G1垃圾回收器介绍
首先要搞清楚G1垃圾回收的过程
主要分为下面4个步骤：
初始标记（Initial Marking） 由于常规标记对象是否直接关联到GC roots，过程简单，停顿时间短
并发标记（Concurrent Marking） 由于可达性性分析，存在较长停顿
最终标记（Final Marking） 由于把并发标记时漏掉的那部分垃圾内存标记起来，比较少，停顿时间短
筛选回收（Live Data Couting and Evacuation） 对标记后的垃圾内存进行整理，按指定计划进行回收。
在下面的案例中，红色部分明确标明了这4个阶段。
日志分析案例
下面一段日志是我在spark计算任务中产生的垃圾回收日志：
2018-11-13T05:32:44.473+0800: 1.857: Application time: 0.2130324 seconds 2018-11-13T05:32:44.571+0800: 1.954: Application time: 0.0970449 seconds 2018-11-13T05:32:44.696+0800: 2.079: Application time: 0.1249397 seconds 2018-11-13T05:32:44.874+0800: 2.258: Application time: 0.1776871 seconds 2.258: [G1Ergonomics (Concurrent Cycles) request concurrent cycle initiation, reason: requested by GC cause, GC cause: Metadata GC Threshold] 2.258: [G1Ergonomics (Concurrent Cycles) initiate concurrent cycle, reason: concurrent cycle initiation requested] 2018-11-13T05:32:44.875+0800: 2.259: [GC pause (Metadata GC Threshold) (young) (initial-mark) 2.260: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 0, predicted base time: 10.00 ms, remaining time: 190.00 ms, target pause time: 200.00 ms] 2.260: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 60 regions, survivors: 0 regions, predicted young region time: 3605.71 ms] 2.260: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 60 regions, survivors: 0 regions, old: 0 regions, predicted pause time: 3615.71 ms, target pause time: 200.00 ms] 2018-11-13T05:32:44.948+0800: 2.331: [SoftReference, 0 refs, 0.0002103 secs]2018-11-13T05:32:44.948+0800: 2.332: [WeakReference, 970 refs, 0.0005817 secs]2018-11-13T05:32:44.949+0800: 2.332: [FinalReference, 4265 refs, 0.0266394 secs] 2018-11-13T05:32:44.975+0800: 2.359: [PhantomReference, 0 refs, 3 refs, 0.0000585 secs]2018-11-13T05:32:44.976+0800: 2.359: [JNI Weak Reference, 0.0001374 secs], 0.1040572 secs] [Parallel Time: 29.1 ms, GC Workers: 38] [GC Worker Start (ms): Min: 2260.1, Avg: 2265.0, Max: 2265.9, Diff: 5.8] [Ext Root Scanning (ms): Min: 0.2, Avg: 18.2, Max: 22.7, Diff: 22.5, Sum: 691.2] [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0] [Processed Buffers: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0] [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.3] [Code Root Scanning (ms): Min: 0.0, Avg: 0.3, Max: 8.6, Diff: 8.6, Sum: 10.0] [Object Copy (ms): Min: 0.0, Avg: 3.9, Max: 22.9, Diff: 22.9, Sum: 147.1] [Termination (ms): Min: 0.0, Avg: 1.3, Max: 2.3, Diff: 2.3, Sum: 50.3] [Termination Attempts: Min: 1, Avg: 3.0, Max: 17, Diff: 16, Sum: 115] [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 1.6] [GC Worker Total (ms): Min: 22.7, Avg: 23.7, Max: 28.6, Diff: 5.9, Sum: 900.6] [GC Worker End (ms): Min: 2288.6, Avg: 2288.7, Max: 2288.8, Diff: 0.2] [Code Root Fixup: 1.0 ms] [Code Root Purge: 0.1 ms] [Clear CT: 1.0 ms] [Other: 72.8 ms] [Choose CSet: 0.0 ms] [Ref Proc: 68.9 ms] [Ref Enq: 0.2 ms] [Redirty Cards: 0.4 ms] [Humongous Register: 0.1 ms] [Humongous Reclaim: 0.1 ms] [Free CSet: 0.5 ms] [Eden: 240.0M(512.0M)->0.0B(496.0M) Survivors: 0.0B->16.0M Heap: 254.0M(10.0G)->31.7M(10.0G)] [Times: user=0.44 sys=0.09, real=0.11 secs] 2018-11-13T05:32:44.980+0800: 2.363: [GC concurrent-root-region-scan-start] 2018-11-13T05:32:44.996+0800: 2.380: [GC concurrent-root-region-scan-end, 0.0167701 secs] 2018-11-13T05:32:44.996+0800: 2.380: [GC concurrent-mark-start] 2018-11-13T05:32:45.013+0800: 2.396: [GC concurrent-mark-end, 0.0160995 secs] 2018-11-13T05:32:45.013+0800: 2.396: Application time: 0.0328728 seconds 2018-11-13T05:32:45.013+0800: 2.397: [GC remark 2018-11-13T05:32:45.013+0800: 2.397: [Finalize Marking, 0.0331140 secs] 2018-11-13T05:32:45.047+0800: 2.430: [GC ref-proc2018-11-13T05:32:45.047+0800: 2.430: [SoftReference, 0 refs, 0.0001959 secs]2018-11-13T05:32:45.047+0800: 2.430: [WeakReference, 0 refs, 0.0001148 secs]2018-11-13T05:32:45.047+0800: 2.430: [FinalReference, 0 refs, 0.0001158 secs]2018-11-13T05:32:45.047+0800: 2.430: [PhantomReference, 0 refs, 0 refs, 0.0002233 secs]2018-11-13T05:32:45.047+0800: 2.431: [JNI Weak Reference, 0.0000572 secs], 0.0008127 secs] 2018-11-13T05:32:45.047+0800: 2.431: [Unloading, 0.0078450 secs], 0.0442183 secs] [Times: user=0.36 sys=0.01, real=0.05 secs] 2018-11-13T05:32:45.058+0800: 2.441: Application time: 0.0001231 seconds 2018-11-13T05:32:45.058+0800: 2.442: [GC cleanup 35M->35M(10G), 0.0134109 secs] [Times: user=0.04 sys=0.00, real=0.01 secs] 2018-11-13T05:32:45.144+0800: 2.528: Application time: 0.0721037 seconds 2018-11-13T05:32:45.392+0800: 2.775: Application time: 0.2464978 seconds 2018-11-13T05:32:45.805+0800: 3.188: Application time: 0.4129881 seconds 2018-11-13T05:32:46.806+0800: 4.190: Application time: 1.0001058 seconds
下面是每条日志含义解析：
Application time： 应用程序时间