content: Add RendererTaskQueueSelector.

content/renderer/scheduler/renderer_task_queue_selector.cc

+// 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.
+
+#include "content/renderer/scheduler/renderer_task_queue_selector.h"
+
+#include "base/logging.h"
+#include "base/pending_task.h"
+
+namespace content {
+
+RendererTaskQueueSelector::RendererTaskQueueSelector() : starvation_count_(0) {
+}
+
+RendererTaskQueueSelector::~RendererTaskQueueSelector() {
+}
+
+void RendererTaskQueueSelector::RegisterWorkQueues(
+    const std::vector<const base::TaskQueue*>& work_queues) {
+  main_thread_checker_.CalledOnValidThread();
+  work_queues_ = work_queues;
+  for (QueuePriority priority = FIRST_QUEUE_PRIORITY;
+       priority < QUEUE_PRIORITY_COUNT;
+       priority = NextPriority(priority)) {
+    queue_priorities_[priority].clear();
+  }
+  // By default, all work queues are set to normal priority.
+  for (size_t i = 0; i < work_queues.size(); i++) {
+    queue_priorities_[NORMAL_PRIORITY].insert(i);
+  }
+}
+
+void RendererTaskQueueSelector::SetQueuePriority(size_t queue_index,
+                                                 QueuePriority priority) {
+  main_thread_checker_.CalledOnValidThread();
+  DCHECK_LT(queue_index, work_queues_.size());
+  DCHECK_LT(priority, QUEUE_PRIORITY_COUNT);
+  DisableQueue(queue_index);
+  queue_priorities_[priority].insert(queue_index);
+}
+
+void RendererTaskQueueSelector::EnableQueue(size_t queue_index,
+                                            QueuePriority priority) {
+  SetQueuePriority(queue_index, priority);
+}
+
+void RendererTaskQueueSelector::DisableQueue(size_t queue_index) {
+  main_thread_checker_.CalledOnValidThread();
+  DCHECK_LT(queue_index, work_queues_.size());
+  for (QueuePriority priority = FIRST_QUEUE_PRIORITY;
+       priority < QUEUE_PRIORITY_COUNT;
+       priority = NextPriority(priority)) {
+    queue_priorities_[priority].erase(queue_index);
+  }
+}
+
+bool RendererTaskQueueSelector::IsOlder(const base::TaskQueue* queueA,
+                                        const base::TaskQueue* queueB) {
+  // Note: the comparison is correct due to the fact that the PendingTask
+  // operator inverts its comparison operation in order to work well in a heap
+  // based priority queue.
+  return queueB->front() < queueA->front();
+}
+
+RendererTaskQueueSelector::QueuePriority
+RendererTaskQueueSelector::NextPriority(QueuePriority priority) {
+  DCHECK(priority < QUEUE_PRIORITY_COUNT);
+  return static_cast<QueuePriority>(static_cast<int>(priority) + 1);
+}
+
+bool RendererTaskQueueSelector::ChooseOldestWithPriority(
+    QueuePriority priority,
+    size_t* out_queue_index) const {
+  bool found_non_empty_queue = false;
+  size_t chosen_queue = 0;
+  for (int queue_index : queue_priorities_[priority]) {
+    if (work_queues_[queue_index]->empty()) {
+      continue;
+    }
+    if (!found_non_empty_queue ||
+        IsOlder(work_queues_[queue_index], work_queues_[chosen_queue])) {
+      found_non_empty_queue = true;
+      chosen_queue = queue_index;
+    }
+  }
+
+  if (found_non_empty_queue) {
+    *out_queue_index = chosen_queue;
+  }
+  return found_non_empty_queue;
+}
+
+bool RendererTaskQueueSelector::SelectWorkQueueToService(
+    size_t* out_queue_index) {
+  main_thread_checker_.CalledOnValidThread();
+  DCHECK(work_queues_.size());
+  // Always service the control queue if it has any work.
+  if (ChooseOldestWithPriority(CONTROL_PRIORITY, out_queue_index)) {
+    return true;
+  }
+  // Select from the normal priority queue if we are starving it.
+  if (starvation_count_ >= kMaxStarvationTasks &&
+      ChooseOldestWithPriority(NORMAL_PRIORITY, out_queue_index)) {
+    starvation_count_ = 0;
+    return true;
+  }
+  // Otherwise choose in priority order.
+  for (QueuePriority priority = HIGH_PRIORITY; priority < QUEUE_PRIORITY_COUNT;
+       priority = NextPriority(priority)) {
+    if (ChooseOldestWithPriority(priority, out_queue_index)) {
+      if (priority == HIGH_PRIORITY) {
+        starvation_count_++;
+      } else {
+        starvation_count_ = 0;
+      }
+      return true;
+    }
+  }
+  return false;
+}
+
+}  // namespace content