理解G1垃圾回收日志

本篇文章主要介绍在-XX:+PrintGCDetails选项的情况下G1 GC log打印的信息。（注：本文假设读者对G1算法的基本原理已经有所了解）

下面是一段G1垃圾收集器相关的log信息

0.522: [GC pause (young), 0.15877971 secs]
   [Parallel Time: 157.1 ms]
      [GC Worker Start (ms):  522.1  522.2  522.2  522.2
       Avg: 522.2, Min: 522.1, Max: 522.2, Diff:   0.1]
      [Ext Root Scanning (ms):  1.6  1.5  1.6  1.9
       Avg:   1.7, Min:   1.5, Max:   1.9, Diff:   0.4]
      [Update RS (ms):  38.7  38.8  50.6  37.3
       Avg:  41.3, Min:  37.3, Max:  50.6, Diff:  13.3]
         [Processed Buffers : 2 2 3 2
          Sum: 9, Avg: 2, Min: 2, Max: 3, Diff: 1]
      [Scan RS (ms):  9.9  9.7  0.0  9.7
       Avg:   7.3, Min:   0.0, Max:   9.9, Diff:   9.9]
      [Object Copy (ms):  106.7  106.8  104.6  107.9
       Avg: 106.5, Min: 104.6, Max: 107.9, Diff:   3.3]
      [Termination (ms):  0.0  0.0  0.0  0.0
       Avg:   0.0, Min:   0.0, Max:   0.0, Diff:   0.0]
         [Termination Attempts : 1 4 4 6
          Sum: 15, Avg: 3, Min: 1, Max: 6, Diff: 5]
      [GC Worker End (ms):  679.1  679.1  679.1  679.1
       Avg: 679.1, Min: 679.1, Max: 679.1, Diff:   0.1]
      [GC Worker (ms):  156.9  157.0  156.9  156.9
       Avg: 156.9, Min: 156.9, Max: 157.0, Diff:   0.1]
      [GC Worker Other (ms):  0.3  0.3  0.3  0.3
       Avg:   0.3, Min:   0.3, Max:   0.3, Diff:   0.0]
   [Clear CT:   0.1 ms]
   [Other:   1.5 ms]
      [Choose CSet:   0.0 ms]
      [Ref Proc:   0.3 ms]
      [Ref Enq:   0.0 ms]
      [Free CSet:   0.3 ms]
   [Eden: 12M(12M)->0B(10M) Survivors: 0B->2048K Heap: 13M(64M)->9739K(64M)]
 [Times: user=0.59 sys=0.02, real=0.16 secs]

上面日志是一段G1垃圾收集器的“整理暂停”阶段的日志，在这个阶段仍然存活的Java对象会从现在所在的regions拷贝到新的regions。
这是一个stop-the-world行为，所有的应用程序线程会在一个合适的安全点停下。

下面我们来看看日志中每行信息的具体意思，

0.522: [GC pause (young), 0.15877971 secs]
该次整理暂停阶段发生在应用程序启动0.522秒后，涉及整理的regions是young区（如果括号里面的内容是mixed，则会同时涉及到young区和old区），这个过程耗时0.15秒

[Parallel Time: 157.1 ms]
GC工作线程在整个阶段花费的时间

[GC Worker Start (ms): 522.1 522.2 522.2 522.2
Avg: 522.2, Min: 522.1, Max: 522.2, Diff: 0.1]
GC工作线程开始工作的时间点，单位是ms

[Ext Root Scanning (ms): 1.6 1.5 1.6 1.9
Avg: 1.7, Min: 1.5, Max: 1.9, Diff: 0.4]
扫描扩展根节点花费的时间，单位ms

[Update RS (ms): 38.7 38.8 50.6 37.3
Avg: 41.3, Min: 37.3, Max: 50.6, Diff: 13.3]
GC工作线程更新Remembered Set花费的时间，Remembered Set标记了heap region存储了哪些java对象

[Processed Buffers : 2 2 3 2
Sum: 9, Avg: 2, Min: 2, Max: 3, Diff: 1]
GC工作线程扫描Remembered Set的数量

[Scan RS (ms): 9.9 9.7 0.0 9.7
Avg: 7.3, Min: 0.0, Max: 9.9, Diff: 9.9]
GC工作线程花费在Remembered Set扫描上面的时间

[Object Copy (ms): 106.7 106.8 104.6 107.9
Avg: 106.5, Min: 104.6, Max: 107.9, Diff: 3.3]
对象拷贝花费的时间

[GC Worker End (ms): 679.1 679.1 679.1 679.1
Avg: 679.1, Min: 679.1, Max: 679.1, Diff: 0.1]
GC工作线程完成操作的时间节点

[Eden: 12M(12M)->0B(13M) Survivors: 0B->2048K Heap: 14M(64M)->9739K(64M)]
本行给出了GC处理后，各个内存区域容量的变化，从上面的日志可以看到Eden区的内存已经全部被回收，而且被G1算法设定的目标值变大

[Times: user=0.59 sys=0.02, real=0.16 secs]
该行记录GC阶段花费的各个角度的时间统计，如果real time与JVM给出的GC耗时相差较大，则表明有其他的服务在消耗机器的资源

像CMS算法一样，G1算法也会有并发标记的处理过程，

1.416: [GC pause (young) (initial-mark), 0.62417980 secs]
…....
2.042: [GC concurrent-root-region-scan-start]
2.067: [GC concurrent-root-region-scan-end, 0.0251507]
2.068: [GC concurrent-mark-start]
3.198: [GC concurrent-mark-reset-for-overflow]
4.053: [GC concurrent-mark-end, 1.9849672 sec]
4.055: [GC remark 4.055: [GC ref-proc, 0.0000254 secs], 0.0030184 secs]（注：remark阶段是stop-the-world的）
 [Times: user=0.00 sys=0.00, real=0.00 secs]
4.088: [GC cleanup 117M->106M(138M), 0.0015198 secs]
 [Times: user=0.00 sys=0.00, real=0.00 secs]
4.090: [GC concurrent-cleanup-start]
4.091: [GC concurrent-cleanup-end, 0.0002721]