RFR: 8338977: Parallel: Improve heap resizing heuristics [v19]

[Zhengyu Gu]

On Sun, 6 Jul 2025 21:07:26 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> src/hotspot/share/gc/parallel/psParallelCompact.cpp line 898:
>> 
>>> 896:       size_t target_capacity_bytes = total_live_words * HeapWordSize
>>> 897:                                    + old_space->capacity_in_bytes() * (MarkSweepDeadRatio / 100.0)
>>> 898:                                    + SpaceAlignment;
>> 
>> Why add `SpaceAlignment`, not `align_up` to `SpaceAlignment`?
>
> The alignment is handled inside the callee. I just wanna leave a minimal delta in the old-gen so that old-gen is not completely full. Otherwise, the next young-gc can not promote anything and will be upgraded to full-gc directly. This is mostly for `UseAdaptiveSizePolicy == false`; if enabled, old-gen will also be resized after full-gc.

After [JDK-8328744](https://bugs.openjdk.org/browse/JDK-8328744),  it no longer uses old-gen capacity, but max old-gen size to estimate if promotion can succeed.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25000#discussion_r2188764546