title: Does exist some official document to introduce the gc log?
body: <p>Today there was an indexing slow log, </p>
  
  <pre><code>[2020-02-12T15:52:37,418][WARN ][i.i.s.index              ] [node-1] [company/KTngnM6ASD-_KdU0FFAWRA] took[22.7s], took_millis[22703], type[_doc], id[20080943028], routing[], source[{...}]
  </code></pre>
  
  <p>then to check <code>gc.log</code> to find out why is so, </p>
  
  <pre><code>[2020-02-12T07:52:37.417+0000][22539][safepoint      ] Total time for which application threads were stopped: 0.0004935 seconds, Stopping threads took: 0.0001389 seconds
  [2020-02-12T07:52:37.586+0000][22539][safepoint      ] Application time: 0.1682439 seconds
  [2020-02-12T07:52:37.586+0000][22539][safepoint      ] Entering safepoint region: GenCollectForAllocation
  [2020-02-12T07:52:37.586+0000][22539][gc,start       ] GC(315124) Pause Young (Allocation Failure)
  [2020-02-12T07:52:37.586+0000][22539][gc,task        ] GC(315124) Using 8 workers of 8 for evacuation
  [2020-02-12T07:52:37.641+0000][22539][gc,age         ] GC(315124) Desired survivor size 34865152 bytes, new threshold 3 (max threshold 6)
  [2020-02-12T07:52:37.641+0000][22539][gc,age         ] GC(315124) Age table with threshold 3 (max threshold 6)
  [2020-02-12T07:52:37.641+0000][22539][gc,age         ] GC(315124) - age   1:   22998672 bytes,   22998672 total
  [2020-02-12T07:52:37.641+0000][22539][gc,age         ] GC(315124) - age   2:    4966112 bytes,   27964784 total
  [2020-02-12T07:52:37.641+0000][22539][gc,age         ] GC(315124) - age   3:   10219520 bytes,   38184304 total
  [2020-02-12T07:52:37.641+0000][22539][gc,age         ] GC(315124) - age   4:    4875304 bytes,   43059608 total
  [2020-02-12T07:52:37.641+0000][22539][gc,heap        ] GC(315124) ParNew: 597611K-&gt;52614K(613440K)
  [2020-02-12T07:52:37.641+0000][22539][gc,heap        ] GC(315124) CMS: 4992477K-&gt;4998973K(16095680K)
  [2020-02-12T07:52:37.641+0000][22539][gc,metaspace   ] GC(315124) Metaspace: 103488K-&gt;103488K(1144832K)
  [2020-02-12T07:52:37.641+0000][22539][gc             ] GC(315124) Pause Young (Allocation Failure) 5459M-&gt;4933M(16317M) 54.724ms
  [2020-02-12T07:52:37.641+0000][22539][gc,cpu         ] GC(315124) User=0.35s Sys=0.00s Real=0.06s
  [2020-02-12T07:52:37.641+0000][22539][safepoint      ] Leaving safepoint region
  </code></pre>
  
  <p>it seems gc is ok, but some log do not understand, e.g.</p>
  
  <pre><code>Entering safepoint region: Cleanup
  Entering safepoint region: RevokeBias
  Entering safepoint region: GenCollectForAllocation
  </code></pre>
  
  <p>what are <code>Cleanup</code>, <code>RevokeBias</code>, <code>GenCollectForAllocation</code> meaning? and What <code>Application time</code> meaning? why are so different?</p>
  
  <pre><code>Application time: 0.1382641 seconds
  Application time: 13.2106552 seconds
  Application time: 106.3031188 seconds 
  </code></pre>