Daniel van Vugt 45244852ac clutter/stage-cogl: Don't skip over the next frame ... The `last_presentation_time` is usually a little in the past (although sometimes in the future depending on the driver). 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 a few milliseconds 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 that frame. **Example** Given: ``` last_presentation_time = now - 3ms sync_delay = 2ms refresh_interval = 16ms next_presentation_time = last_presentation_time + refresh_interval = now + 13ms -3ms now +13ms +29ms +45ms ----|--+------------|---------------|---------------|---- : : last_presentation_time next_presentation_time ``` Old algorithm: ``` update_time = last_presentation_time + sync_delay = now - 1ms while (update_time < now) (now - 1ms < now) update_time = now - 1ms + 16ms update_time = now + 15ms next_presentation_time = now + 13ms available_render_time = next_presentation_time - max(now, update_time) = (now + 13ms) - (now + 15ms) = -2ms so the next frame will be skipped. -3ms now +13ms +29ms +45ms ----|--+------------|-+-------------|---------------|---- : : : : : update_time (too late) : : last_presentation_time next_presentation_time (a missed frame) ``` New algorithm: ``` min_render_time_allowed = refresh_interval / 2 = 8ms max_render_time_allowed = refresh_interval - sync_delay = 14ms target_presentation_time = last_presentation_time + refresh_interval = now - 3ms + 16ms = now + 13ms while (target_presentation_time - min_render_time_allowed < now) (now + 13ms - 8ms < now) (5ms < 0ms) # loop is never entered update_time = target_presentation_time - max_render_time_allowed = now + 13ms - 14ms = now - 1ms next_presentation_time = now + 13ms available_render_time = next_presentation_time - max(now, update_time) = (now + 13ms) - now = 13ms which is plenty of render time. -3ms now +13ms +29ms +45ms ----|-++------------|---------------|---------------|---- : : : : update_time : : : last_presentation_time next_presentation_time ``` The reason nobody noticed these missed frames very often was because mutter has some 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 (till e9e4b2b72). * Even though Wayland support is now implemented the native backend provides a `last_presentation_time` much faster than Xorg sessions (being in the same process) and so is less likely to spuriously enter the while loop to miss a frame. * For Xorg sessions we are accidentally triple buffering (#334). This is a good way to avoid the missed frames, but is also an accident. * `sync_delay` is presently just high enough (2ms by coincidence is very close to common values of `now - last_presentation_time`) to push the `update_time` into the future in some cases, which avoids entering the while loop. This is why the same missed frames problem was also noticed when experimenting with `sync_delay = 0`. v2: adjust variable names and code style. Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=789186 and most of https://gitlab.gnome.org/GNOME/mutter/issues/571 https://gitlab.gnome.org/GNOME/mutter/merge_requests/520		2019-05-16 22:13:54 +00:00
..
clutter	clutter/stage-cogl: Don't skip over the next frame	2019-05-16 22:13:54 +00:00
tests	Use free_full on GSList's instead of foreach + free	2019-05-15 14:49:56 -05:00
.gitignore	clutter: Remove clutter specific version	2018-11-06 17:17:36 +01:00
meson.build	meson: Define srcdir and builddir using meson functions	2019-05-02 19:56:23 +00:00