RFR: 8276887: G1: Move precleaning to Concurrent Mark From Roots subphase [v5]

[Albert Mingkun Yang]

On Sun, 14 Nov 2021 23:08:09 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Simple change of moving the "Preclean" subphase into "Mark from roots" subphase so that precleaning becomes parallel automatically. Evaluation using a contrived java program shows that multiple GC threads do precleaning. More detailed testing results are available in the JBS ticket.
>> 
>> Test: hotspot_gc
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

In the scenario where discovery is sequential && parallel ref-processing is disabled (e.g. `ParallelRefProcEnabled == false`) && `ParallelGCThreads > 1`, all iterations except the first one in `for (uint j = 0; j < _num_queues; ++j) {` are no-op. Will be fixed by 
https://bugs.openjdk.java.net/browse/JDK-8279456

Therefore, I will keep this PR open until then.

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PR: https://git.openjdk.java.net/jdk/pull/6327