Draft: Wayland Compositor: Reduce latency by sending frame callbacks before painting

Frame Callbacks

wl_surface:request:frame provides a callback to the compositor, that is intended to be used to signal the wayland client "when it is a good time to draw and commit the next frame of animation".

KWin currently emits wl_surface::frame callbacks after completing frame rendering/painting. Frame rendering is done in WaylandCompositor::composite, which is scheduled by RenderLoopPrivate::compositeTimer, so that it finishes just before the next estimated presentation time.

With frame callbacks being emitted shortly after rendering, clients immediately start commiting new frames right after presetation. These early commits will be presented at least a full refresh cycle later, resulting in unneccesarily high latency.

Approach

This MR attempts to reduce latency by splitting the framePass in WaylandCompositor::composite into two separate passes:

1. frameCallbackPass: emits frameCallbacks to "request" clients to commit frames just before painting/rendering
2. frameFeedbackPass: emits presentationFeedback after rendering

Next Steps

Marking this as draft to get some early feedback on the idea. I havent done much c++ yet, so I might have missed something.

Some possible improvements / concerns:

• Painting immediately after emitting frame callbacks might be problematic, as the client will take some time to act on the callback.
• Perhaps rendering (GlFinish, acquiring buffers etc.) should be moved completely out of WaylandCompositor::composite as is the case with explicit sync (?), so that WaylandCompositor::composite really only deals with compositing. In that case we could:
  • trigger commits by using a timer renderTimer to schedule frameCallbacks at nextPresentationTime - renderDuration - compositeDuration
    • (or multiple timers and render duration estimations for per-window latency optimization)
  • asynchronously acquire buffers ASAP on commit
  • use a second timer compositeTimer to schedule compositing at nextPresentationTime - compositeDuration while only using readily acquired buffers