diff --git a/gfx/layers/ipc/CompositorParent.cpp b/gfx/layers/ipc/CompositorParent.cpp
index 08d4fde3b624..e67091b0382e 100644
--- a/gfx/layers/ipc/CompositorParent.cpp
+++ b/gfx/layers/ipc/CompositorParent.cpp
@@ -682,6 +682,8 @@ CompositorParent::CompositorParent(nsIWidget* aWidget,
     mCompositorScheduler = new CompositorSoftwareTimerScheduler(this);
   }
 
+  gfxPlatform::GetPlatform()->ComputeTileSize();
+
   LayerScope::SetPixelScale(mWidget->GetDefaultScale().scale);
 }
 
@@ -1733,6 +1735,7 @@ public:
     , mNotifyAfterRemotePaint(false)
   {
     MOZ_ASSERT(NS_IsMainThread());
+    gfxPlatform::GetPlatform()->ComputeTileSize();
   }
 
   // IToplevelProtocol::CloneToplevel()
diff --git a/gfx/tests/gtest/TestTiledLayerBuffer.cpp b/gfx/tests/gtest/TestTiledLayerBuffer.cpp
index c5e9b0f41d31..03b18ea71be5 100644
--- a/gfx/tests/gtest/TestTiledLayerBuffer.cpp
+++ b/gfx/tests/gtest/TestTiledLayerBuffer.cpp
@@ -11,6 +11,8 @@ namespace mozilla {
 namespace layers {
 
 TEST(TiledLayerBuffer, TileStart) {
+  gfxPlatform::GetPlatform()->ComputeTileSize();
+
   ASSERT_EQ(RoundDownToTileEdge(10, 256), 0);
   ASSERT_EQ(RoundDownToTileEdge(-10, 256), -256);
 }
diff --git a/gfx/thebes/gfxPlatform.cpp b/gfx/thebes/gfxPlatform.cpp
index c0c5adfe483b..f746df0d83af 100644
--- a/gfx/thebes/gfxPlatform.cpp
+++ b/gfx/thebes/gfxPlatform.cpp
@@ -504,8 +504,6 @@ gfxPlatform::Init()
     InitLayersAccelerationPrefs();
     InitLayersIPC();
 
-    gPlatform->ComputeTileSize();
-
     nsresult rv;
 
     bool usePlatformFontList = true;
@@ -1004,7 +1002,7 @@ gfxPlatform::ComputeTileSize()
   // The tile size should be picked in the parent processes
   // and sent to the child processes over IPDL GetTileSize.
   if (!XRE_IsParentProcess()) {
-    return;
+    NS_RUNTIMEABORT("wrong process.");
   }
 
   int32_t w = gfxPrefs::LayersTileWidth();
diff --git a/gfx/thebes/gfxPlatform.h b/gfx/thebes/gfxPlatform.h
index 02fe6082c2cc..7f835a516a5b 100644
--- a/gfx/thebes/gfxPlatform.h
+++ b/gfx/thebes/gfxPlatform.h
@@ -307,6 +307,12 @@ public:
     int GetTileWidth();
     int GetTileHeight();
     void SetTileSize(int aWidth, int aHeight);
+    /**
+     * Calling this function will compute and set the ideal tile size for the
+     * platform. This should only be called in the parent process; child processes
+     * should be updated via SetTileSize to match the value computed in the parent.
+     */
+    void ComputeTileSize();
 
     /**
      * Rebuilds the any cached system font lists
@@ -754,13 +760,6 @@ private:
 
     virtual void GetPlatformCMSOutputProfile(void *&mem, size_t &size);
 
-    /**
-     * Calling this function will compute and set the ideal tile size for the
-     * platform. This will only have an effect in the parent process; child processes
-     * should be updated via SetTileSize to match the value computed in the parent.
-     */
-    void ComputeTileSize();
-
     nsRefPtr<gfxASurface> mScreenReferenceSurface;
     nsTArray<uint32_t> mCJKPrefLangs;
     nsCOMPtr<nsIObserver> mSRGBOverrideObserver;