日志配置

在VM argument选项中可配置如下：

-Xms256m –jvm堆的最小值

-Xmx256m –jvm堆的最大值

-XX:+PrintGCTimeStamps — 打印出GC的时间信息

-XX:+PrintGCDetails –打印出GC的详细信息

-verbose:gc –开启gc日志

-Xloggc:d:/gc.log — gc日志的存放位置

-Xmn10M — 新生代内存区域的大小

-XX:SurvivorRatio=8 –新生代内存区域中Eden和Survivor的比例

PS日志

[GC(Allocation Failure)
>[GC表示YGC，否则是FGC（GC原因

[DefNew:5698->129k(7268k),0.03735498 secs]
>[年代：回收前年轻代空间->回收后年轻代空间（年轻代总大小），耗时]

5698k->3465k(1984k),0.09856210 secs]**
>回收前堆占用空间->回收后堆占用空间（堆总空间），耗时]

[Times:user=0.00 sys=0.09,rel=0.09 secs]
>[Linux times：用户耗时，系统耗时，真正耗时]

eden space 5632K, 94% used [0x00000000ff980000,0x00000000ffeb3e28,0x00000000fff00000)
> 后面的内存地址指的是，起始地址，使用空间结束地址，整体空间结束地址

Metaspace used 3529k,capacity 5569k,committed 5864k,reserved 116795l
> 已经使用，总容量，虚拟内存占用，虚拟内存保留

CMS日志

使用命令 -XX:+PrintGCDetails -XX:+UseConcMarkSweepGC 可以表示打印GC日志详情，使用CMS垃圾回收器

[GC (Allocation Failure) [ParNew: 6144K->640K(6144K), 0.0265885 secs] 6585K->2770K(19840K), 0.0268035 secs] [Times: user=0.02 sys=0.00, real=0.02 secs] 
>ParNew：年轻代回收器

[GC (CMS Initial Mark) [1 CMS-initial-mark: 8511K(13696K)] 9866K(19840K), 0.0040321 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] 
>CMS-initial-mark表示mixed模式开启，混合回收。8511 (13696) : 老年代使用（最大），9866 (19840) : 整个堆使用（最大）

[CMS-concurrent-mark-start]
[CMS-concurrent-mark: 0.018/0.018 secs] [Times: user=0.01 sys=0.00, real=0.02 secs] 
>这里的时间意义不大，因为是并发执行

[CMS-concurrent-preclean-start]
[CMS-concurrent-preclean: 0.000/0.000 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
>标记Card为Dirty，也称为Card Marking

[GC (CMS Final Remark) [YG occupancy: 1597 K (6144 K)][Rescan (parallel) , 0.0008396 secs][weak refs processing, 0.0000138 secs][class unloading, 0.0005404 secs][scrub symbol table, 0.0006169 secs][scrub string table, 0.0004903 secs][1 CMS-remark: 8511K(13696K)] 10108K(19840K), 0.0039567 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
>STW阶段，YG occupancy:年轻代占用及容量
>[Rescan (parallel)：STW下的存活对象标记
>weak refs processing: 弱引用处理
>class unloading: 卸载用不到的class
>scrub symbol(string) table: 
>cleaning up symbol and string tables which hold class-level metadata and 
>internalized string respectively
>CMS-remark: 8511K(13696K): 阶段过后的老年代占用及容量
>10108K(19840K): 阶段过后的堆占用及容量

[CMS-concurrent-sweep-start]
[CMS-concurrent-sweep: 0.005/0.005 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
>标记已经完成，进行并发清理

[CMS-concurrent-reset-start]
[CMS-concurrent-reset: 0.000/0.000 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
>重置内部结构，为下次GC做准备

G1日志

[GC pause (G1 Evacuation Pause) (young) (initial-mark), 0.0015790 secs]
>young -> 年轻代 Evacuation-> 复制存活对象 
>initial-mark 混合回收的阶段，这里是YGC混合老年代回收
   [Parallel Time: 1.5 ms, GC Workers: 1] 
>一个GC线程
      [GC Worker Start (ms):  92635.7]
      [Ext Root Scanning (ms):  1.1]
      [Update RS (ms):  0.0]
         [Processed Buffers:  1]
      [Scan RS (ms):  0.0]
      [Code Root Scanning (ms):  0.0]
      [Object Copy (ms):  0.1]
      [Termination (ms):  0.0]
         [Termination Attempts:  1]
      [GC Worker Other (ms):  0.0]
      [GC Worker Total (ms):  1.2]
      [GC Worker End (ms):  92636.9]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.0 ms]
   [Other: 0.1 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.0 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.0 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 0.0B(1024.0K)->0.0B(1024.0K) Survivors: 0.0B->0.0B Heap: 18.8M(20.0M)->18.8M(20.0M)]
 [Times: user=0.00 sys=0.00, real=0.00 secs] 
>以下是混合回收其他阶段
[GC concurrent-root-region-scan-start]
[GC concurrent-root-region-scan-end, 0.0000078 secs]
[GC concurrent-mark-start]
>无法evacuation，进行FGC
[Full GC (Allocation Failure)  18M->18M(20M), 0.0719656 secs]
   [Eden: 0.0B(1024.0K)->0.0B(1024.0K) Survivors: 0.0B->0.0B Heap: 18.8M(20.0M)->18.8M(20.0M)], [Metaspace: 38
76K->3876K(1056768K)] [Times: user=0.07 sys=0.00, real=0.07 secs]