Skip to content

WIP: clutter-stage-cogl: Don't skip past the next frame [performance]

Daniel van Vugt requested to merge vanvugt/mutter:avoid-skipping-frames-v3 into master

The last_presentation_time is usually a little in the past, although sometimes slightly in the future. When it's over 2ms (sync_delay) in the past that would trigger the while loop to count up so that the next update_time is in the future.

The problem with that is for common values of last_presentation_time which are only 2-8ms ago, incrementing update_time by refresh_interval also means counting past the next physical frame that we haven't rendered yet. And so mutter would skip a frame.

The reason nobody noticed these missed frames very often was because the bug had three accidental workarounds built-in:

  • Prior to 3.32, the offending code was only reachable in Xorg sessions. It was never reached in Wayland sessions because it hadn't been implemented yet (e9e4b2b7).

  • For Xorg sessions, frames are being over-scheduled with triple buffering. This avoids missed frames by often prerendering one more than intended. (#334 (closed)).

  • sync_delay is presently high enough (2ms) to push the update_time into the future in some cases, often avoiding the erroneous while loop.

Now we modify the while loop to accept an update_time that's still slightly in the past. So providing there's still at least half a frame of render time we try to make the next frame instead of skipping over it. The master clock already supports update times in the past just fine, as it treats them the same as being told to wake up immediately.

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=789186

Edited by Jeff Fortin

Merge request reports