RFR: 8199407: Skip Rebuild Remset Phase if there are no rebuild candidates

[Thomas Schatzl]

On Mon, 12 Apr 2021 11:14:20 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

> Hi all,
> 
> Please review this change that skips the rebuild remset phase and the associated updates after rebuild, If during Remark, G1 finds that there are no remembered sets to rebuild.
> 
> Test: tier1-5

Would it be possible to add a test that checks for this case?
I.e. allocate some objects of the approximate amount of a region, keeping them live, do a full gc to move them to old gen, then start a concurrent cycle that should print that new message?

src/hotspot/share/gc/g1/g1ConcurrentMark.cpp line 1324:

> 1322: 
> 1323:   verify_during_pause(G1HeapVerifier::G1VerifyCleanup, VerifyOption_G1UsePrevMarking, "Cleanup before");
> 1324:   bool has_rebuilt_remembered_sets = (total_selected_for_rebuild() > 0);

Please add a newline before.

src/hotspot/share/gc/g1/g1ConcurrentMark.hpp line 460:

> 458:   // set phase at all.
> 459:   HeapWord* volatile* _top_at_rebuild_starts;
> 460:   uint _total_selected_for_rebuild;

Something like `// Number of regions selected for rebuild in the Remark pause` should be added here.
It seems unnecessary to actually store that number, but sufficient that there is any need to rebuild regions. That would simplify the conditions in two places.

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PR: https://git.openjdk.java.net/jdk/pull/3434