RFR: 8345323: Parallel GC does not handle UseLargePages and UseNUMA gracefully [v2]

[Thomas Schatzl]

On Mon, 9 Dec 2024 10:27:53 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> This patch reverts the default value of `OldSize` to its previous setting prior to being obsoleted in [JDK-8333962](https://bugs.openjdk.org/browse/JDK-8333962). The change addresses an issue where `OldSize` being set to zero results in a default `MinHeapSize` that is too small to handle LargePages correctly. This problem is exemplified by `ParallelArguments::initialize_heap_flags_and_sizes`, as identified in [JDK-8345323](https://bugs.openjdk.org/browse/JDK-8345323).
>> 
>> Changing the default value of `OldSize` may have broader implications due to the complexity of the logic that determines default values for various flags. Altering one default can lead to cascading effects and potential breakages elsewhere. For these reasons, this patch restores the previous default value of `OldSize` to mitigate such risks.
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update src/hotspot/share/gc/shared/genArguments.cpp
>   
>   Co-authored-by: Stefan Johansson <54407259+kstefanj at users.noreply.github.com>

src/hotspot/share/gc/shared/genArguments.cpp line 43:

> 41: // together with NewSize, is used to derive them.
> 42: // Using the same value when it was a configurable flag to avoid breakage.
> 43: // See more in JDK-8345323

I do not like referrals to the bug tracker in the code, and/or referring to some code the past ("when it was configurable").
Better explicitly state the problem with heap sizing and large pages and file a follow-up RFE (not mentioning it here).

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PR Review Comment: https://git.openjdk.org/jdk/pull/22575#discussion_r1877886973