5 files changed, 32 insertions, 0 deletions
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 3b97191..fd0d1d3 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1102,6 +1102,9 @@ DrawResult LayerTreeHostImpl::PrepareToDraw(FrameData* frame) {
   }
   need_to_update_visible_tiles_before_draw_ = true;
 
+  UMA_HISTOGRAM_CUSTOM_COUNTS(
+      "Compositing.NumActiveLayers", active_tree_->NumLayers(), 1, 400, 20);
+
   bool ok = active_tree_->UpdateDrawProperties();
   DCHECK(ok) << "UpdateDrawProperties failed during draw";
 
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 908e993..bf0a996 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -561,6 +561,10 @@ void LayerTreeImpl::UnregisterLayer(LayerImpl* layer) {
   layer_id_map_.erase(layer->id());
 }
 
+size_t LayerTreeImpl::NumLayers() {
+  return layer_id_map_.size();
+}
+
 void LayerTreeImpl::PushPersistedState(LayerTreeImpl* pending_tree) {
   pending_tree->SetCurrentlyScrollingLayer(
       LayerTreeHostCommon::FindLayerInSubtree(pending_tree->root_layer(),
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index ea636ca..13746c3 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -205,6 +205,8 @@ class CC_EXPORT LayerTreeImpl {
   void RegisterLayer(LayerImpl* layer);
   void UnregisterLayer(LayerImpl* layer);
 
+  size_t NumLayers();
+
   AnimationRegistrar* animationRegistrar() const;
 
   void PushPersistedState(LayerTreeImpl* pending_tree);
diff --git a/cc/trees/layer_tree_impl_unittest.cc b/cc/trees/layer_tree_impl_unittest.cc
index 9edf838..d215349 100644
--- a/cc/trees/layer_tree_impl_unittest.cc
+++ b/cc/trees/layer_tree_impl_unittest.cc
@@ -2319,5 +2319,20 @@ TEST_F(LayerTreeImplTest, SelectionBoundsForScaledLayers) {
   EXPECT_TRUE(right_output.visible);
 }
 
+TEST_F(LayerTreeImplTest, NumLayersTestOne) {
+  EXPECT_EQ(0u, host_impl().active_tree()->NumLayers());
+  scoped_ptr<LayerImpl> root = LayerImpl::Create(host_impl().active_tree(), 1);
+  EXPECT_EQ(1u, host_impl().active_tree()->NumLayers());
+}
+
+TEST_F(LayerTreeImplTest, NumLayersSmallTree) {
+  EXPECT_EQ(0u, host_impl().active_tree()->NumLayers());
+  scoped_ptr<LayerImpl> root = LayerImpl::Create(host_impl().active_tree(), 1);
+  root->AddChild(LayerImpl::Create(host_impl().active_tree(), 2));
+  root->AddChild(LayerImpl::Create(host_impl().active_tree(), 3));
+  root->child_at(1)->AddChild(LayerImpl::Create(host_impl().active_tree(), 4));
+  EXPECT_EQ(4u, host_impl().active_tree()->NumLayers());
+}
+
 }  // namespace
 }  // namespace cc
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 1374c6a..6a00a56 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -2743,6 +2743,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Compositing.NumActiveLayers">
+  <owner>dneto@chromium.org</owner>
+  <summary>
+    The number of layers in the active tree for each compositor frame. This is
+    logged once per frame, before the frame is drawn.
+  </summary>
+</histogram>
+
 <histogram name="ConnectivityDiagnostics.ChromeOsSignalStrength"
     units="percent">
   <owner>ebeach@google.com</owner>