Bug 873378 - Add a limit on mobile of 20 layers per container layer. r=roc

layout/base/FrameLayerBuilder.cpp

@@ -2041,6 +2041,9 @@ ContainerState::ProcessDisplayItems(const nsDisplayList& aList,
     topLeft = lastActiveScrolledRoot->GetOffsetToCrossDoc(mContainerReferenceFrame);
   }
 
+  int32_t maxLayers = nsDisplayItem::MaxActiveLayers();
+  int layerCount = 0;
+
   for (nsDisplayItem* item = aList.GetBottom(); item; item = item->GetAbove()) {
     NS_ASSERTION(mAppUnitsPerDevPixel == AppUnitsPerDevPixel(item),
       "items in a container layer should all have the same app units per dev pixel");
@@ -2083,6 +2086,10 @@ ContainerState::ProcessDisplayItems(const nsDisplayList& aList,
       }
     }
 
+    if (maxLayers != -1 && layerCount >= maxLayers) {
+      forceInactive = true;
+    }
+
     // Assign the item to a layer
     if (layerState == LAYER_ACTIVE_FORCE ||
         (layerState == LAYER_INACTIVE && !mManager->IsWidgetLayerManager()) ||
@@ -2090,6 +2097,8 @@ ContainerState::ProcessDisplayItems(const nsDisplayList& aList,
          (layerState == LAYER_ACTIVE_EMPTY ||
           layerState == LAYER_ACTIVE))) {
 
+      layerCount++;
+
       // LAYER_ACTIVE_EMPTY means the layer is created just for its metadata.
       // We should never see an empty layer with any visible content!
       NS_ASSERTION(layerState != LAYER_ACTIVE_EMPTY ||

layout/base/nsDisplayList.cpp

@@ -1497,6 +1497,20 @@ nsDisplayItem::ForceActiveLayers()
   return sForce;
 }
 
+/* static */ int32_t
+nsDisplayItem::MaxActiveLayers()
+{
+  static int32_t sMaxLayers = false;
+  static bool sMaxLayersCached = false;
+
+  if (!sMaxLayersCached) {
+    Preferences::AddIntVarCache(&sMaxLayers, "layers.max-active", -1);
+    sMaxLayersCached = true;
+  }
+
+  return sMaxLayersCached;
+}
+
 bool
 nsDisplayItem::RecomputeVisibility(nsDisplayListBuilder* aBuilder,
                                    nsRegion* aVisibleRegion) {

layout/base/nsDisplayList.h

@@ -1003,6 +1003,12 @@ public:
    */
   static bool ForceActiveLayers();
 
+  /**
+   * Returns the maximum number of layers that should be created
+   * or -1 for no limit. Requires setting the pref layers.max-acitve.
+   */
+  static int32_t MaxActiveLayers();
+
   /**
    * @return LAYER_NONE if BuildLayer will return null. In this case
    * there is no layer for the item, and Paint should be called instead

mobile/android/app/mobile.js

@@ -546,6 +546,12 @@ pref("layers.async-video.enabled", true);
 pref("layers.progressive-paint", true);
 pref("layers.low-precision-buffer", true);
 pref("layers.low-precision-resolution", 250);
+// We want to limit layers for two reasons:
+// 1) We can't scroll smoothly if we have to many draw calls
+// 2) Pages that have too many layers consume too much memory and crash.
+// By limiting the number of layers on mobile we're making the main thread
+// work harder keep scrolling smooth and memory low.
+pref("layers.max-active", 20);
 
 pref("notification.feature.enabled", true);
 pref("dom.webnotifications.enabled", true);

modules/libpref/src/init/all.js

@@ -4053,6 +4053,10 @@ pref("layers.acceleration.draw-fps", false);
 pref("layers.draw-borders", false);
 pref("layers.frame-counter", false);
 
+// Max number of layers per container. See Overwrite in mobile prefs.
+pref("layers.max-active", -1);
+
+
 #ifdef XP_MACOSX
 pref("layers.offmainthreadcomposition.enabled", true);
 #else