使用Serial收集器发生了GC但是使用Parallel Scavenge收集器却没有

具体如下，请问 是什么原因？

package test1;

class TestGc {

private static final int _1MB = 1024 * 1024;

/**
-verbose:gc
-Xms20m
-Xmx20m
-Xmn10m
-XX:+UseSerialGC
-XX:+PrintGCDetails
-XX:SurvivorRatio=8

* @param args
*/
public static void main(String[] args) {
byte[] allocation1, allocation2, allocation3, allocation4;
allocation1  = new byte[2 * _1MB];
allocation2  = new byte[2 * _1MB];
allocation3  = new byte[2 * _1MB];
allocation4  = new byte[4 * _1MB];
}

}



运行结果：
[GC [DefNew: 6487K->160K(9216K), 0.0155080 secs] 6487K->6305K(19456K), 0.0155918 secs] [Times: user=0.00 sys=0.02, real=0.02 secs]
Heap
def new generation   total 9216K, used 4584K [0x00000000f9a00000, 0x00000000fa400000, 0x00000000fa400000)
  eden space 8192K,  54% used [0x00000000f9a00000, 0x00000000f9e51f98, 0x00000000fa200000)
  from space 1024K,  15% used [0x00000000fa300000, 0x00000000fa3283d0, 0x00000000fa400000)
  to   space 1024K,   0% used [0x00000000fa200000, 0x00000000fa200000, 0x00000000fa300000)
tenured generation   total 10240K, used 6144K [0x00000000fa400000, 0x00000000fae00000, 0x00000000fae00000)
   the space 10240K,  60% used [0x00000000fa400000, 0x00000000faa00030, 0x00000000faa00200, 0x00000000fae00000)
compacting perm gen  total 21248K, used 2981K [0x00000000fae00000, 0x00000000fc2c0000, 0x0000000100000000)
   the space 21248K,  14% used [0x00000000fae00000, 0x00000000fb0e96a0, 0x00000000fb0e9800, 0x00000000fc2c0000)
No shared spaces configured.

去掉-XX:+UseSerialGC后（默认采用的是Parallel Scavenge收集器）没有发生GC
Heap
PSYoungGen      total 9216K, used 6651K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 81% used [0x00000000ff600000,0x00000000ffc7eec8,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
PSOldGen        total 10240K, used 4096K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  object space 10240K, 40% used [0x00000000fec00000,0x00000000ff000010,0x00000000ff600000)
PSPermGen       total 21248K, used 2972K [0x00000000f9a00000, 0x00000000faec0000, 0x00000000fec00000)
  object space 21248K, 13% used [0x00000000f9a00000,0x00000000f9ce71e0,0x00000000faec0000)

一个人也没有么？