Re: [web-vitals-feedback] FP / FCP time and other events

Michal Mocny

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Dec 17, 2024, 7:03:13 AM 12/17/24

to Szczepan Bielecki, web-vitals-feedback

This is just the difference between paint time (main thread scheduling) and presentation time (compositor, GPU, frame swap, etc).

See spec details: https://www.w3.org/TR/paint-timing/#painttimingmixin

On Tue, Dec 17, 2024, 11:27 'Szczepan Bielecki' via web-vitals-feedback < web-vital...@googlegroups.com > wrote:

I'm trying to figure out why FP / FCP is delayed in relation to the first Paint on the main thread.

The first requestAnimationFrame event also is much sooner than FP.

Here is a trace:
https://trace.cafe/t/v2nsepHQPT

I'm looking for information that lets me better understand this behavior.--
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Szczepan Bielecki (Pimko)

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Dec 19, 2024, 10:20:00 AM 12/19/24

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Thank Michal for Your response.

It's forced me to get deeper about this presentation time.

I would like to prove that there is something more job to do after the main thread paint and commit event ends.

I have looked for some information about that in DevTools but nothing was found.

Then I found some interesting sources about frames:

https://chromium.googlesource.com/chromium/src.git/+/main/docs/life_of_a_frame.md#life-of-a-frame

There is something like PipelineReporter that could give me some more information about drawing a frame.

I made some traces and started an investigation.

I created a screenshot with a description to show what I found:

Of course there is compositor, GPU Track, but on PipelineReporter we see all things in one track to better understand that.

Could you tell me if I have missed something?

Michal Mocny

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Dec 19, 2024, 1:45:05 PM 12/19/24

to Szczepan Bielecki (Pimko), web-vitals-feedback

I don't see your attached screenshot, but you are exactly right. The PipelineReporter trace events are a "flat" view of the "pipeline" of asynchronous steps that it takes to produce a single animation frame.

Although there will only be one active "frame" in each "stage" of rendering at a time, there will often be multiple overlapping "PipelineReporter" events.

To view this discussion visit https://groups.google.com/d/msgid/web-vitals-feedback/f484b4fb-0661-4a29-9b4a-89187efae189n%40googlegroups.com .

Szczepan Bielecki (Pimko)

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Dec 20, 2024, 8:09:08 AM 12/20/24

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Screenshot:
FCP Perfetto ui.png

FCP Perfetto ui.png

Michal Mocny

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Dec 21, 2024, 7:34:23 AM 12/21/24

to Szczepan Bielecki (Pimko), web-vitals-feedback

Excellent.

To view this discussion visit https://groups.google.com/d/msgid/web-vitals-feedback/6d73cc3d-d6ff-4bce-b798-056c3d5c470en%40googlegroups.com .

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