JVM CMS GC日志详解

# JDK8
-Xms20M -Xmx20M -XX:+PrintGCDetails -XX:+UseConcMarkSweepGC

[GC (Allocation Failure) [ParNew: 5504K->640K(6144K), 0.0136830 secs] 5504K->1556K(19840K), 0.0137738 secs] [Times: user=0.06 sys=0.01, real=0.01 secs] 
[GC (Allocation Failure) [ParNew: 6144K->544K(6144K), 0.0012950 secs] 7060K->2102K(19840K), 0.0013310 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [ParNew: 6048K->640K(6144K), 0.0175172 secs] 7606K->4311K(19840K), 0.0176172 secs] [Times: user=0.07 sys=0.01, real=0.01 secs] 
[GC (Allocation Failure) [ParNew: 6144K->640K(6144K), 0.0163793 secs] 9815K->7694K(19840K), 0.0164765 secs] [Times: user=0.10 sys=0.01, real=0.01 secs] 
# ParNew：年轻代收集器   6144->640：收集前后的对比 （6144）：整个年轻代容量  9815 ->7694：整个堆的情况（19840）：整个堆大小

[GC (CMS Initial Mark) [1 CMS-initial-mark: 7054K(13696K)] 7806K(19840K), 0.0023289 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
# 初始标记，STW, 7054K(13696K)  老年代使用空间、最大空间       7806K(19840K)   整个堆使用空间、最大空间
[CMS-concurrent-mark-start]
[CMS-concurrent-mark: 0.015/0.015 secs] [Times: user=0.05 sys=0.00, real=0.01 secs] 
# 并发标记，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 Markin
[GC (CMS Final Remark) [YG occupancy: 1865 K (6144 K)][Rescan (parallel) , 0.0015431 secs][weak refs processing, 0.0000870 secs][class unloading, 0.0017562 secs][scrub symbol table, 0.0012416 secs][scrub string table, 0.0006834 secs][1 CMS-remark: 7054K(13696K)] 8919K(19840K), 0.0054720 secs] [Times: user=0.01 sys=0.01, real=0.01 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: 7054K(13696K): 阶段过后的老年代占用及容量
   # 8919K(19840K) : 阶段过后的堆占用及容量
[CMS-concurrent-sweep-start]
[CMS-concurrent-sweep: 0.010/0.010 secs] [Times: user=0.02 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