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

[Thomas Schatzl]

On Mon, 18 Aug 2025 09:24:55 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

>> 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:

Afaics yes :)

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PR Review Comment: https://git.openjdk.org/jdk/pull/26351#discussion_r2290428890