jvm eden space 为什么会为 0K

设置新生代大小为 1M，-XX:SurvivorRatio=2, 结果 eden space 为 0K，from space 为 512K，to space 为 512K。是因为 from 和 to 有最小值限制吗？不太明白，求各位大佬指教~

vm 参数: -Xmx20m -Xms20m -Xmn1m -XX:SurvivorRatio=2 -XX:+PrintGCDetails

版本：jdk1.7.0_79

代码和结果如下:

public class NewSizeDemo {
    public static void main(String[] args){
        byte[] b = null;

        for (int i=0; i<10; i++){
            b = new byte[1*1024*1024];
        }
    }
}

Heap
 PSYoungGen      total 512K, used 0K [0x00000007fff00000, 0x0000000800000000, 0x0000000800000000)
  eden space 0K, -2147483648% used [0x00000007fff00000,0x00000007fff00000,0x00000007fff00000)
  from space 512K, 0% used [0x00000007fff80000,0x00000007fff80000,0x0000000800000000)
  to   space 512K, 0% used [0x00000007fff00000,0x00000007fff00000,0x00000007fff80000)
 ParOldGen       total 19456K, used 11573K [0x00000007fec00000, 0x00000007fff00000, 0x00000007fff00000)
  object space 19456K, 59% used [0x00000007fec00000,0x00000007ff74d5a0,0x00000007fff00000)
 PSPermGen       total 21504K, used 2996K [0x00000007f9a00000, 0x00000007faf00000, 0x00000007fec00000)
  object space 21504K, 13% used [0x00000007f9a00000,0x00000007f9ced030,0x00000007faf00000)