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

[Hamlin]

Hi,

Would you please help to review an improvement in G1 full GC?

bug: https://bugs.openjdk.java.net/browse/JDK-8262068

webrev: 
https://github.com/openjdk/jdk/pull/2760/commits/4be6c18e2201fc8d22ee0f31d11ca7892be21a43

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;
- add hr's with high percentage of live bytes into a "no moving" set 
rather the normal compaction set in prepare phase, and fill dummy 
objects in the places of dead objects in these hr's;
- nothing special is done in adjust phase;
- just compact the regions in compaction set;

VM options added
-----------

- G1FullGCNoMoving: enable "no moving region" mode in G1 Full GC.
- G1NoMovingRegionLiveBytesLowerThreshold: the lower threshold (percent) 
of heap region live bytes to skip compaction

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

- specjbb2015: no regression
- dacapo: 3%-11% improvement of full gc pause. Attachment is the dacapo 
h2 full gc pause.

$ 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


Thanks,

Hamlin