RFR: 8366082: Improve queue size computation in CPU-time sampler

[Francesco Andreuzzi]

On Mon, 25 Aug 2025 14:21:30 GMT, Johannes Bechberger <jbechberger at openjdk.org> wrote:

>> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 186:
>> 
>>> 184:     if (factor > 1) {
>>> 185:       u4 new_capacity = _capacity * factor > CPU_TIME_QUEUE_MAX_CAPACITY ? CPU_TIME_QUEUE_MAX_CAPACITY : capacity * factor;
>>> 186:       set_capacity(new_capacity);
>> 
>> Is it a problem if another thread changes `_capacity` since when you read it at L171, and the update is overwritten?
>
> No other thread can change it. The queue capacity is only changed when
> - a thread is created (so queue drainage is possible in parallel
> - when the queue is drained and there is sample loss (but this happens only with a dequeue lock)
> - a thread is terminated: I added synchronisation to prevent concurrency

Thanks for the clarification

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PR Review Comment: https://git.openjdk.org/jdk/pull/26926#discussion_r2298259049