Configure CPUs for profiling

[pveentjer]

It is possible to configure particular CPU's for profiling?

With perf I frequently check what a particular CPU is doing by passing an -C <cpu>.

Does async-profiler has such an ability? I could not find anything in the documentation.

If it doesn't have this capability, I would not mind picking up this task.

[apangin]

Async-profiler currently does not support CPU selection.
It's easy to add support for profiling one specific CPU. Profiling a subset of CPUs is not that trivial though, since it does not fit the existing event-per-thread architecture.

What is a use case for such an option? I can't imagine how -C can be helpful, unless threads are pinned to a particular CPU. But if they are pinned, you can as well profile selected threads rather than selected CPUs.

[pveentjer]

The use case is that I want to see what is happening on a single core without knowing which threads are running.

If I see an imbalance in the CPU load between CPUs, I just want to have a quick peek. Normally I use 'perf top' for that, but it doesn't work for Java applications.

Not all architectures have pinned threads and even when threads are pinned, there can always be threads (e.g. GC) that are not pinned. So I want to see what is running on a single CPU without being too specific. Otherwise I'm at risk of filtering out relevant profiling info and chasing down the wrong problem.

I already had a look at the implementation. Passing a CPU would be simple. But indeed, let the logic work over multiple CPUs is a bit more complicated.

The key question is: will you accept a feature where the CPU(s) is configured for profiling?

[apangin]

I don't mind adding an option for profiling only one selected CPU, as the change is rather small and straightforward.
Extending the feature to multiple CPUs isn't worth complication though.