jvm eden space 为什么会为 0K

[sun1992]

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)

[sun1992]

详细参数
-XX:InitialHeapSize=20971520 -XX:MaxHeapSize=20971520 -XX:MaxNewSize=1048576 -XX:NewSize=1048576 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:SurvivorRatio=2 -XX:+UseCompressedOops -XX:+UseParallelGC

[sun1992]

这个是 jdk7u40 版本左右开始的 bug，jdk8 中已经修复

http://www.oracle.com/technetwork/java/javase/7u40-relnotes-2004172.html
jdk7u40 的 Update Release Notes 中有这个问题说明

````
Area: hotspot/gc
Synopsis: New minimum young generation size is not properly checked by the JVM.

In JDK 7u40, the minimum size of the young generation for the parallel garbage collector was increased from 192 KB to 768 KB in a 32-bit JVM, and to 1536 KB in a 64-bit JVM. This new minimum size is not properly checked by the JVM. If a young generation size that is smaller than the new minimum is specified on the command line, it can result in either a crash or degraded performance.

The young generation size is set by the options -XX:NewSize= and -XX:MaxNewSize=, or by the option -Xmn (the latter option is equivalent to setting both NewSize and MaxNewSize to ). If the above options are not used, then the young generation size is computed as a fraction of the maximum heap size.

Workaround: Use a young generation size that is at least 768 KB (for 32-bit JVM) or 1536 KB (for 64-bit JVM).
````