cc: Optimize decode scheduling for checker-images.

cc/tiles/checker_image_tracker.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TILES_CHECKER_IMAGE_TRACKER_H_
 #define CC_TILES_CHECKER_IMAGE_TRACKER_H_
 
+#include <queue>
 #include <unordered_map>
 #include <vector>
 
 #include "cc/base/cc_export.h"
 #include "cc/playback/image_id.h"
 #include "cc/tiles/image_controller.h"
 #include "third_party/skia/include/core/SkImage.h"
 
 namespace cc {
 
@@ skipping 26 matching lines @@
   // The entries remaining in |images| are for images for which a cached decode
   // from the image decode service is available, or which must be decoded before
   // before this tile can be rasterized.
   void FilterImagesForCheckeringForTile(std::vector<DrawImage>* images,
                                         ImageIdFlatSet* checkered_images,
                                         WhichTree tree);
 
   // Returns the set of images to invalidate on the sync tree.
   const ImageIdFlatSet& TakeImagesToInvalidateOnSyncTree();
 
+  void ResetImageDecodeQueue();
+
   void DidActivateSyncTree();
 
  private:
   void DidFinishImageDecode(ImageId image_id,
                             ImageController::ImageDecodeRequestId request_id,
                             ImageController::ImageDecodeResult result);
 
   // Returns true if the decode for |image| will be deferred to the image decode
   // service and it should be be skipped during raster.
   bool ShouldCheckerImage(const sk_sp<const SkImage>& image,
                           WhichTree tree) const;
 
   void ScheduleImageDecodeIfNecessary(const sk_sp<const SkImage>& image);
+  void RequestNextImageDecode();

[vmpstr, 2017/03/06 20:45:25]

Comment, and if this is what I think it is, then it should be called
ScheduleNextImageDecode

[Khushal, 2017/03/07 00:31:08]

Done.

 
   ImageController* image_controller_;
   CheckerImageTrackerClient* client_;
   const bool enable_checker_imaging_;
 
   // A set of images which have been decoded and are pending invalidation for
   // raster on the checkered tiles.
   ImageIdFlatSet images_pending_invalidation_;
 
   // A set of images which were invalidated on the current sync tree.
   ImageIdFlatSet invalidated_images_on_current_sync_tree_;
 
-  // A set of images which are currently pending decode from the image decode
-  // service.
-  // TODO(khushalsagar): This should be a queue that gets re-built each time we
-  // do a PrepareTiles? See crbug.com/689184.
+  // The set of images in the image decode queue.
   ImageIdFlatSet pending_image_decodes_;
 
+  // The queue of images pending decode. We maintain a queue to ensure that the
+  // order in which images are decoded is aligned with the priority of the tiles
+  // dependent on these images.
+  std::queue<sk_sp<const SkImage>> image_decode_request_queue_;

[vmpstr, 2017/03/06 20:45:25]

std::queue's underlying type is std::deque which is a big memory hog as we've
discovered. Since this is just sk_sp (small objects), I think we should just use
a vector. 

[Khushal, 2017/03/07 00:31:09]

Done.

+
   // A set of images which have been decoded at least once from the
   // ImageDecodeService and should not be checkered again.
   // TODO(khushalsagar): Limit the size of this set.
   // TODO(khushalsagar): Plumb navigation changes here to reset this. See
   // crbug.com/693228.
   std::unordered_set<ImageId> images_decoded_once_;
 
   // A map of image id to image decode request id for images to be unlocked.
   std::unordered_map<ImageId, ImageController::ImageDecodeRequestId>
       image_id_to_decode_request_id_;
 
   base::WeakPtrFactory<CheckerImageTracker> weak_factory_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TILES_CHECKER_IMAGE_TRACKER_H_