RFR: 8342818: Implement CPU Time Profiling for JFR

[Erik Gahlin]

On Tue, 10 Sep 2024 14:57:22 GMT, Jaroslav Bachorik <jbachorik at openjdk.org> wrote:

>> This happens when the JVM is unable to walk a specific stack trace, but the queues are not full, and when the JVM is unable to resolve `Method` pointers.
>> 
>> The main idea behind it is that we still record that the thread runs on the CPU at this point in time; we just don't have any proper information on the stack trace. Only knowing that a thread run is still valuable information.
>
> Indeed. This is important for getting the correct distribution. Since we are sampling per thread cpu time, having one global 'event loss' is suboptimal as it is not possible to easily assign the lost events to threads proportionally.
> But because keeping the event loss per thread would be too expensive we settled for this solution. OTOH, for unwalkable samples we can 'cheaply' record them and maintain the distribution.

Could we add a field, i.e. "failed" and "Failed Stack Walk", to indicate failure instead of using null, which could be because stack trace has been disabled.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20752#discussion_r1769540699