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  1. JDK
  2. JDK-8262068

Improve G1 Full GC by skipping compaction for regions with high survival ratio

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    • Subcomponent:
      gc
    • Resolved In Build:
      b18

      Description

      Summary
      -----------

      Improve G1 Full GC by skip compaction for regions with high survival ratio.

      Backgroud
      -----------

      There are 4 steps in full gc of G1 GC.
      - mark live objects
      - prepare forwardee
      - adjust pointers
      - compact

      When full gc occurs, there may be very high percentage of live bytes in some regions. For these regions, it's not efficient to compact them and better to skip them, as there are little space to save but many objects to copy.

      Description
      -----------

      We enhance the full gc implementation for the above situation through following steps:
      - accumulate live bytes of every hr in mark phase; (already done by JDK-8263495)
      - skip adding regions with high survial ratio, and set the region with high survival ratio as pinned in _region_attr_table during prepare phase;
      - nothing special is done in adjust phase, regions with high survial ratio are skipped because of pin setting in the above step;
      - nothing special is done in compact phase, regions with high survival ratio are skipped because these regions are skipped when adding regions to compaction set in the prepare phase;

      VM options related
      -----------

      - MarkSweepDeadRatio: we reuse this exising vm option to indicate the high survial ratio threhold (100-MarkSweepDeadRatio) in G1.
           - default value of MarkSweepDeadRatio: 5

      Test
      -----------

      - specjbb2015: no regression
      - dacapo: (Attachment dacapo.v3.png is the dacapo h2 full gc pause.)
           - 95% of full gc pauses: 10%-19% improvement.
           - 5% of full gc pauses: 1.2% improvement.
           - 0.1% of full gc pauses: -6.16% improvement.

      $ java -Xmx1g -Xms1g -XX:ParallelGCThreads=4 -Xlog:gc*=info:file=gc.log -jar dacapo-9.12-bach.jar --iterations 5 --size huge --no-pre-iteration-gc h2

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              Assignee:
              mli Hamlin Li
              Reporter:
              mli Hamlin Li
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