1730997 - Consider making "cpu" feature non-optional

Status: NEW

(Core :: Gecko Profiler, task, P2)

Description

[Gerald Squelart (he/him) (not at Mozilla since 2022-09-15)]

A recent profile https://share.firefox.dev/3EnYIHm (from bug 1730907) was done with stack sampling, but no CPU utilization, which makes it less useful, especially when trying to diagnose janks.

We should probably remove the "cpu" option and just make it always run when stack sampling is turned on.

"nostacksampling" (aka No Periodic Sampling in about:profiling) is mostly useful to considerably reduce the profiler overhead (mostly due to stopping each thread when it's sampled). In this case, "cpu" is still low overhead so I think it would be useful to keep, which would give us CPU graphs but without the stacks.

We should investigate the actual overhead of "cpu", and if it's not negligible on low-end hardware, maybe we'd want a "nocpu" option to help there; but in such a way that we can't have samples without CPU measurements.
But note that this overhead could always be reduced by increasing the "Sampling interval", so measurements happen less often, but there would still be a useful CPU graph, albeit less precise. So an alternative to a "nocpu" option would be to remind users that they can reduce the overhead by increasing the sampling interval.

(Thanks Florian for the suggestion.)