848071 - Optimize circular buffer usage of JS frames

Status: NEW

(Core :: Gecko Profiler, task, P3)

Description

[Benoit Girard (:BenWa)]

The profiler uses a circular buffer to store samples. It copies the JS frames into the circular buffer which consumes a disproportionate amount of space compared to c++ frames. On low memory b2g devices we reduce the circular buffer for memory constraint meaning the JS frames wrap the circular buffer very quickly.

Comment 1

[Benoit Girard (:BenWa)]

I think the best option is to modify the lifetime of JS labels such that they don't need to be copied for every sample. Perhaps allow the profiler to AddRef the JS labels it currently has in it's circular buffer. This isn't simple because the AddRef/ReleaseRef needs to be perform from a signal handler

Comment 2

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

Benoit, do you know if this old bug is still valid? (If you don't have the time to check, please just cancel the NI.) TIA.

Comment 3

[Markus Stange [:mstange]]

It is still valid, but it's unknown how much impact it has / how much skew it causes.

Basically, for JIT frames, we only store an address in the buffer. But interpreter frames use profiler label frames, which store the entire function string in the buffer. That's the whole function name and the URL. It is possible that this causes skew, attributing too much time to interpreted JS, see bug 1330576.