1889737 - Ensure we don't collect overflowed largest_contentful_paint_from_response_start

Status: RESOLVED FIXED

(Core :: Performance Engineering, defect)

Comment 1

[Sean Feng [:sefeng211]]

Attachment: Bug 1889737 - Ensure we don't collect overflowed largest_contentful_paint_from_response_start

We seem to be collecting some overflowed durations based on the Glean
data. This is a sanity check for that.

Comment 2

[BugBot [:suhaib / :marco/ :calixte]]

The severity field is not set for this bug.
:fdoty, could you have a look please?

For more information, please visit BugBot documentation.

Comment 3

[BugBot [:suhaib / :marco/ :calixte]]

The following patch is waiting for review from an inactive reviewer:

ID	Title	Author	Reviewer Status
D206689	Bug 1889737 - Ensure we don't collect overflowed largest_contentful_paint_from_response_start	sefeng211	bas: Disabled

:sefeng211, could you please find another reviewer or abandon the patch if it is no longer relevant?

For more information, please visit BugBot documentation.

Comment 4

[Justin Link]

According to Claude and ChatGPT, this could be caused by precision reduction in GetLargestContentfulRenderTimeStamp(). I was able to reproduce this scenario by setting privacy.resistFingerprinting.reduceTimerPrecision.microseconds to a large value in about:config and reloading example.com a few times.

One way to fix this would be to use a high-res timestamp instead but, while I'm not an expert in the world of fingerprinting, this seems like it would potentially make LCP times able to be used for fingerprinting.

So, at least for the moment, I am in favor of something similar to sefeng's patch here but, rather than completely bail on reporting LCP for this case, we should just report an LCP of zero (since that is an appropriate approximation of the correct answer).

I will put up a patch for that shortly.

Comment 5

[Justin Link]

Attachment: Bug 1889737: Ensure that we don't report underflowed values in largest_contentful_paint_* metrics r=denispal,bas

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/121593b924f0
https://hg.mozilla.org/integration/autoland/rev/4df68b9f93bd
Ensure that we don't report underflowed values in largest_contentful_paint_* metrics r=denispal

Comment 7

[amarc]

https://hg.mozilla.org/mozilla-central/rev/4df68b9f93bd