RFR: Allow young collection to suspend marking in old generation [v3]

[Roman Kennke]

On Fri, 26 Feb 2021 19:41:08 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> We had some discussion on how to handle degenerated cycles. Having the degenerated cycle complete the generation in-progress would probably shorten the pause times, but running the degenerated cycle on the entire heap would reclaim the most memory. At any rate, it's not an irreversible decision and it's something to experiment with.
>> 
>> I'll add JTreg tests to exercise these new flags and create a new configuration to run the tests we expect to pass in generational mode.
>
> This looks much better! My tests now die when attempting full-GC, which is expected (and not going to be addressed in this PR). hotspot_gc_shenandoah also seems stable now. I'll give it an in-depth review next week.

You should enable GitHub actions in your fork under Settings->Actions->Allow all actions. Then you'll get presubmit-tests which runs a whole bunch of stuff on various platforms. A great sanity check.

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PR: https://git.openjdk.java.net/shenandoah/pull/19