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// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2018 Google Inc. All rights reserved.
// http://ceres-solver.org/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// * Redistributions of source code must retain the above copyright notice,
// this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above copyright notice,
// this list of conditions and the following disclaimer in the documentation
// and/or other materials provided with the distribution.
// * Neither the name of Google Inc. nor the names of its contributors may be
// used to endorse or promote products derived from this software without
// specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
// POSSIBILITY OF SUCH DAMAGE.
//
// Author: vitus@google.com (Michael Vitus)
#ifndef CERES_INTERNAL_THREAD_POOL_H_
#define CERES_INTERNAL_THREAD_POOL_H_
#include <functional>
#include <mutex>
#include <thread>
#include <vector>
#include "ceres/concurrent_queue.h"
namespace ceres {
namespace internal {
// A thread-safe thread pool with an unbounded task queue and a resizable number
// of workers. The size of the thread pool can be increased but never decreased
// in order to support the largest number of threads requested. The ThreadPool
// has three states:
//
// (1) The thread pool size is zero. Tasks may be added to the thread pool via
// AddTask but they will not be executed until the thread pool is resized.
//
// (2) The thread pool size is greater than zero. Tasks may be added to the
// thread pool and will be executed as soon as a worker is available. The
// thread pool may be resized while the thread pool is running.
//
// (3) The thread pool is destructing. The thread pool will signal all the
// workers to stop. The workers will finish all of the tasks that have already
// been added to the thread pool.
//
class ThreadPool {
public:
// Returns the maximum number of hardware threads.
static int MaxNumThreadsAvailable();
// Default constructor with no active threads. We allow instantiating a
// thread pool with no threads to support the use case of single threaded
// Ceres where everything will be executed on the main thread. For single
// threaded execution this has two benefits: avoid any overhead as threads
// are expensive to create, and no unused threads shown in the debugger.
ThreadPool();
// Instantiates a thread pool with min(MaxNumThreadsAvailable, num_threads)
// number of threads.
explicit ThreadPool(int num_threads);
// Signals the workers to stop and waits for them to finish any tasks that
// have been scheduled.
~ThreadPool();
// Resizes the thread pool if it is currently less than the requested number
// of threads. The thread pool will be resized to min(MaxNumThreadsAvailable,
// num_threads) number of threads. Resize does not support reducing the
// thread pool size. If a smaller number of threads is requested, the thread
// pool remains the same size. The thread pool is reused within Ceres with
// different number of threads, and we need to ensure we can support the
// largest number of threads requested. It is safe to resize the thread pool
// while the workers are executing tasks, and the resizing is guaranteed to
// complete upon return.
void Resize(int num_threads);
// Adds a task to the queue and wakes up a blocked thread. If the thread pool
// size is greater than zero, then the task will be executed by a currently
// idle thread or when a thread becomes available. If the thread pool has no
// threads, then the task will never be executed and the user should use
// Resize() to create a non-empty thread pool.
void AddTask(const std::function<void()>& func);
// Returns the current size of the thread pool.
int Size();
private:
// Main loop for the threads which blocks on the task queue until work becomes
// available. It will return if and only if Stop has been called.
void ThreadMainLoop();
// Signal all the threads to stop. It does not block until the threads are
// finished.
void Stop();
// The queue that stores the units of work available for the thread pool. The
// task queue maintains its own thread safety.
ConcurrentQueue<std::function<void()>> task_queue_;
std::vector<std::thread> thread_pool_;
std::mutex thread_pool_mutex_;
};
} // namespace internal
} // namespace ceres
#endif // CERES_INTERNAL_THREAD_POOL_H_
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