threadpool.h

//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#pragma once

#include <functional>

#include "rocksdb/rocksdb_namespace.h"

namespace ROCKSDB_NAMESPACE {

/*
 * ThreadPool is a component that will spawn N background threads that will
 * be used to execute scheduled work, The number of background threads could
 * be modified by calling SetBackgroundThreads().
 * */
class ThreadPool {
 public:
  virtual ~ThreadPool() {}

  // Wait for all threads to finish.
  // Discard those threads that did not start
  // executing
  virtual void JoinAllThreads() = 0;

  // Set the number of background threads that will be executing the
  // scheduled jobs.
  virtual void SetBackgroundThreads(int num) = 0;
  virtual int GetBackgroundThreads() = 0;

  // Get the number of jobs scheduled in the ThreadPool queue.
  virtual unsigned int GetQueueLen() const = 0;

  // Waits for all jobs to complete those
  // that already started running and those that did not
  // start yet. This ensures that everything that was thrown
  // on the TP runs even though
  // we may not have specified enough threads for the amount
  // of jobs
  virtual void WaitForJobsAndJoinAllThreads() = 0;

  // Submit a fire and forget jobs
  // This allows to submit the same job multiple times
  virtual void SubmitJob(const std::function<void()>&) = 0;
  // This moves the function in for efficiency
  virtual void SubmitJob(std::function<void()>&&) = 0;

  // Reserve available background threads. This function does not ensure
  // so many threads can be reserved, instead it will return the number of
  // threads that can be reserved against the desired one. In other words,
  // the number of available threads could be less than the input.
  virtual int ReserveThreads(int /*threads_to_be_reserved*/) { return 0; }

  // Release a specific number of reserved threads
  virtual int ReleaseThreads(int /*threads_to_be_released*/) { return 0; }
};

// NewThreadPool() is a function that could be used to create a ThreadPool
// with `num_threads` background threads.
extern ThreadPool* NewThreadPool(int num_threads);

}  // namespace ROCKSDB_NAMESPACE