Add Thread Standby Policy SchedulerWorkerPoolImpl

base/task_scheduler/scheduler_worker_pool_params.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_PARAMS_H_
 #define BASE_TASK_SCHEDULER_SCHEDULER_WORKER_POOL_PARAMS_H_
 
 #include <string>
 
 #include "base/macros.h"
 #include "base/threading/platform_thread.h"
 
 namespace base {
 
 class TimeDelta;
 
 class BASE_EXPORT SchedulerWorkerPoolParams final {
  public:
   enum class IORestriction {
     ALLOWED,
     DISALLOWED,
   };
 
+  enum class StandbyThreadPolicy {
+    // Create threads as needed on demand, reclaimed as necessary.
+    LAZY,
+    // When possible, keep one idle thread alive on standby, reclaimed as
+    // necessary.
+    ONE,
+  };
+
   // Construct a scheduler worker pool parameter object. |name| will be used to
   // label the pool's threads ("TaskScheduler" + |name| + index) and histograms
   // ("TaskScheduler." + histogram name + "." + |name| + extra suffixes). The
   // pool will contain up to |max_threads|. |priority_hint| is the preferred
   // thread priority; the actual thread priority depends on shutdown state and
   // platform capabilities. |io_restriction| indicates whether Tasks on the pool
   // are allowed to make I/O calls. |suggested_reclaim_time| sets a suggestion
   // on when to reclaim idle threads. The pool is free to ignore this value for
   // performance or correctness reasons.
   SchedulerWorkerPoolParams(const std::string& name,
                             ThreadPriority priority_hint,
                             IORestriction io_restriction,
+                            StandbyThreadPolicy standby_thread_policy,
                             int max_threads,
                             const TimeDelta& suggested_reclaim_time);
   SchedulerWorkerPoolParams(SchedulerWorkerPoolParams&& other);
   SchedulerWorkerPoolParams& operator=(SchedulerWorkerPoolParams&& other);
 
   const std::string& name() const { return name_; }
   ThreadPriority priority_hint() const { return priority_hint_; }
   IORestriction io_restriction() const { return io_restriction_; }
+  StandbyThreadPolicy standby_thread_policy() const {
+    return standby_thread_policy_;
+  }
   size_t max_threads() const { return max_threads_; }
   const TimeDelta& suggested_reclaim_time() const {
     return suggested_reclaim_time_;
   }
 
  private:
   std::string name_;
   ThreadPriority priority_hint_;
   IORestriction io_restriction_;
+  StandbyThreadPolicy standby_thread_policy_;
   size_t max_threads_;
   TimeDelta suggested_reclaim_time_;
 
   DISALLOW_COPY_AND_ASSIGN(SchedulerWorkerPoolParams);
 };
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_SCHEDULER_WORKER_POOL_PARAMS_H_