Commits on Jan 8, 2019

1. Heap size startup hints …

   Initial contribution of feature where on PHASE_NOT_STARTUP phase change
   GC will store current heap sizes (optionally, averaged with already
   existing stored values) into Shared Cache. During early startup GC will
   use these hints to expand heap beyond default or even explicitly
   specified sizes.
   
   This should help large workloads to avoid early GCs while heap is
   aggressively expanding, which in turn may help with startup times.
   
   In this contribution, Standard group of GC policies are covered, with
   Balanced GC following at a later point.
   
   For Flat heap configurations (optthruput/optavgpause) only the total
   heap size is maintained/expanded, while for generational configuration,
   Tenure and Nursery are separately maintained/expanded.
   
   The feature is disabled by default, but can be enabled by specifying
   value [1-100] to -XXgc:heapSizeStatupHintWeightNewValue= command line
   option. Reasonable value is 80, which should typically take about 5
   restarts for hints to converge to stable values. Value 0 (currently
   defaults) means that new values are never accounted for, which means
   that values are always 0, which effectively means that feature is
   disabled. Value 100 would mean that only use the hints from the most
   recent run, hence no historical averaging.
   
   Another tuning options is -XXgc:heapSizeStatupHintConservativeFactor=
   Default value is 70. It means that conservatively only 70% of the hint
   values are taken.
   
   An example of verbose GC stanzas reporting the expand just after
   initialized stanza is complete, before even first GC:
   
   ```
   .....
   </initialized>
   
   <heap-resize id="2" type="expand" space="nursery" amount="1461583872"
   count="1" timems="1.738" reason="hint from previous runs"
   timestamp="2019-01-03T12:59:54.909" />
   <heap-resize id="3" type="expand" space="tenure" amount="200212480"
   count="1" timems="0.930" reason="hint from previous runs"
   timestamp="2019-01-03T12:59:54.910" />
   ```
   
   Signed-off-by: Aleksandar Micic <amicic@ca.ibm.com>

   

   amicic committed Jan 8, 2019
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