RFR: 8342818: Implement JEP 509: JFR CPU-Time Profiling [v5]

[Johannes Bechberger]

On Sun, 25 May 2025 10:08:44 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

>> Johannes Bechberger has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix compilation
>
> src/hotspot/share/jfr/metadata/metadata.xml line 966:
> 
>> 964: 
>> 965:   <Event name="CPUTimeSample" category="Java Virtual Machine, Profiling" label="CPU Time Method Sample"
>> 966:     description="Snapshot of a threads state from the CPU time sampler. The throttle can be either an upper bound for the event emission rate, e.g. 100/s, or the cpu-time period, e.g. 10ms, with s, ms, us and ns supported as time units."
> 
> What is the minimal practical sampling period allowed?

Depends on your system: It is configured via `CONFIG_HZ` at build time. This value is usually between 250 and 1000 Hz. We could proactively lower-bound the interval to 1ms or just state it in the description.

> src/hotspot/share/jfr/metadata/metadata.xml line 972:
> 
>> 970:     <Field type="boolean" name="failed" label="Failed" description="Failed to obtain the stack trace" />
>> 971:     <Field type="Tickspan" name="samplingPeriod" label="CPU Time Sampling Period"/>
>> 972:     <Field type="boolean" name="biased" label="Biased" description="The sample is biased towards the frame at safepoint" />
> 
> "The sample is biased towards the frame at safepoint". What does that mean? Are there three things involved? A sample, a frame and a safepoint? How are they related? See if you can clarify what you intend to communicate here.

Maybe better: "The sample is safepoint-biased"

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PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2106652540
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2106653359