jvm垃圾收集的问题

Question

VM参数：-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:-HandlePromotionFailure

这里是主要代码：
allocation1 = new byte[2 * _1MB]; //分配2MB，下面一样
allocation2 = new byte[2 * _1MB];

allocation3 = new byte[2 * _1MB];

allocation1 = null;

allocation4 = new byte[2 * _1MB]; //到这里就有YGC了，新生代不是有9MB可以用么？

[GC [DefNew: 6471K->135K(9216K), 0.0042747 secs] 6471K->4231K(19456K), 0.0043168 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

Answer 1

刚开始就已经用了2MB了
[code="java"]
public static final int _1MB = 1024 * 1024;

@SuppressWarnings("unused")
public static void main(String ars[]) throws InterruptedException {
    showMemory();
    byte[] a = new byte[2 * _1MB];
    showMemory();
    byte[] b = new byte[2 * _1MB];
    showMemory();
    byte[] c = new byte[2 * _1MB];
    showMemory();
    byte[] d = new byte[2 * _1MB];
    showMemory();
    a = null;
    showMemory();
    byte[] e = new byte[2 * _1MB];
    showMemory();
}

private static void showMemory() throws InterruptedException {
    long total;
    long free;
    total = Runtime.getRuntime().totalMemory();
    free = Runtime.getRuntime().freeMemory();
    System.out.println("total:" + total + "\tfree:" + free + "\tused:"
            + (total - free));
    Thread.sleep(1000);
}

[/code]