cc: Decrease transfer buffer memory limit to a single frame of uploads.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index e293a5aa9ad34723305fe31def6293258d0cad95..377e05cf61b155b3e2e1ec036a64cc3527e23f5a 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -89,7 +89,8 @@ void DidVisibilityChange(cc::LayerTreeHostImpl* id, bool visible) {
   TRACE_EVENT_ASYNC_END0("webkit", "LayerTreeHostImpl::SetVisible", id);
 }
 
-size_t GetMaxTransferBufferUsageBytes(cc::ContextProvider* context_provider) {
+size_t GetMaxTransferBufferUsageBytes(cc::ContextProvider* context_provider,
+                                      double refresh_rate) {
   // Software compositing should not use this value in production. Just use a
   // default value when testing uploads with the software compositor.
   if (!context_provider)
@@ -100,13 +101,17 @@ size_t GetMaxTransferBufferUsageBytes(cc::ContextProvider* context_provider) {
   // the pipeline.
   // For reference Chromebook Pixel can upload 1MB in about 0.5ms.
   const size_t kMaxBytesUploadedPerMs = 1024 * 1024 * 2;
-  // Assuming a two frame deep pipeline between CPU and GPU and we are
-  // drawing 60 frames per second which would require us to draw one
-  // frame in 16 milliseconds.
-  const size_t kMaxTransferBufferUsageBytes = 16 * 2 * kMaxBytesUploadedPerMs;
+
+  // This is the number of bytes the compositor would be able to upload before
+  // the next frame deadline.

[reveman, 2014/08/30 00:38:39]

maybe worth mentioning that this assumes no more than a one frame deep pipeline
between CPU and GPU. and if we have more than that it might cause us to issue
less uploads than we can perform.  

[danakj, 2014/08/30 00:51:38]

I added something, let me know if you think it's a lie :)

+  const size_t ms_per_frame = std::floor(1000.0 / refresh_rate);
+  const size_t max_transfer_buffer_usage_bytes =
+      ms_per_frame * kMaxBytesUploadedPerMs;

[reveman, 2014/08/30 00:38:39]

nit: up to you but I'd get rid of the 'const' modifier here

[danakj, 2014/08/30 00:42:32]

Done.

+
+  // The context may request a higher limit based on the device capabilities.

[reveman, 2014/08/30 00:38:39]

nit: lower limit based on the device capabilities.

[danakj, 2014/08/30 00:42:32]

I would have to change std::min to std::max then. Right now it can only increase
the number. Maybe this points to a bug, I will file one.

[danakj, 2014/08/30 00:44:28]

Oh, no I agree with you. Fridays~

   return std::min(
       context_provider->ContextCapabilities().max_transfer_buffer_usage_bytes,
-      kMaxTransferBufferUsageBytes);
+      max_transfer_buffer_usage_bytes);
 }
 
 unsigned GetMapImageTextureTarget(cc::ContextProvider* context_provider) {
@@ -1939,7 +1944,7 @@ void LayerTreeHostImpl::CreateAndSetTileManager() {
 
   ContextProvider* context_provider = output_surface_->context_provider();
   transfer_buffer_memory_limit_ =
-      GetMaxTransferBufferUsageBytes(context_provider);
+      GetMaxTransferBufferUsageBytes(context_provider, settings_.refresh_rate);
 
   if (use_gpu_rasterization_ && context_provider) {
     resource_pool_ =
@@ -2066,9 +2071,6 @@ bool LayerTreeHostImpl::InitializeRenderer(
 
   CreateAndSetRenderer();
 
-  transfer_buffer_memory_limit_ =

[danakj, 2014/08/30 00:29:00]

this is also set in CreateAndSetTileManager() which is called next.

-      GetMaxTransferBufferUsageBytes(output_surface_->context_provider());
-
   if (settings_.impl_side_painting)
     CreateAndSetTileManager();