content: Add RendererScheduler.

content/renderer/scheduler/renderer_scheduler_selector.h

+// Copyright 2014 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 CONTENT_RENDERER_SCHEDULER_RENDERER_SCHEDULER_POLICY_H_
+#define CONTENT_RENDERER_SCHEDULER_RENDERER_SCHEDULER_POLICY_H_
+
+#include <set>
+
+#include "base/threading/thread_checker.h"
+#include "content/renderer/scheduler/task_queue_selector.h"
+
+namespace content {
+
+class RendererSchedulerSelector : public TaskQueueSelector {
+ public:
+  enum QueuePriority {
+    kControlPriority,
+    kHighPriority,
+    kNormalPriority,
+    kBestEffortPriority,

[alexclarke, 2014/10/24 15:56:30]

Please document what kBestEffortPriority does (i.e. tasks from a
kBestEffortPriority queue are only run when the other queues are empty)

[rmcilroy, 2014/10/27 16:25:01]

Added documentation for all priorities (in other CL).

+    // Must be the last entry.
+    kQueuePriorityCount,
+    kFirstQueuePriority = kControlPriority,
+  };
+
+  RendererSchedulerSelector();
+  virtual ~RendererSchedulerSelector();
+
+  // Set the priority of |queue_index| to |set_priority|.
+  void SetQueuePriority(size_t queue_index, QueuePriority priority);
+
+  // Enable the |queue_index| with a priority of |priority|. By default all
+  // queues are enabled with normal priority.
+  void EnableQueue(size_t queue_index, QueuePriority priority);
+
+  // Disable the |queue_index|.
+  void DisableQueue(size_t queue_index);
+
+  // TaskQueueSelector implementation:
+  virtual void RegisterWorkQueues(
+      const std::vector<const base::TaskQueue*>& work_queues) override;
+  virtual bool SelectWorkQueueToService(size_t* out_queue_index) override;
+
+ private:
+  // Returns true if queueA contains an older task than queueB.
+  bool IsOlder(const base::TaskQueue* queueA,
+               const base::TaskQueue* queueB) const;
+
+  // Return true if |out_queue_index| indicates the index of the queue with
+  // the oldest pending task from the set of queues of |priority|, or
+  // false if all queues of that priority are empty.
+  bool ChooseOldestWithPriority(QueuePriority priority,
+                                size_t* out_queue_index) const;
+
+  // Returns true if |queue_index| is enabled with the given |priority|.
+  bool QueueEnabledWithPriority(size_t queue_index,
+                                QueuePriority priority) const;
+
+  // Number of high priority tasks which can be run before a normal priority
+  // task should be selected to prevent starvation.
+  // TODO(rmcilroy): Check if this is a good value.
+  static const int kMaxStarvationTasks = 5;
+
+  base::ThreadChecker main_thread_checker_;
+  std::vector<const base::TaskQueue*> work_queues_;
+  std::set<size_t> queue_priorities_[kQueuePriorityCount];
+  int normal_queue_starvation_count_;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_RENDERER_SCHEDULER_RENDERER_SCHEDULER_POLICY_H_