RFR: 8236073: G1: Use SoftMaxHeapSize to guide GC heuristics

[Man Cao]

On Mon, 24 Mar 2025 23:07:08 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.

This probably requires fixing https://bugs.openjdk.org/browse/JDK-8352765 before users try to use `SoftMaxHeapSize`. Otherwise, setting a small `SoftMaxHeapSize` could trigger premature OutOfMemoryError.

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PR Comment: https://git.openjdk.org/jdk/pull/24211#issuecomment-2749664966