RFR: 8303252: G1: Return early from Full-GC if no regions are selected for compaction. [v2]

[Albert Mingkun Yang]

On Tue, 28 Feb 2023 13:58:57 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

>> Please review this change to return early from full-gc in case no regions have been selected as targets for compaction. 
>> To skip  `phase3_adjust_pointers` and `phase4_do_compaction()`, we need to decouple cleanup work that was piggybacked onto these phases. Thus, the `phase5_` is introduced to do the cleanup.
>> 
>> Testing: Tier 1-3.
>
> Ivan Walulya has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Thomas review

src/hotspot/share/gc/g1/g1FullCollector.cpp line 217:

> 215:     phase4_do_compaction();
> 216:   } else {
> 217:     log_info(gc, phases) ("No Regions selected for compaction. Skipping Phase 3: Adjust pointers and Phase 4: Compact heap");

A comment on when this will be the case would be nice. As I understand it, it's `all regions are almost full & max-compaction == false`.

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PR: https://git.openjdk.org/jdk/pull/12768