RFR: 8359348: G1: Improve cpu usage measurements for heap sizing [v3]

[Ivan Walulya]

On Wed, 6 Aug 2025 21:12:52 GMT, Man Cao <manc at openjdk.org> wrote:

>>> Which is very similar to the current approach to divide concurrent-cpu-usage by some arbitrary threading factor, without needing to know that factor, and incurring inaccuracies because of that.
>> 
>> Effectively it divides that concurrent-cpu-usage by the number of active processors if I am not mistaken :)
>
> The `#active-processors * time-since-last-pause` approach sounds worth experimenting.
> 
> One potential concern is the heap sizing behavior could be more heavily affected by #active-processors, which could be surprising comparing to G1's current behavior. Admittedly, G1's current heap sizing behavior could already be affected by #active-processors (mainly via default values for ParallelGCThreads/ConcGCThreads), but it is likely to a much lesser extent.
> 
> If a prototype of this approach exists, we should probably test how much difference in heap sizing there is for an application running with a low value of `ActiveProcessorCount` vs a high value. My main worry is that `#active-processors * time-since-last-pause` could inflate the divisor too much if `#active-processors` is very high (e.g. >100), which could make the heap much smaller, which is quite counter-intuitive.

So iiuc, we have narrowed down to 

Suggestion:

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PR Review Comment: https://git.openjdk.org/jdk/pull/26351#discussion_r2281829373