RFR: 8338977: Parallel: Improve heap resizing heuristics [v3]

[Zhengyu Gu]

On Sun, 6 Jul 2025 20:59:49 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Returning `null` outside of a safe point does have unexpected negative effects. e.g. `HeapDumpOnOutOfMemoryError` may not get good heap dump, as other threads can come in and run additional GCs, then the heap dump may contain confusingly little live data.
>> 
>> I wonder if you can improve this situtation.
>
> Let me try to rephrase your concern to ensure that I understand you correctly -- after `_gc_overhead_counter >= GCOverheadLimitThreshold`, it's possible that another GC is triggered, which reclaims much free memory, and resets `_gc_overhead_counter` to zero. Then, `HeapDumpOnOutOfMemoryError` will not be able to get the intended heap snapshot.
> 
> Is my understanding correct?
> 
> (Seems that baseline resets the condition and can encounter the same problem as well.)
> 
> 
>         if (limit_exceeded && softrefs_clear) {
>           *gc_overhead_limit_was_exceeded = true;
>           size_policy()->set_gc_overhead_limit_exceeded(false);

Yes. I don't think it is exceeding overhead limit only problem, but OOM and heap dump have to be, in a sense of `atomic`. We encountered  this problem in real production system.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25000#discussion_r2188761699