Bug 214866

Summary:	Profiler resolution is too low
Product:	WebKit	Reporter:	Radek Pietruszewski <radexpl>
Component:	Web Inspector	Assignee:	Nobody <webkit-unassigned>
Status:	NEW ---
Severity:	Enhancement	CC:	fpizlo, inspector-bugzilla-changes, saam, webkit-bug-importer, ysuzuki
Priority:	P2	Keywords:	InRadar
Version:	Safari 13
Hardware:	Mac
OS:	Unspecified

Description Radek Pietruszewski 2020-07-28 01:53:11 PDT

Hi. It appears that the Safari/webkit web inspector profiler samples stack traces approximately every 1ms. This is slow, and not suitable for many profiling tasks, since I'm only going to get a handful of samples when trying to hit 60fps, or react to an event in 50ms. By comparison, it appears that Chrome's Timeline profiler samples stacks every 100us.

Is there a technical reason why WebKit's profiler can't go up to 10kHz? If there's a tradeoff (slow processing, significantly degraded performance), can this be made configurable?

Comment 1 Radar WebKit Bug Importer 2020-07-28 16:59:24 PDT

<rdar://problem/66245375>

Comment 2 Saam Barati 2020-07-28 18:23:52 PDT

(In reply to radexpl from comment #0)
> Hi. It appears that the Safari/webkit web inspector profiler samples stack
> traces approximately every 1ms. This is slow, and not suitable for many
> profiling tasks, since I'm only going to get a handful of samples when
> trying to hit 60fps, or react to an event in 50ms. By comparison, it appears
> that Chrome's Timeline profiler samples stacks every 100us.
> 
> Is there a technical reason why WebKit's profiler can't go up to 10kHz? If
> there's a tradeoff (slow processing, significantly degraded performance),
> can this be made configurable?

Last time I measured this it did negatively impact performance of the script being run, and we were trying to find a balance where we didn't introduce so much perturbation that the samples you got were unrepresentative.

I also remember it being difficult to be woken up reliably in 100us.

However, this was a long time ago. We should check again the impact of increasing the resolution.

Comment 3 Radek Pietruszewski 2021-03-13 23:57:44 PST

Hi Saam! I wanted to bump this thread and ask again whether it's possible to increase WebKit's profiler resolution -- or at least make it a configurable option.

I've just upgraded my Mac to an M1 machine, which exacerbates the issue even more -- the faster a dev machine is, the fewer samples for analysis I get.

BTW: I'm not very familiar with the ins and outs of sampling profiling, but I do wonder if instead of being a hardcoded timer, the profiler sampler rate shouldn't be calculated/estimated off of CPU's speed - so that a program would get roughly the same number of samples regardless of how fast a machine is.