fix(mmserver): drop application frames if the application is too slow

This produces roughly the same number of frames - meaning if an
application is slow, we still wait for it to encode - but prevents the
compositor thread from blocking while it does. Instead, we just try
again next frame.

Author: colinmarc

mm-server/src/session/compositor.rs

@@ -6,7 +6,7 @@ use std::{collections::BTreeMap, sync::Arc};
 
 use protocols::*;
 use slotmap::SlotMap;
-use tracing::{instrument, trace};
+use tracing::{debug, instrument, trace};
 use wayland_protocols::{
     wp::{
         fractional_scale::v1::server::wp_fractional_scale_manager_v1,
@@ -172,8 +172,11 @@ impl Compositor {
         &mut self,
         video_pipeline: &mut video::EncodePipeline,
     ) -> anyhow::Result<()> {
-        let now = EPOCH.elapsed().as_millis() as u32;
-        unsafe { video_pipeline.begin()? };
+        let ready = unsafe { video_pipeline.begin()? };
+        if !ready {
+            debug!("dropped frame because of backpressure");
+            return Ok(());
+        }
 
         // Iterate backwards to find the first fullscreen window.
         let first_visible_idx = self
@@ -186,8 +189,9 @@ impl Compositor {
             })
             .unwrap_or_default();
 
-        let mut presentation_feedback =
-            Vec::with_capacity(self.surface_stack.len() - first_visible_idx);
+        let num_surfaces = self.surface_stack.len() - first_visible_idx;
+        let mut presentation_feedback = Vec::with_capacity(num_surfaces);
+
         for id in self.surface_stack[first_visible_idx..].iter() {
             let surface = &mut self.surfaces[*id];
 

mm-server/src/session/video.rs

@@ -146,10 +146,18 @@ impl EncodePipeline {
     // }
 
     #[instrument(level = "trace", skip_all)]
-    pub unsafe fn begin(&mut self) -> anyhow::Result<()> {
+    pub unsafe fn begin(&mut self) -> anyhow::Result<bool> {
         let device = &self.vk.device;
         let frame = &mut self.swap[self.swap_idx];
 
+        let ready = frame.tp_clear.poll()?;
+
+        // If the previous frame isn't ready, drop this one to let the app
+        // catch up.
+        if !ready {
+            return Ok(false);
+        }
+
         // Trace on on the GPU side.
         if let Some(ref ctx) = self.vk.graphics_queue.tracy_context {
             if let Some(span) = frame.staging_span.take() {
@@ -167,10 +175,6 @@ impl EncodePipeline {
         }
 
         frame.texture_semas_used = 0;
-
-        // Wait for the frame to no longer be in flight, and then establish new timeline
-        // points.
-        frame.tp_clear.wait()?;
         frame.tp_staging_done += 10;
         frame.tp_render_done = &frame.tp_staging_done + 1;
         frame.tp_clear = &frame.tp_render_done + 1;
@@ -203,7 +207,7 @@ impl EncodePipeline {
             vk::AccessFlags2::COLOR_ATTACHMENT_WRITE,
         );
 
-        Ok(())
+        Ok(true)
     }
 
     /// Adds a surface to be drawn. Returns the timeline point when the texture
@@ -355,6 +359,8 @@ impl EncodePipeline {
         Ok(release)
     }
 
+    /// End the current frame and submit it to the GPU. Returns the timeline
+    /// point indicating when rendering and encoding have both completed.
     #[instrument(skip_all)]
     pub unsafe fn end_and_submit(&mut self) -> anyhow::Result<VkTimelinePoint> {
         let device = &self.vk.device;