RFR: JDK-8262068: Improve G1 Full GC by skipping compaction for regions with high survival ratio [v12]

[Thomas Schatzl]

On Sat, 27 Mar 2021 12:02:47 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> 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 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
>
> Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   refine the code.

I will run this through internal testing before approving.

src/hotspot/share/gc/g1/g1FullGCPrepareTask.cpp line 160:

> 158:   }
> 159:   // High live ratio region will not be compacted.
> 160:   return false;

`return live_words <= live_words_thresholds` should be sufficient here.

-------------

PR: https://git.openjdk.java.net/jdk/pull/2760