RFR: 8364282: ZGC: Improve ZPageAllocation JFR event sending

[Joel Sikström]

On Tue, 29 Jul 2025 14:01:43 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Hello,
>> 
>> This is a follow up to [JDK-8350441](https://bugs.openjdk.org/browse/JDK-8350441), improving on how the ZPageAllocation JFR event is sent. 
>> 
>> The starting time for the event is measured separately from creating a local event varaible (EventZPageAllocation). This means it is better to use the anonymous EventZPageAllocation::commit rather than creating an event and sending that event, which would record the starting time again, unnecessarily.
>> 
>> Additionally, if the event is not enabled (either specifically or if JFR is disabled), we should not waste time sampling statistics for the event.
>> 
>> Testing:
>> * Oracle's tier1-4
>
> Marked as reviewed by stefank (Reviewer).

Thank you for the reviews! @stefank @xmas92

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PR Comment: https://git.openjdk.org/jdk/pull/26534#issuecomment-3138879922