1880744 - Leaving devtools (netmonitor) open in a background tab janks the parent process main thread

Status: NEW

(DevTools :: Netmonitor, defect, P3)

Description

[Florian Quèze [:florian]]

Here's a profile: https://share.firefox.dev/3SLJcgw

Steps to reproduce:

• I left devtools open on the netmonitor tab for the entire night on a web page that updates itself using a XHR every second (a bit more than 13,000 XHR requests over the night)

Expected result:
No consequence on browser responsiveness.

Actual result:
The entire browser is slow. I noticed because attempting to make a selection in the profiler was janky (mouse events are not dispatched smoothly to the content process). The parent process main thread is using a 100% of a CPU core.
Profiling shows that this is the netmonitor doing JS work and causing long reflows.
I think this should be faster in general (I think there are already bugs covering switching the netmonitor to a virtual list) and avoided entirely when devtools is not on screen (docked inside a background tab).

Comment 1

[Hubert Boma Manilla (:bomsy)]

Thanks for filing! This on our roadmap as part of our work for performance this year.

Comment 3

[Mike Conley (:mconley) (:⚙️)]

The Performance Impact Calculator has determined this bug's performance impact to be low. If you'd like to request re-triage, you can reset the Performance Impact flag to "?" or needinfo the triage sheriff.

Platforms: [x] Windows [x] macOS [x] Linux
Impact on browser: Causes noticeable jank
Configuration: Rare
[x] Affects animation smoothness

Comment 4

[Alex Thayer [:alexical] (she/her)]

Nicolas, is this a bit of a dupe of what we're seeing in bug 1896461? I.e., if the visibilityChange notifications were working as intended, would we still be seeing this?

Comment 5

[Nicolas Chevobbe [:nchevobbe][PTO - back on October 21]]

(In reply to Alex Thayer [:alexical] (she/her) from comment #4)

Nicolas, is this a bit of a dupe of what we're seeing in bug 1896461? I.e., if the visibilityChange notifications were working as intended, would we still be seeing this?

Florian didn't said if the tab that had DevTools was the active one.
If it was active, then Bug 1896461 wouldn't really help, Bug 1594378 or Bug 1308441 would help here.

Not sure if this bug is worth keeping open since we have all the other ones.
Florian, do you think it's still relevant to have this open?

Comment 6

[Florian Quèze [:florian]]

(In reply to Nicolas Chevobbe [:nchevobbe] from comment #5)

(In reply to Alex Thayer [:alexical] (she/her) from comment #4)

Nicolas, is this a bit of a dupe of what we're seeing in bug 1896461? I.e., if the visibilityChange notifications were working as intended, would we still be seeing this?

Florian didn't said if the tab that had DevTools was the active one.

I don't remember, but looking at the profile linked in comment 0, I see the screenshots (and user interactions) are for the profiler tab that doesn't have devtools in it, so it's a fair assumption that devtools was in a background tab.

Not sure if this bug is worth keeping open since we have all the other ones.
Florian, do you think it's still relevant to have this open?

It's not an exact duplicate of bug 1896461 as there's no visibilitychange event involved in the profile here, the jank happens here when a new network request occurs. But it's likely that fixing visibility handling for devtools would fix both bugs at once. I would keep both open until it's fixed, but I don't have a strong opinion.

Comment 9

[Julian Descottes [:jdescottes]]

Another similar profile: https://share.firefox.dev/3YUs8d3