RFR: 8048180: G1: Eager reclaim of humongous objects with references [v7]

[Benjamin Peterson]

On Thu, 9 Oct 2025 14:24:12 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this change to G1 that enables eager reclaim of humongous objects with references, either objArrays or just really large objects.
>> 
>> It works the same as other humongous object's eager reclaim, with the caveat that I disabled it during concurrent mark to avoid problems with SATB. There is fairly extensive documentation about why I think this works, and how in the comment in `g1YoungCollector.cpp`.
>> 
>> The test case exhaustively tests all combinations of when a particular type of object should be able to be reclaimed depending on when it is allocated (only for `typeArray` and `objArray` - huge objects work the same as the latter).
>> 
>> I added a diagnostic flag `G1EagerReclaimWithRefs` to allow disabling this particular functionality for diagnostic purposes, but I think it's not really necessary, and I'm open to remove it.
>> 
>> The following screenshot shows pause times before (purple) and after (brown) of one of the [Optaplanner](https://www.optaplanner.org/blog/2021/09/15/HowMuchFasterIsJava17.html) benchmarks as a result of this change.
>>  
>> <img width="702" height="485" alt="Screenshot 2025-09-26 at 13 59 05" src="https://github.com/user-attachments/assets/995e9e27-f861-44bd-8ced-3bd952e314e8" />
>> 
>> Testing: new test cases, gha, tier1-5
>> 
>> Thanks,
>>   Thomas
>
> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
> 
>   * remove G1EagerReclaimWithRefs flag
>   * improve readability of condition to make a humongous object eager reclaim candidate

src/hotspot/share/gc/g1/g1RemSetTrackingPolicy.cpp line 56:

> 54: 
> 55:   bool selected_for_rebuild = false;
> 56:   // Humongous regions containing type-array objs are remset-tracked to

comment needs an update?

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PR Review Comment: https://git.openjdk.org/jdk/pull/27525#discussion_r2417196485