Make it easier to identify samples where we were _only_ running script

[mikeconley]

This is distinct from the "JS Only" view in the Tree View, since that feature shows us stacks where JS is involved. What I'm requesting here is slightly deeper - what I'm hoping for is something that highlights samples where the deepest part of the stack is still JS, and that the JS isn't calling into platform-y things.

I think this would help us identify things like "this page is just running super expensive algorithms in JS".

/cc jrmuizel

┆Issue is synchronized with this Jira Task

[gregtatum]

It sounds like you are saying "Drop samples with non-JS leaf nodes"? This could get implemented as a transform.

I think we need to resolves #586 first (which needs a Gecko patch: https://bugzilla.mozilla.org/show_bug.cgi?id=1426124).

cpp -> js -> js
cpp -> js -> js -> cpp
cpp
cpp -> js

Would then get transformed to:

cpp -> js -> js
cpp -> js

[mikeconley]

It sounds like you are saying "Drop samples with non-JS leaf nodes"?

Yeah, I think that sounds right.

[upsuper]

Would it be more helpful the other way? I mean, drop samples with JS-leaf nodes, so that we can see how time is distributed inside the platform-y things. And from there, we can further identify any performance issue inside the platform code without things from JS.

[julienw]

Maybe that's what "c++ only" should really do. Is the current behavior of "c++ only" actually useful ?

[upsuper]

That's probably true? The current behavior of "C++ only" for me is somehow useful to shallow the call tree... but I don't see other usefulness there indeed.

[bholley]

I agree it would be very useful to separate JS execution from platform calls.

I always assumed that the "C++ only" thing referred to the stack type (C++ stacks vs JS stacks). I never really explored that stuff, but we should be wary about muddying the difference between a JS callstack and a C++ callstack in the interpreter.

[gregtatum]

This should be implemented with this: https://github.com/devtools-html/perf.html/issues/911

[julienw]

Does the activity graph answer this question? If it's yellow at the top of the graph, it means we're doing JS. Is that good enough?