1379178 - requestIdleCallback() reports less idle time when there is no rendering happening

Status: NEW

(Core :: DOM: Core & HTML, defect, P3)

Description

[Ben Kelly [:bkelly, not reviewing]]

STR:

1. Open https://idle-no-render.glitch.me/ in a fresh browser instance
2. Observe that firefox executes around 15 to 25 rIC executions per second
3. Select some text with the mouse and move your mouse a round.  This forces some rendering.
4. Observe that rIC executions *increase* up to 60 per second.

It seems very strange that rendering would produce more idle time compared to not rendering.

In contrast, chrome maxes out the CPU at ~170k rIC executions per second on my machine on this site.

Comment 1

[Andreas Farre [:farre]]

What does timeRemaining say? ~20 * 50ms = 1000ms, while 60 * 16.7 = 1000ms. That is, it's the difference between long and short idle periods. If we expect more calls during long period.

Comment 2

[Ben Kelly [:bkelly, not reviewing]]

I changed the demo to sum the timeRemaining() and average it over a second.  On my desktop its showing my something along the lines of:

  requestIdleCallback rate (executions / second):
  13
  requestIdleCallback deadline (total idle milliseconds / second):
  650

So I believe it is setting a timeRemaining() of 50ms.  Its not running quite at 20Hz though.

If I force rendering the summed deadline time also goes up:

  requestIdleCallback rate (executions / second):
  60
  requestIdleCallback deadline (total idle milliseconds / second):
  825

Its not as dramatic as the callback rate looked, but its still a bit unexpected that more work produces more idle time here.

Comment 3

[Ben Kelly [:bkelly, not reviewing]]

Updated the title to reflect that the overall deadline time remaining is reported drops when rendering is not happening.  The fact that fewer callbacks are made is not a problem.

Comment 4

[Sylvestre Ledru [:Sylvestre]]

Moving to p3 because no activity for at least 1 year(s).
See https://github.com/mozilla/bug-handling/blob/master/policy/triage-bugzilla.md#how-do-you-triage for more information