Increase the graphics pipeline depth to 2

Previously, we weren't overlapping any work between the UI and the GPU threads
because each waited for the other to finish. After this patch, we now have a
pipeline depth of 2, which means we can be working on two frames at once, one
for each thread.  In the future, we should dynamically adjust the pipeline
depth.

sky/shell/ui/animator.cc

@@ -11,11 +11,14 @@
 namespace sky {
 namespace shell {
 
+const int kPipelineDepth = 2;
+
 Animator::Animator(const Engine::Config& config, Engine* engine)
     : config_(config),
       engine_(engine),
+      outstanding_draw_requests_(0),
+      did_defer_frame_request_(false),
       engine_requested_frame_(false),
-      frame_in_progress_(false),
       paused_(false),
       weak_factory_(this) {
 }
@@ -27,15 +30,19 @@ void Animator::RequestFrame() {
   if (engine_requested_frame_)
     return;
 
+  DCHECK(!did_defer_frame_request_);
+
   TRACE_EVENT_ASYNC_BEGIN0("sky", "Frame request pending", this);
   engine_requested_frame_ = true;
 
-  if (!frame_in_progress_) {
-    frame_in_progress_ = true;
-    base::MessageLoop::current()->PostTask(
-        FROM_HERE,
-        base::Bind(&Animator::BeginFrame, weak_factory_.GetWeakPtr()));
+  if (outstanding_draw_requests_ >= kPipelineDepth) {
+    did_defer_frame_request_ = true;
+    return;
   }
+
+  base::MessageLoop::current()->PostTask(
+      FROM_HERE,
+      base::Bind(&Animator::BeginFrame, weak_factory_.GetWeakPtr()));
 }
 
 void Animator::Stop() {
@@ -49,31 +56,30 @@ void Animator::Start() {
 }
 
 void Animator::BeginFrame() {
-  DCHECK(frame_in_progress_);
-  // There could be a request in the message loop at time of cancel.
-  if (!engine_requested_frame_) {
-    frame_in_progress_ = false;
+  if (!engine_requested_frame_)
     return;
-  }
-
   engine_requested_frame_ = false;
   TRACE_EVENT_ASYNC_END0("sky", "Frame request pending", this);
 
   engine_->BeginFrame(base::TimeTicks::Now());
+  skia::RefPtr<SkPicture> picture = engine_->Paint();
+
+  outstanding_draw_requests_++;
+  DCHECK(outstanding_draw_requests_ <= kPipelineDepth);
   config_.gpu_task_runner->PostTaskAndReply(
       FROM_HERE,
-      base::Bind(&GPUDelegate::Draw, config_.gpu_delegate, engine_->Paint()),
+      base::Bind(&GPUDelegate::Draw, config_.gpu_delegate, picture),
       base::Bind(&Animator::OnFrameComplete, weak_factory_.GetWeakPtr()));
 }
 
 void Animator::OnFrameComplete() {
-  DCHECK(frame_in_progress_);
-  frame_in_progress_ = false;
+  DCHECK(outstanding_draw_requests_ > 0);
+  --outstanding_draw_requests_;
   if (paused_)
     return;
 
-  if (engine_requested_frame_) {
-    frame_in_progress_ = true;
+  if (engine_requested_frame_ && did_defer_frame_request_) {
+    did_defer_frame_request_ = false;
     BeginFrame();
   }
 }

sky/shell/ui/animator.h

@@ -27,8 +27,9 @@ class Animator {
 
   Engine::Config config_;
   Engine* engine_;
+  int outstanding_draw_requests_;
+  bool did_defer_frame_request_;
   bool engine_requested_frame_;
-  bool frame_in_progress_;
   bool paused_;
 
   base::WeakPtrFactory<Animator> weak_factory_;