RFR: 8236073: G1: Use SoftMaxHeapSize to guide GC heuristics [v5]

[Ivan Walulya]

On Tue, 1 Apr 2025 08:44:55 GMT, Man Cao <manc at openjdk.org> wrote:

>> Hi all,
>> 
>> I have implemented SoftMaxHeapSize for G1 as attached. It is completely reworked compared to [previous PR](https://github.com/openjdk/jdk/pull/20783), and excludes code for `CurrentMaxHeapSize`. I believe I have addressed all direct concerns from [previous email thread](https://mail.openjdk.org/pipermail/hotspot-gc-dev/2024-November/050214.html), such as:
>> 
>> - does not respect `MinHeapSize`;
>> - being too "blunt" and does not respect other G1 heuristics and flags for resizing, such as `MinHeapFreeRatio`, `MaxHeapFreeRatio`;
>> - does not affect heuristcs to trigger a concurrent cycle;
>> 
>> [This recent thread](https://mail.openjdk.org/pipermail/hotspot-gc-dev/2025-March/051619.html) also has some context.
>
> Man Cao has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revise test summary

With the changes to `young_collection_expansion_amount()`, once we reach the `SoftMaxHeapSize`, we cannot expand the heap except during GC where expansion can happen without regard for `SoftMaxHeapSize`. Thus, after exceeding  `SoftMaxHeapSize` we go into a phase of repeated GCs where we expand the heap almost one region at a time. Is this the expected effect of the `SoftMaxHeapSize` as implemented by this patch?

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PR Comment: https://git.openjdk.org/jdk/pull/24211#issuecomment-2768966455