If there is a pending frame, the RenderLoop will delay all schedule
repaint requests to the next vblank event. This means that the render
loop needs to be notified when a frame has been presented or failed.
At the moment, the RenderLoop is notified only about successfully
presented frames. If some frame fails, no repaints will be scheduled
on that output.
In order to make frame scheduling robust, the RenderLoop has to be
notified if a frame has failed.
Currently, we estimate the expected render time purely based on the
latency policy.
The problem with doing so is that the real render time might be larger,
this can result in frame drops.
In order to avoid frame drops, we need to take into account previous
render times while estimating the next render time. For now, we just
measure how long it takes to record rendering commands on the CPU.
In the future, we might want consider using OpenGL timer queries for
measuring the real render time, but for now, it's good enough.
At the moment, our frame scheduling infrastructure is still heavily
based on Xinerama-style rendering. Specifically, we assume that painting
is driven by a single timer, etc.
This change introduces a new type - RenderLoop. Its main purpose is to
drive compositing on a specific output, or in case of X11, on the
overlay window.
With RenderLoop, compositing is synchronized to vblank events. It
exposes the last and the next estimated presentation timestamp. The
expected presentation timestamp can be used by effects to ensure that
animations are synchronized with the upcoming vblank event.
On Wayland, every outputs has its own render loop. On X11, per screen
rendering is not possible, therefore the platform exposes the render
loop for the overlay window. Ideally, the Scene has to expose the
RenderLoop, but as the first step towards better compositing scheduling
it's good as is for the time being.
The RenderLoop tries to minimize the latency by delaying compositing as
close as possible to the next vblank event. One tricky thing about it is
that if compositing is too close to the next vblank event, animations
may become a little bit choppy. However, increasing the latency reduces
the choppiness.
Given that, there is no any "silver bullet" solution for the choppiness
issue, a new option has been added in the Compositing KCM to specify the
amount of latency. By default, it's "Medium," but if a user is not
satisfied with the upstream default, they can tweak it.
