RFR: 8280396: G1: Full gc mark stack draining should prefer to make work available to other threads [v2]

[Thomas Schatzl]

On Mon, 24 Jan 2022 14:15:22 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> Thomas Schatzl has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
>> 
>>  - Merge branch 'master' into 8280396-g1-fullgc-stack-drain
>>  - Remove unused methods
>>  - Initial commit
>
> src/hotspot/share/gc/g1/g1FullGCMarker.inline.hpp line 162:
> 
>> 160:         follow_object(obj);
>> 161:       }
>> 162:     }
> 
> What do you think about instead of extracting all this into `drain_stack()` have something like `transfer_from_overflow()` as the first statement in in `pop_object()`? It would either return an object that can't be transferred or push it back to the overflow and just let pop do what it does today. 
> 
> The problem I see with that is that `pop_object(...)` would have side effects that you wouldn't really expect, so if doing that maybe we should call it `drain_object_stack(...)` or something like that.

Refactored the code a bit, to hopefully something better.

I *think* there are some additional gains to be made by not draining the regular oop task queue fully when trying to move work from its overflow stack to the shared one, however measurements were within a few percent so far. I will file an issue for working on that idea - the current change is already much better than what has been there before.

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PR: https://git.openjdk.java.net/jdk/pull/7175