1295505 - Background tab slowdown causing very long browser hangs compared to same tab in foreground

Status: RESOLVED INCOMPLETE

(Core :: JavaScript Engine, defect, P3)

Description

[Hugh Nougher [:Hughman]]

With this I cannot give access to the website which is causing this to happen for me (MSP Manager) but have fairly specific steps.

Suspected problem(s):
1. The background tab JS (or something) slowdown causes Firefox to hang on large scripts.
2. The background slowdown does not stop on existing execution when going back to a tab stuck in script/execution (either by manual change or by slow script dialogue).

Steps:
1. Have a tab which will run lots of JS on load (like >5 seconds in one block and less than the slow script timeout) and a second  simple tab that you can swap to (like google).
2. Reload the first page and verify that it only takes the number of seconds.
3. Reload the first page and quickly change to the second.
3a. Additional for my case, if I can time my change back to the first tab to after the main block of script starts executing, the same thing happens.
4. Firefox hangs. After a few seconds the "Slow Script" dialogue appears on the first tab. Press continue.
5. Repeat step 4 a few more times (in my case its usually between 4 and 12 times).

Tested on both v48 and Nightly with same results.

Comment 1

[Ben Kelly [:bkelly, not reviewing]]

This would be solved by multiple child content processes.  Some of the event loop bucketing plans may help as well depending on what the problem window is doing.

Comment 2

[Ben Kelly [:bkelly, not reviewing]]

I mean to save multiple child content processes would help, but "solve" is probably too strong a word.  Here is the event loop improvement bug.

Comment 3

[Arkadiusz Michalski (Spirit)]

In single process you can test older version because new has terrible bottleneck for background tabs (https://bugzilla.mozilla.org/show_bug.cgi?id=1140892). But I must write that now e10s (an more proces, 8 or more) finally works better than older older older version (I notes better times + responsiveness).

Comment 4

[Andrew McCreight (out until 2026) [:mccr8]]

Mitigated by e10s-multi. With Fission, you'll only get hangs by other tabs for the same site, which at least might be more comprehensible to the average user.