content: Add RendererTaskQueueSelector.

content/renderer/scheduler/renderer_scheduler_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_scheduler_selector.h"
+
+#include "base/logging.h"
+#include "base/pending_task.h"
+
+namespace content {
+
+RendererSchedulerSelector::RendererSchedulerSelector() : starvation_count_(0) {
+}
+
+RendererSchedulerSelector::~RendererSchedulerSelector() {
+}
+
+void RendererSchedulerSelector::RegisterWorkQueues(
+    const std::vector<const base::TaskQueue*>& work_queues) {
+  main_thread_checker_.CalledOnValidThread();
+  work_queues_ = work_queues;
+  for (QueuePriority priority = kFirstQueuePriority;
+       priority < kQueuePriorityCount;
+       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_[kNormalPriority].insert(i);
+  }
+}
+
+void RendererSchedulerSelector::SetQueuePriority(size_t queue_index,
+                                                 QueuePriority priority) {
+  main_thread_checker_.CalledOnValidThread();
+  DCHECK_LT(queue_index, work_queues_.size());
+  DCHECK_LT(priority, kQueuePriorityCount);
+  DisableQueue(queue_index);
+  queue_priorities_[priority].insert(queue_index);
+}
+
+void RendererSchedulerSelector::EnableQueue(size_t queue_index,
+                                            QueuePriority priority) {
+  SetQueuePriority(queue_index, priority);
+}
+
+void RendererSchedulerSelector::DisableQueue(size_t queue_index) {
+  main_thread_checker_.CalledOnValidThread();
+  DCHECK_LT(queue_index, work_queues_.size());
+  for (QueuePriority priority = kFirstQueuePriority;
+       priority < kQueuePriorityCount;
+       priority = NextPriority(priority)) {
+    queue_priorities_[priority].erase(queue_index);
+  }
+}
+
+bool RendererSchedulerSelector::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();
+}
+
+RendererSchedulerSelector::QueuePriority
+RendererSchedulerSelector::NextPriority(QueuePriority priority) {
+  DCHECK(priority < kQueuePriorityCount);
+  return static_cast<QueuePriority>(static_cast<int>(priority) + 1);
+}
+
+bool RendererSchedulerSelector::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]) {

[no sievers, 2014/10/28 01:21:00]

Regarding the implementation, isn't it simpler to just maintain one ordered list
of tasks queue indices (ordered by priority)?

[rmcilroy, 2014/10/28 12:37:41]

This wouldn't quite work as we want.  We want to have multiple queues that have
the same priority, and choose from a particular priority based on task age of
all the queues with that priority.  A simple ordered list would give each queue
a different priority depending on it's position in the list, which would make it
difficult to do this task-age choice for queue's which are meant to be of the
same priority.

We also want different starving semantics for each priority - control should
starve everything, but high should not starve normal, and best effort is allowed
to be starved by any other queue.  Again I think this would be tricky to
incorporate in a single ordered list.

+    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 RendererSchedulerSelector::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(kControlPriority, out_queue_index)) {
+    return true;
+  }
+  // Select from the normal priority queue if we are starving it.
+  if (starvation_count_ >= kMaxStarvationTasks &&
+      ChooseOldestWithPriority(kNormalPriority, out_queue_index)) {
+    starvation_count_ = 0;
+    return true;
+  }
+  // Otherwise choose in priority order.
+  for (QueuePriority priority = kHighPriority; priority < kQueuePriorityCount;
+       priority = NextPriority(priority)) {
+    if (ChooseOldestWithPriority(priority, out_queue_index)) {
+      if (priority == kHighPriority) {
+        starvation_count_++;
+      } else {
+        starvation_count_ = 0;
+      }
+      return true;
+    }
+  }
+  return false;
+}
+
+}  // namespace content