Tighter sync and faster ACK on SwapBuffers/PostSubBuffers

ui/aura/root_window.cc

Index: ui/aura/root_window.cc
diff --git a/ui/aura/root_window.cc b/ui/aura/root_window.cc
index 3bd7ccae8bd9d007cd2bbbf570078efee112c43e..60f9f1fa47db6caafbfe476e2a38f289da8b9169 100644
--- a/ui/aura/root_window.cc
+++ b/ui/aura/root_window.cc
@@ -131,8 +131,7 @@ RootWindow::RootWindow(const gfx::Rect& initial_bounds)
       mouse_move_hold_count_(0),
       should_hold_mouse_moves_(false),
       compositor_lock_(NULL),
-      draw_on_compositor_unlock_(false),
-      draw_trace_count_(0) {
+      draw_on_compositor_unlock_(false) {
   SetName("RootWindow");
   should_hold_mouse_moves_ = !CommandLine::ForCurrentProcess()->HasSwitch(
       switches::kAuraDisableHoldMouseMoves);
@@ -243,9 +242,9 @@ void RootWindow::Draw() {
   }
   waiting_on_compositing_end_ = true;
 
-  TRACE_EVENT_ASYNC_BEGIN0("ui", "RootWindow::Draw", draw_trace_count_++);
+  TRACE_EVENT_ASYNC_BEGIN0("ui", "RootWindow::Draw",
+                           compositor_->last_started_id() + 1);
 
-  defer_draw_scheduling_ = false;
   compositor_->Draw(false);
 }
 
@@ -589,8 +588,13 @@ void RootWindow::ScheduleDraw() {
 void RootWindow::OnCompositingStarted(ui::Compositor*) {
 }
 
+void RootWindow::OnCompositingWillEnd(ui::Compositor*) {
+  defer_draw_scheduling_ = false;

[piman, 2012/06/04 18:20:39]

A possible issue with this is that if other observers want to trigger a redraw
in their own OnCompositingWillEnd callback, then the ability to do so depends on
the observer order (leading to subtle bugs...).
I was thinking of moving some of the ScheduleDraw culling logic to Compositor -
in a way, that's where it belongs - where it'd be easier to do this.

[jonathan.backer, 2012/06/05 13:35:58]

Your point is taken. It's a potential problem.

I need to cull the ScheduleDraw's for the browser prioritization in the GPU
process to land (otherwise the browser draws 2 frames for every 1 from a
renderer). I'm happy to keep this or temporarily break your aura_bench suite.

Given that kicking ACKs on OnCompositingWillEnd is inherently dangerous, I
prefer the latter (breaking aura_bench). WDYT?

+}
+
 void RootWindow::OnCompositingEnded(ui::Compositor*) {
-  TRACE_EVENT_ASYNC_END0("ui", "RootWindow::Draw", draw_trace_count_);
+  TRACE_EVENT_ASYNC_END0("ui", "RootWindow::Draw",
+                         compositor_->last_ended_id());
   waiting_on_compositing_end_ = false;
   if (draw_on_compositing_end_) {
     draw_on_compositing_end_ = false;