Currently the Gecko Profiler would profile all processes together, however people may not always need that. Sometimes people may only want to profile some processes (content process / chrome process / GPU process), or even one specific process. It's probably helpful to have an extra button beside "Start" to adjust what process to profile.
Isn't it what kats added with the "pid:" syntax ? (I don't find the bug # at the moment)
Bug 1444765 - that syntax allows profiling only a single process, but it'll end up profiling *all* threads of that process. We don't currently have a way to ask for a specific set of threads, of a given process. We also don't have a way of matching only certain "types" of processes.
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