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

[Roman Kennke]

On Mon, 1 Mar 2021 14:19:43 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
>
> That is a huge change! Good work!
> First-pass review follows.
> 
> (I need to remember to scan for 'HEY!' in addition to 'TODO' ;-) )

Do you plan to enable reference processing again on old-gen marking? Because I think it's a bad idea to not do it. Ideally, we'd do reference processing on both young-gen and old-gen markings, if that is not possible, then we should at least do it on old-gen markings (I believe other GCs don't do it in young-gen).

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