Currently, ClutterFrameClock uses g_source_set_ready_time() to determine
the usec timing of the next frame. That translates into a poll() with a
millisecond timeout if no trigger occurs to break the poll() out early.
To avoid spinning the CPU, GLib always rounds *up* to the next millisecond
value unless a timeout of 0 was provided by a GSource.
This means that timeouts for the ClutterFrameClock can easily skew beyond
their expected time as the precision is too coarse.
This applies the same concept as GNOME/glib!3949 but just for the
ClutterFrameClock. That may be more ideal than adding a timerfd for every
GMainContext, but we'll see if that lands upstream. I wanted to provide
this here because it could easily be cherry-picked in the mean time if
this is found to be useful.
From a timer stability perspective, this improves things from erratically
jumping between 100s and 1000s off of the expected awake time to single
or low double digits.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3636>
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