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 <unordered_map>
 #include <vector>
 
@@ skipping 33 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();
 
+  // Resets any pending image decode requests that have not been scheduled for
+  // decode already. Note that calling this means that tiles which have already
+  // been checker-imaged need to be filtered on the tracker again to
+  // re-rasterize them.
+  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);
 
+  // Called when the next request in the |image_decode_queue_| should be
+  // scheduled with the image decode service.
+  void ScheduleNextImageDecode();
+
   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.
+  // The first image in this queue, if any, is the currently outstanding decode
+  // request given to the queue.
+  std::list<sk_sp<const SkImage>> image_decode_request_queue_;

[vmpstr, 2017/03/07 19:25:08]

Can you write a cc_perftest for repeated schedules/cancels to see if std::list
performs better than std::vector (this can be a one-off)

[Khushal, 2017/03/13 20:35:08]

I guess not necessary anymore. We iterate through the queue to pull the next
image to decode and just clear it at the end.

+
   // 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_