RFR: Allow young collection to suspend marking in old generation [v14]

[earthling-amzn]

On Mon, 1 Mar 2021 10:37:24 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> earthling-amzn has updated the pull request incrementally with three additional commits since the last revision:
>> 
>>  - Encode active generation and old generation marking status
>>  - Do not unset preemption request flag prematurely
>>  - Disable reference processing generational mode
>
> src/hotspot/share/gc/shenandoah/mode/shenandoahGenerationalMode.cpp line 46:
> 
>> 44:   // HEY! Disabled while M7 work is in progress.
>> 45:   FLAG_SET_ERGO(ShenandoahUnloadClassesFrequency, 0);
>> 46:   FLAG_SET_ERGO(RegisterReferences, false);
> 
> This doesn't work in release build, unfortunately.

I also tried some runs with reference processing enabled and they succeeded. As I think about it, the reference processing itself will probably be fine with these changes. My concern would be that finalizers in the old generation might not run. I disabled it out of caution, but we can try leaving it enabled.

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