Bug 1745996 - Send a memory pressure event when deactivating sessions. r=jnicol,calu

WebRender retains about 50MBs of memory for every window. When switching
between lots of tabs on Android (where 1 tab = 1 window), this can cause
problems as the app will consume a significant amount of memory.

To avoid this problem, we send a memory pressure event whenever a session is
deactivated, which signals to WebRender that it should deallocate the memory.

This message is sent on a delay of 10s to avoid interfering with tab switching,
and we cancel the message if the tab becomes active again before we fire the
memory pressure event.

Co-Authored-By: Cathy Lu <calu@mozilla.com>
Co-Authored-By: Jonathan Almeida [:jonalmeida] <jonalmeida942@gmail.com>

Differential Revision: https://phabricator.services.mozilla.com/D136965

gfx/layers/ipc/CompositorBridgeParent.cpp

@@ -520,6 +520,11 @@ mozilla::ipc::IPCResult CompositorBridgeParent::RecvFlushRendering(
   return IPC_OK();
 }
 
+mozilla::ipc::IPCResult CompositorBridgeParent::RecvNotifyMemoryPressure() {
+  NotifyMemoryPressure();
+  return IPC_OK();
+}
+
 mozilla::ipc::IPCResult CompositorBridgeParent::RecvFlushRenderingAsync(
     const wr::RenderReasons& aReasons) {
   if (mWrBridge) {

gfx/layers/ipc/CompositorBridgeParent.h

@@ -242,6 +242,7 @@ class CompositorBridgeParentBase : public PCompositorBridgeParent,
       const LayersId& id, CompositorOptions* compositorOptions) = 0;
   virtual mozilla::ipc::IPCResult RecvFlushRendering(
       const wr::RenderReasons& aReasons) = 0;
+  virtual mozilla::ipc::IPCResult RecvNotifyMemoryPressure() = 0;
   virtual mozilla::ipc::IPCResult RecvWaitOnTransactionProcessed() = 0;
   virtual mozilla::ipc::IPCResult RecvStartFrameTimeRecording(
       const int32_t& bufferSize, uint32_t* startIndex) = 0;
@@ -324,6 +325,7 @@ class CompositorBridgeParent final : public CompositorBridgeParentBase,
     return IPC_OK();
   }
 
+  mozilla::ipc::IPCResult RecvNotifyMemoryPressure() override;
   mozilla::ipc::IPCResult RecvBeginRecording(
       const TimeStamp& aRecordingStart,
       BeginRecordingResolver&& aResolve) override;

gfx/layers/ipc/ContentCompositorBridgeParent.cpp

@@ -242,6 +242,12 @@ ContentCompositorBridgeParent::RecvMapAndNotifyChildCreated(
   return IPC_FAIL_NO_REASON(this);
 }
 
+mozilla::ipc::IPCResult
+ContentCompositorBridgeParent::RecvNotifyMemoryPressure() {
+  // This can only be called from the browser process.
+  return IPC_FAIL_NO_REASON(this);
+}
+
 mozilla::ipc::IPCResult ContentCompositorBridgeParent::RecvCheckContentOnlyTDR(
     const uint32_t& sequenceNum, bool* isContentOnlyTDR) {
   *isContentOnlyTDR = false;

gfx/layers/ipc/ContentCompositorBridgeParent.h

@@ -84,6 +84,8 @@ class ContentCompositorBridgeParent final : public CompositorBridgeParentBase {
     return IPC_OK();
   }
 
+  mozilla::ipc::IPCResult RecvNotifyMemoryPressure() override;
+
   mozilla::ipc::IPCResult RecvCheckContentOnlyTDR(
       const uint32_t& sequenceNum, bool* isContentOnlyTDR) override;
 

gfx/layers/ipc/PCompositorBridge.ipdl

@@ -173,6 +173,8 @@ parent:
   sync NotifyChildRecreated(LayersId id)
     returns (CompositorOptions compositorOptions);
 
+  async NotifyMemoryPressure();
+
   // Make sure any pending composites are started immediately and
   // block until they are completed.
   sync FlushRendering(RenderReasons aReasons);

mobile/android/geckoview/src/main/java/org/mozilla/gecko/util/ThreadUtils.java

@@ -60,6 +60,14 @@ public final class ThreadUtils {
     sUiHandler.post(runnable);
   }
 
+  public static void postToUiThreadDelayed(final Runnable runnable, final long delayMillis) {
+    sUiHandler.postDelayed(runnable, delayMillis);
+  }
+
+  public static void removeUiThreadCallbacks(final Runnable runnable) {
+    sUiHandler.removeCallbacks(runnable);
+  }
+
   public static Handler getBackgroundHandler() {
     return GeckoBackgroundThread.getHandler();
   }

mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java

@@ -90,6 +90,19 @@ public class GeckoSession {
 
   private static final int DATA_URI_MAX_LENGTH = 2 * 1024 * 1024;
 
+  // Delay running compositor memory pressure by 10s to avoid interfering with tab switching.
+  private static final int NOTIFY_MEMORY_PRESSURE_DELAY_MS = 10 * 1000;
+
+  private final Runnable mNotifyMemoryPressure =
+      new Runnable() {
+        @Override
+        public void run() {
+          if (mCompositorReady) {
+            mCompositor.notifyMemoryPressure();
+          }
+        }
+      };
+
   private enum State implements NativeQueue.State {
     INITIAL(0),
     READY(1);
@@ -195,6 +208,9 @@ public class GeckoSession {
     @WrapForJNI(calledFrom = "ui", dispatchTo = "gecko")
     public native void setDynamicToolbarMaxHeight(int height);
 
+    @WrapForJNI(calledFrom = "ui", dispatchTo = "gecko")
+    public native void notifyMemoryPressure();
+
     // Gecko thread pauses compositor; blocks UI thread.
     @WrapForJNI(calledFrom = "ui", dispatchTo = "current")
     public native void syncPauseCompositor();
@@ -2090,6 +2106,10 @@ public class GeckoSession {
 
     if (!active) {
       mEventDispatcher.dispatch("GeckoView:FlushSessionState", null);
+      ThreadUtils.postToUiThreadDelayed(mNotifyMemoryPressure, NOTIFY_MEMORY_PRESSURE_DELAY_MS);
+    } else {
+      // Delete any pending memory pressure events since we're active again.
+      ThreadUtils.removeUiThreadCallbacks(mNotifyMemoryPressure);
     }
 
     ThreadUtils.runOnUiThread(() -> getAutofillSupport().onActiveChanged(active));

widget/android/nsWindow.cpp

@@ -1117,6 +1117,21 @@ class LayerViewSupport final
     gkWindow->Resize(aLeft, aTop, aWidth, aHeight, /* repaint */ false);
   }
 
+  void NotifyMemoryPressure() {
+    MOZ_ASSERT(NS_IsMainThread());
+    auto acc = mWindow.Access();
+    if (!acc) {
+      return;  // Already shut down.
+    }
+
+    nsWindow* gkWindow = acc->GetNsWindow();
+    if (!gkWindow) {
+      return;
+    }
+
+    gkWindow->mCompositorBridgeChild->SendNotifyMemoryPressure();
+  }
+
   void SetDynamicToolbarMaxHeight(int32_t aHeight) {
     MOZ_ASSERT(NS_IsMainThread());
     auto acc = mWindow.Access();