[reference - do not submit] Always create four pools in TaskSchedulerImpl.

base/task_scheduler/scheduler_worker_pool_impl.h

 // Copyright 2016 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 BASE_TASK_SCHEDULER_SCHEDULER_WORKER_POOL_IMPL_H_
 #define BASE_TASK_SCHEDULER_SCHEDULER_WORKER_POOL_IMPL_H_
 
 #include <stddef.h>
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/synchronization/atomic_flag.h"
 #include "base/synchronization/condition_variable.h"
 #include "base/task_scheduler/priority_queue.h"
 #include "base/task_scheduler/scheduler_lock.h"
 #include "base/task_scheduler/scheduler_worker.h"
 #include "base/task_scheduler/scheduler_worker_pool.h"
 #include "base/task_scheduler/scheduler_worker_stack.h"
 #include "base/task_scheduler/sequence.h"
 #include "base/task_scheduler/task.h"
+#include "base/threading/platform_thread.h"
 #include "base/time/time.h"
 
 namespace base {
 
 class HistogramBase;
 class SchedulerWorkerPoolParams;
 class TaskTraits;
 
 namespace internal {
 
 class DelayedTaskManager;
 class TaskTracker;
 
 // A pool of workers that run Tasks. This class is thread-safe.
 class BASE_EXPORT SchedulerWorkerPoolImpl : public SchedulerWorkerPool {
  public:
   // Callback invoked when a Sequence isn't empty after a worker pops a Task
   // from it.
   using ReEnqueueSequenceCallback = Callback<void(scoped_refptr<Sequence>)>;
 
   // Destroying a SchedulerWorkerPoolImpl returned by Create() is not allowed in
   // production; it is always leaked. In tests, it can only be destroyed after
   // JoinForTesting() has returned.
   ~SchedulerWorkerPoolImpl() override;
 
-  // Creates a SchedulerWorkerPoolImpl following the |worker_pool_params|
-  // specification. |re_enqueue_sequence_callback| will be invoked after a
-  // worker of this worker pool tries to run a Task. |task_tracker| is used to
-  // handle shutdown behavior of Tasks. |delayed_task_manager| handles Tasks
-  // posted with a delay. Returns nullptr on failure to create a worker pool
-  // with at least one thread.
+  // Creates a SchedulerWorkerPoolImpl. |name| is used to label the pool's
+  // threads ("TaskScheduler" + |name| + index) and histograms ("TaskScheduler."
+  // + histogram name + "." + |name|). |priority_hint| is the preferred thread
+  // priority; the actual thread priority depends on shutdown state and platform
+  // capabilities. |params| contains additional thread creation params.
+  // |re_enqueue_sequence_callback| is invoked after a worker tries to run a
+  // Task. |task_tracker| is used to handle shutdown behavior of Tasks.
+  // |delayed_task_manager| handles Tasks posted with a delay. Returns nullptr
+  // on failure to create a worker pool with at least one thread.
   static std::unique_ptr<SchedulerWorkerPoolImpl> Create(
+      const std::string& name,
+      ThreadPriority priority_hint,
       const SchedulerWorkerPoolParams& params,
       const ReEnqueueSequenceCallback& re_enqueue_sequence_callback,
       TaskTracker* task_tracker,
       DelayedTaskManager* delayed_task_manager);
 
   // SchedulerWorkerPool:
   scoped_refptr<TaskRunner> CreateTaskRunnerWithTraits(
       const TaskTraits& traits) override;
   scoped_refptr<SequencedTaskRunner> CreateSequencedTaskRunnerWithTraits(
       const TaskTraits& traits) override;
@@ skipping 32 matching lines @@
   // threads, and as a result, threads running after JoinForTesting().
   void DisallowWorkerDetachmentForTesting();
 
   // Returns the number of workers alive in this worker pool. The value may
   // change if workers are woken up or detached during this call.
   size_t NumberOfAliveWorkersForTesting();
 
  private:
   class SchedulerWorkerDelegateImpl;
 
-  SchedulerWorkerPoolImpl(const SchedulerWorkerPoolParams& params,
+  SchedulerWorkerPoolImpl(const std::string& name,
+                          const SchedulerWorkerPoolParams& params,
                           TaskTracker* task_tracker,
                           DelayedTaskManager* delayed_task_manager);
 
   bool Initialize(
+      ThreadPriority priority_hint,
       const SchedulerWorkerPoolParams& params,
       const ReEnqueueSequenceCallback& re_enqueue_sequence_callback);
 
   // Wakes up the last worker from this worker pool to go idle, if any.
   void WakeUpOneWorker();
 
   // Adds |worker| to |idle_workers_stack_|.
   void AddToIdleWorkersStack(SchedulerWorker* worker);
 
   // Peeks from |idle_workers_stack_|.
   const SchedulerWorker* PeekAtIdleWorkersStack() const;
 
   // Removes |worker| from |idle_workers_stack_|.
   void RemoveFromIdleWorkersStack(SchedulerWorker* worker);
 
   // Returns true if worker thread detachment is permitted.
   bool CanWorkerDetachForTesting();
 
-  // The name of this worker pool, used to label its worker threads.
+  // The name of this worker pool, used to label threads and histograms.
   const std::string name_;
 
   // All worker owned by this worker pool. Only modified during initialization
   // of the worker pool.
   std::vector<scoped_refptr<SchedulerWorker>> workers_;
 
   // PriorityQueue from which all threads of this worker pool get work.
   PriorityQueue shared_priority_queue_;
 
   // Suggested reclaim time for workers.
@@ skipping 43 matching lines @@
   TaskTracker* const task_tracker_;
   DelayedTaskManager* const delayed_task_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(SchedulerWorkerPoolImpl);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_SCHEDULER_WORKER_POOL_IMPL_H_