RFR: 8302215: G1: Last-ditch Full GC should do serial compaction for tail regions in per thread compaction points. [v2]

[Ivan Walulya]

On Wed, 15 Feb 2023 15:03:32 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> I can add a comment if its unclear, however, redoing the filtering as above can be extremely unnecessary, especially where heap has very many pinned regions as in the test-case `vmTestbase/vm/gc/compact/Compact_InternedStrings_NonbranchyTree/TestDescription.java`. You have a few regions for serial compaction, and the sorting is sufficient.  So I would rather keep the current implementation.
>
> An alternative, similar to what Albert suggests, would be to change `move_regions_with_higher_hrm_index(...)` to `remove_above(lowest_hr, bitmap)` and when removing the regions from the compaction point store what regions were in use in a bitmap. Then you could use that bitmap to add the regions in order to the serial compaction point.

Nice, that would solve the concerns I raised above.

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PR: https://git.openjdk.org/jdk/pull/12529