GC时旧生代空间大小的疑问.

-Xms18m
-Xmx18m
-Xmn16m
-XX:+UseSerialGC -XX:-UseAdaptiveSizePolicy
-XX:SurvivorRatio=6
-verbose:gc 
-XX:+PrintGC
-XX:+PrintGCDetails

使用上面的JVM参数运行下面的代码时,GC日志中旧生代总容量变成4096K4M),与设置的2M(18-16)不一样

[Tenured: 3072K->3072K(4096K), 0.0143395 secs]

注意这里,括号里面4096K表示旧生代所有空间为4M,而不是设置的2M.

如果设置 -Xms24m -Xmx24m,则由正常了(输出[Tenured: 6144K->6144K(8192K), 0.0129542 secs]).

这个到底嘛原因呢?

输出如下:

Minor GC should happen

[GC [DefNew (promotion failed): 11509K->11659K(14336K), 0.0123390 secs][Tenured: 3072K->3072K(4096K), 0.0143395 secs] 11509K->11413K(18432K), [Perm : 2075K->2075K(12288K)], 0.0333981 secs] [Times: user=0.01 sys=0.00, real=0.03 secs] 

[Full GC [Tenured: 3072K->3072K(4096K), 0.0097443 secs] 13461K->13461K(18432K), [Perm : 2081K->2081K(12288K)], 0.0097931 secs] [Times: user=0.02 sys=0.00, real=0.02 secs] 

[Full GC [Tenured: 3072K->3072K(4096K), 0.0177553 secs] 13461K->13448K(18432K), [Perm : 2081K->2075K(12288K)], 0.0178095 secs] [Times: user=0.01 sys=0.00, real=0.02 secs] 

java.lang.OutOfMemoryError: Java heap space

Dumping heap to D:/D/myeclipse_dump.bin ...

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

  at jvm.MultiMajorGCLog$MemoryObject.<init>(MultiMajorGCLog.java:48)

  at jvm.MultiMajorGCLog.testMain(MultiMajorGCLog.java:34)

  at jvm.MultiMajorGCLog.main(MultiMajorGCLog.java:39)

Unable to create D:/D/myeclipse_dump.bin: File exists

Heap

 def new generation   total 14336K, used 10748K [0x02a00000, 0x03a00000, 0x03a00000)

  eden space 12288K,  87% used [0x02a00000, 0x0347f088, 0x03600000)

  from space 2048K,   0% used [0x03800000, 0x03800000, 0x03a00000)

  to   space 2048K,   0% used [0x03600000, 0x03600000, 0x03800000)

 tenured generation   total 4096K, used 3072K [0x03a00000, 0x03e00000, 0x03e00000)

   the space 4096K,  75% used [0x03a00000, 0x03d00010, 0x03d00200, 0x03e00000)

 compacting perm gen  total 12288K, used 2082K [0x03e00000, 0x04a00000, 0x07e00000)

   the space 12288K,  16% used [0x03e00000, 0x04008840, 0x04008a00, 0x04a00000)

No shared spaces configured.

package jvm;

public class MultiMajorGCLog
{

  public static void testMain()
  {
    try
    {
      Thread.sleep(40);
    }
    catch (Exception e)
    {
      e.printStackTrace();
    }

    MemoryObject object1 = new MemoryObject(1024 * 1024 * 3);
    MemoryObject object2 = new MemoryObject(1024 * 1024 * 3);
    MemoryObject object3 = new MemoryObject(1024 * 1024 * 3);
    MemoryObject object4 = new MemoryObject(1024 * 1024 * 2);
    System.out.println("Minor GC should happen");

    MemoryObject object5 = new MemoryObject(1024 * 1024 * 2);
    MemoryObject object6 = new MemoryObject(1024 * 1024 * 2);
  }

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

  private static class MemoryObject
  {
    private byte[] bytes;

    public MemoryObject(int objectSize)
    {
      this.bytes = new byte[objectSize];
    }
  }
}