java G1 gc parameter explanation

gc parameter tuning

nohup java -jar -Xms4096m  -Xmx4096m -XX:+UseG1GC -XX:MaxGCPauseMillis=200 -XX:InitiatingHeapOccupancyPercent=40 -XX:G1HeapRegionSize=4 -XX:MaxTenuringThreshold=13 -XX:+UseStringDeduplication -XX:ConcGCThreads=2 -XX:AutoBoxCacheMax=20000 -XX:MetaspaceSize=128m -XX:MaxMetaspaceSize=256m -XX:+PrintGCApplicationStoppedTime -XX:+PrintPromotionFailure -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=$APP_HOME/java_pid${pid}.hprof -Xloggc:$APP_HOME/trade-gc.log -Duser.timezone=GMT+8 -verbose:gc -XX:+PrintGCDateStamps -XX:+PrintGCDetails ${JAR_FILE}  >>/dev/null 2>&1 &

• -Xms: initial heap size
• -Xmx: set the new generation size
• -XX:+UseG1GC: Use the G1 garbage collector
• -XX:MaxGCPauseMillis: The maximum time for each garbage collection, unit ms
• -XX:InitiatingHeapOccupancyPercent: If the usage of the entire heap exceeds this value, G1 will trigger a concurrency cycle
• -XX:G1HeapRegionSize: When using G1, the Java heap will be divided into regions of uniform size. This parameter can specify the size of each heap area
• XX:MaxTenuringThreshold: Controls how many GC promotions the new generation needs to undergo to reach the maximum threshold in the old generation.
• -XX:+UseStringDeduplication: Enable string deduplication
• -XX:ConcGCThreads: Number of threads for concurrent garbage collection
• -XX:AutoBoxCacheMax: Integer cache maximum value
• -XX:MetaspaceSize: The value size of the initialized Metaspace, which will trigger gc
• -XX:MaxMetaspaceSize: Maximum value of Metaspace space
• -XX:+PrintGCApplicationStoppedTime: Displays the time the program was paused during garbage collection
• -XX:+PrintPromotionFailure: Displays how many new generation objects failed to be promoted to the old generation, thus triggering Full GC.
• -XX:+HeapDumpOnOutOfMemoryError: When a JVM memory overflow occurs, a DUMP file with error information is automatically generated.
• -XX:HeapDumpPath: Specify the path and file name of the generated DUMP file
• Xloggc: output path and file name of gc log file
• -Duser.timezone: specify time zone
• -verbose: Print the details of GC in the jvm virtual machine
• -XX:+PrintGCDateStamps: print gc trigger time
• -XX:+PrintGCDetails: print GC details

2. gc log analysis

2020-06-19T10:47:33.199+0000: 15.533: [GC pause (G1 Evacuation Pause) (young) (to-space exhausted), 0.0189107 secs]
本次清理是在JVM启动1553.3ms 时开始的，清理的region区域是年轻代，即eden区域和survivor区域，暂停在JVM启动1553.3ms开始，完成这一次收集花费了0.0189107秒。

   [Parallel Time: 14.5 ms, GC Workers: 4]
表明后面的活动由4个 Worker 线程并行执行, 消耗时间为14.5ms

      [GC Worker Start (ms): Min: 15533.5, Avg: 15537.6, Max: 15539.4, Diff: 5.8]
GC的worker线程开始启动时，相对于 pause 开始的时间戳。如果 Min 和 Max 差别很大，则表明本机其他进程所使用的线程数量过多，挤占了GC的CPU时间。
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.8, Max: 3.1, Diff: 3.1, Sum: 3.1]
用了多长时间来扫描堆外(non-heap)的root, 如 classloaders, JNI引用, JVM的系统root等。后面显示了运行时间, “Sum” 指的是CPU时间。
      [Update RS (ms): Min: 3.7, Avg: 5.1, Max: 6.1, Diff: 2.5, Sum: 20.3]
         [Processed Buffers: Min: 8, Avg: 14.2, Max: 27, Diff: 19, Sum: 57]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
用了多长时间来扫描实际代码中的 Root。
      [Object Copy (ms): Min: 3.4, Avg: 4.4, Max: 5.1, Diff: 1.6, Sum: 17.6]
用了多长时间来拷贝收集区内的存活对象。
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
GC的worker线程用了多长时间来确保自身可以安全地停止, 这段时间什么也不用做, stop 之后该线程就终止运行了。
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 4]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
一些琐碎的小活动,在GC日志中不值得单独列出来。
      [GC Worker Total (ms): Min: 8.5, Avg: 10.3, Max: 14.3, Diff: 5.8, Sum: 41.0]
GC的worker 线程的工作时间总计。
      [GC Worker End (ms): Min: 15547.8, Avg: 15547.8, Max: 15547.8, Diff: 0.0]
GC的worker 线程完成作业的时间戳，通常来说这部分数字应该大致相等，否则就说明有太多的线程被挂起
   [Code Root Fixup: 0.0 ms]
释放用于管理并行活动的内部数据。一般都接近于零。这是串行执行的过程。
   [Code Root Purge: 0.0 ms]
清理其他部分数据，也是非常快的, 但如非必要则几乎等于零。这是串行执行的过程。
   [String Dedup Fixup: 0.9 ms, GC Workers: 4]
      [Queue Fixup (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Table Fixup (ms): Min: 0.8, Avg: 0.8, Max: 0.8, Diff: 0.0, Sum: 3.2]
   [Clear CT: 0.2 ms]
   [Other: 3.3 ms]
其他活动消耗的时间, 其中有很多是并行执行的。
      [Evacuation Failure: 2.6 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.2 ms]
处理非强引用(non-strong)的时间: 进行清理或者决定是否需要清理。
      [Ref Enq: 0.0 ms]
用来将剩下的 non-strong 引用排列到合适的 ReferenceQueue中。
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.2 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.1 ms]
将回收集中被释放的小堆归还所消耗的时间, 以便他们能用来分配新的对象。

   [Eden: 2048.0K(4096.0M)->0.0B(4096.0M) Survivors: 0.0B->0.0B Heap: 4096.0M(4096.0M)->4096.0M(4096.0M)]
 Eden: 使用量(eden区总量)->回收后使用量(eden区总量) Survivors: 回收前使用量->回收后使用量    Heap: 回收前使用量(Heap总量)->回收后使用量(Heap总量)，可以看出这个Heap 总量就是配置的量
 
 [Times: user=0.04 sys=0.00, real=0.02 secs]
GC线程消耗的CPU总时间，系统调用和系统等待时间，应用暂停时间。