JVM七种垃圾收集器实战分析

Serial收集器

在运行时通过-Xms20M、-Xmx20M、-Xmn10M这三个参数限制了Java堆大小为20MB，不可扩展，其中10MB分配给新生代，10MB分配给老年代。

/**
 * VM参数： -verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SruvivorRation=8 -XX:+UseSerialGC
 * -XX:+UseSerialGC 指定新生代使用Serial收集器
 */
public class AllocationTest {
    private static final int _1MB =1024 * 1024;
    
    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];
    }
}

GC日志

[GC[DefNew: 6981K->477K(9216K), 0.0063639 secs] 6981K->6621K(19456K), 0.0064132 secs] [Times: user=0.01 sys=0.00, real=0.02 secs] 
Heap
 def new generation   total 9216K, used 4983K [0x00000000f9a00000, 0x00000000fa400000, 0x00000000fa400000)
  eden space 8192K,  55% used [0x00000000f9a00000, 0x00000000f9e667b0, 0x00000000fa200000)
  from space 1024K,  46% used [0x00000000fa300000, 0x00000000fa377768, 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 2551K [0x00000000fae00000, 0x00000000fc2c0000, 0x0000000100000000)
   the space 21248K,  12% used [0x00000000fae00000, 0x00000000fb07dea8, 0x00000000fb07e000, 0x00000000fc2c0000)
No shared spaces configured.

从GC日志中看出，执行中分配allocation4对象的语句时会发生一次Minor GC，这次GC的结果是新生代使用的内存容量由GC前的6981K变为GC后的477K，而总内存占用量几乎没有减少（因为allocation1、allocation2、allocation3三个对象都是存活的，虚拟机几乎没有找到可以回收的对象）。发生GC的原因是给allocation4分配内存时，发现Eden已经被占用了6MB，剩余空间已不足以分配allocation4所需的4MB内存，因此发生了Minor GC。GC期间虚拟机又发现已有的3个2MB大小的对象全部无法放进Survivor空间（1MB），所以通过分配担保机制提前转移到老年代去。
这次GC结束后，4MB的allocation4对象顺利分配在Eden中，因此程序执行完的结果是Eden占用4MB（allocation4），Survivor空闲，老年代被占用6MB（allocation1、allocation2、allocation3）。通过GC日志可以证实。

ParNew收集器

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

GC日志

[GC[ParNew: 6981K->519K(9216K), 0.0833662 secs] 6981K->6663K(19456K), 0.0834041 secs] [Times: user=0.00 sys=0.00, real=0.08 secs] 
Heap
 par new generation   total 9216K, used 5025K [0x00000000f9a00000, 0x00000000fa400000, 0x00000000fa400000)
  eden space 8192K,  55% used [0x00000000f9a00000, 0x00000000f9e667b0, 0x00000000fa200000)
  from space 1024K,  50% used [0x00000000fa300000, 0x00000000fa381c70, 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 2551K [0x00000000fae00000, 0x00000000fc2c0000, 0x0000000100000000)
   the space 21248K,  12% used [0x00000000fae00000, 0x00000000fb07dea8, 0x00000000fb07e000, 0x00000000fc2c0000)
No shared spaces configured.

Parallel Scavenge收集器

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

GC日志

Heap
 PSYoungGen      total 9216K, used 7145K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 87% used [0x00000000ff600000,0x00000000ffcfa498,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 10240K, used 4096K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  object space 10240K, 40% used [0x00000000fec00000,0x00000000ff000010,0x00000000ff600000)
 PSPermGen       total 21504K, used 2548K [0x00000000f9a00000, 0x00000000faf00000, 0x00000000fec00000)
  object space 21504K, 11% used [0x00000000f9a00000,0x00000000f9c7d300,0x00000000faf00000)

从GC日志中看出，这次执行到allocation4 = new byte[4 * _1MB];时没有发生GC，Eden区使用时87%内存，稍大于6MB，也就是说allocation1、allocation2、allocation3对象是存活在Eden区的，而剩下的空间不足以分配给4MB的allocation4，Survivor区仅有1MB也不足以分配给它，此时Parallel Scavenge收集器直接将allocation4放进老年代中。从GC日志中的ParOldGen使用了4096K刚好证明了这一点。

G1收集器

VM参数：-verbose:gc -Xms20M -Xmx20M -Xmn10M  -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+UseG1GC

GC日志

GC pause (young) (initial-mark), 0.0023456 secs]
   [Parallel Time: 1.7 ms, GC Workers: 8]
      [GC Worker Start (ms): Min: 254.8, Avg: 254.8, Max: 254.9, Diff: 0.1]
      [Ext Root Scanning (ms): Min: 0.7, Avg: 0.8, Max: 1.4, Diff: 0.7, Sum: 6.3]
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
         [Processed Buffers: Min: 0, Avg: 1.0, Max: 8, Diff: 8, Sum: 8]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 0.0, Avg: 0.6, Max: 0.7, Diff: 0.7, Sum: 4.6]
      [Termination (ms): Min: 0.0, Avg: 0.1, Max: 0.1, Diff: 0.1, Sum: 0.6]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.1, Diff: 0.1, Sum: 0.6]
      [GC Worker Total (ms): Min: 1.5, Avg: 1.5, Max: 1.6, Diff: 0.1, Sum: 12.2]
      [GC Worker End (ms): Min: 256.3, Avg: 256.4, Max: 256.4, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Clear CT: 0.4 ms]
   [Other: 0.3 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.2 ms]
      [Ref Enq: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 1024.0K(10.0M)->0.0B(9216.0K) Survivors: 0.0B->1024.0K Heap: 6836.8K(20.0M)->6768.1K(20.0M)]
 [Times: user=0.00 sys=0.00, real=0.01 secs] 
[GC concurrent-root-region-scan-start]
[GC concurrent-root-region-scan-end, 0.0007035 secs]
[GC concurrent-mark-start]
Heap
 garbage-first heap   total 20480K, used 10864K [0x00000000f9a00000, 0x00000000fae00000, 0x00000000fae00000)
  region size 1024K, 2 young (2048K), 1 survivors (1024K)
 compacting perm gen  total 20480K, used 2551K [0x00000000fae00000, 0x00000000fc200000, 0x0000000100000000)
   the space 20480K,  12% used [0x00000000fae00000, 0x00000000fb07dea8, 0x00000000fb07e000, 0x00000000fc200000)
No shared spaces configured.