text: /*
   * Copyright 2017 Facebook, Inc.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  #pragma once
  
  #include <atomic>
  
  #include <folly/executors/IOExecutor.h>
  #include <folly/executors/ThreadPoolExecutor.h>
  #include <folly/io/async/EventBaseManager.h>
  
  namespace folly {
  
  /**
   * A Thread Pool for IO bound tasks
   *
   * @note Uses event_fd for notification, and waking an epoll loop.
   * There is one queue (NotificationQueue specifically) per thread/epoll.
   * If the thread is already running and not waiting on epoll,
   * we don't make any additional syscalls to wake up the loop,
   * just put the new task in the queue.
   * If any thread has been waiting for more than a few seconds,
   * its stack is madvised away. Currently however tasks are scheduled round
   * robin on the queues, so unless there is no work going on,
   * this isn't very effective.
   * Since there is one queue per thread, there is hardly any contention
   * on the queues - so a simple spinlock around an std::deque is used for
   * the tasks. There is no max queue size.
   * By default, there is one thread per core - it usually doesn't make sense to
   * have more IO threads than this, assuming they don't block.
   *
   * @note ::getEventBase() will return an EventBase you can schedule IO work on
   * directly, chosen round-robin.
   *
   * @note N.B. For this thread pool, stop() behaves like join() because
   * outstanding tasks belong to the event base and will be executed upon its
   * destruction.
   */
  class IOThreadPoolExecutor : public ThreadPoolExecutor, public IOExecutor {
   public:
    explicit IOThreadPoolExecutor(
        size_t numThreads,
        std::shared_ptr<ThreadFactory> threadFactory =
            std::make_shared<NamedThreadFactory>("IOThreadPool"),
        folly::EventBaseManager* ebm = folly::EventBaseManager::get(),
        bool waitForAll = false);
  
    ~IOThreadPoolExecutor() override;
  
    void add(Func func) override;
    void add(
        Func func,
        std::chrono::milliseconds expiration,
        Func expireCallback = nullptr) override;
  
    folly::EventBase* getEventBase() override;
  
    static folly::EventBase* getEventBase(ThreadPoolExecutor::ThreadHandle*);
  
    folly::EventBaseManager* getEventBaseManager();
  
   private:
    struct FOLLY_ALIGN_TO_AVOID_FALSE_SHARING IOThread : public Thread {
      IOThread(IOThreadPoolExecutor* pool)
          : Thread(pool), shouldRun(true), pendingTasks(0) {}
      std::atomic<bool> shouldRun;
      std::atomic<size_t> pendingTasks;
      folly::EventBase* eventBase;
      std::mutex eventBaseShutdownMutex_;
    };
  
    ThreadPtr makeThread() override;
    std::shared_ptr<IOThread> pickThread();
    void threadRun(ThreadPtr thread) override;
    void stopThreads(size_t n) override;
    uint64_t getPendingTaskCountImpl(const RWSpinLock::ReadHolder&) override;
  
    std::atomic<size_t> nextThread_;
    folly::ThreadLocal<std::shared_ptr<IOThread>> thisThread_;
    folly::EventBaseManager* eventBaseManager_;
  };
  
  } // namespace folly
  