 21504K, 12% used [0x00000000f4600000,0x00000000f48afc08,0x00000000f5b00000)

可是时间去哪了？我在网上查了很多资料，都会打印时间的。
我用的系统是 mint ， jdk 是 openjdk 1.7.0_79

[qinglangee]

-Xms20M 值太大，还没触发 GC ， 改成 -Xms8M 试试

[binbing]

System.gc()

[eightqueen]

@qinglangee @binbing bytes4 直接被分配到了老年代

[qinglangee]

我是说把 -Xms 值设小才会触发 GC