A simple C++11 Thread Pool implementation
使用起来非常方便：
// create thread pool with 4 worker threads
ThreadPool pool(4);
// enqueue and store future
auto result = pool.enqueue([](int answer) { return answer; }, 42);
// get result from future
std::cout << result.get() << std::endl;
如果中间要等待任务完全执行完成使用下面两个函数：
pool.wait_until_empty();
pool.wait_until_nothing_in_flight ();
源码：
// -*- C++ -*-
// Copyright (c) 2012-2015 Jakob Progsch
//
// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.
//
// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:
//
// 1. The origin of this software must not be misrepresented; you must not
// claim that you wrote the original software. If you use this software
// in a product, an acknowledgment in the product documentation would be
// appreciated but is not required.
//
// 2. Altered source versions must be plainly marked as such, and must not be
// misrepresented as being the original software.
//
// 3. This notice may not be removed or altered from any source
// distribution.
//
// Modified for log4cplus, copyright (c) 2014-2015 Václav Zeman.
#ifndef THREAD_POOL_H_7ea1ee6b_4f17_4c09_b76b_3d44e102400c
#define THREAD_POOL_H_7ea1ee6b_4f17_4c09_b76b_3d44e102400c
#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <atomic>
#include <functional>
#include <stdexcept>
#include <algorithm>
#include <cassert>
namespace progschj {
class would_block
: public std::runtime_error
{
using std::runtime_error::runtime_error;
};
class ThreadPool {
public:
template <typename F, typename... Args>
using return_type =
#if defined(__cpp_lib_is_invocable) && __cpp_lib_is_invocable >= 201703L
typename std::invoke_result<F&&, Args&&...>::type;
#else
typename std::result_of<F&& (Args&&...)>::type;
#endif
explicit ThreadPool(std::size_t threads
= (std::max)(2u, std::thread::hardware_concurrency()));
template <typename F, typename... Args>
auto enqueue_block(F&& f, Args&&... args) -> std::future<return_type<F, Args...>>;
template <typename F, typename... Args>
auto enqueue(F&& f, Args&&... args) -> std::future<return_type<F, Args...>>;
void wait_until_empty();
void wait_until_nothing_in_flight();
void set_queue_size_limit(std::size_t limit);
void set_pool_size(std::size_t limit);
~ThreadPool();
private:
void start_worker(std::size_t worker_number,
std::unique_lock<std::mutex> const &lock);
template <typename F, typename... Args>
auto enqueue_worker(bool, F&& f, Args&&... args) -> std::future<return_type<F, Args...>>;
template <typename T>
static std::future<T> make_exception_future (std::exception_ptr ex_ptr);
// need to keep track of threads so we can join them
std::vector< std::thread > workers;
// target pool size
std::size_t pool_size;
// the task queue
std::queue< std::function<void()> > tasks;
// queue length limit
std::size_t max_queue_size = 100000;
// stop signal
bool stop = false;
// synchronization
std::mutex queue_mutex;
std::condition_variable condition_producers;
std::condition_variable condition_consumers;
std::mutex in_flight_mutex;
std::condition_variable in_flight_condition;
std::atomic<std::size_t> in_flight;
struct handle_in_flight_decrement
{
ThreadPool & tp;
handle_in_flight_decrement(ThreadPool & tp_)
: tp(tp_)
{ }
~handle_in_flight_decrement()
{
std::size_t prev
= std::atomic_fetch_sub_explicit(&tp.in_flight,
std::size_t(1),
std::memory_order_acq_rel);
if (prev == 1)
{
std::unique_lock<std::mutex> guard(tp.in_flight_mutex);
tp.in_flight_condition.notify_all();
}
}
};
};
// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(std::size_t threads)
: pool_size(threads)
, in_flight(0)
{
std::unique_lock<std::mutex> lock(this->queue_mutex);
for (std::size_t i = 0; i != threads; ++i)
start_worker(i, lock);
}
// add new work item to the pool and block if the queue is full
template<class F, class... Args>
auto ThreadPool::enqueue_block(F&& f, Args&&... args) -> std::future<return_type<F, Args...>>
{
return enqueue_worker (true, std::forward<F> (f), std::forward<Args> (args)...);
}
// add new work item to the pool and return future with would_block exception if it is full
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args) -> std::future<return_type<F, Args...>>
{
return enqueue_worker (false, std::forward<F> (f), std::forward<Args> (args)...);
}
template <typename F, typename... Args>
auto ThreadPool::enqueue_worker(bool block, F&& f, Args&&... args) -> std::future<return_type<F, Args...>>
{
auto task = std::make_shared< std::packaged_task<return_type<F, Args...>()> >(
std::bind(std::forward<F>(f), std::forward<Args>(args)...)
);
std::future<return_type<F, Args...>> res = task->get_future();
std::unique_lock<std::mutex> lock(queue_mutex);
if (tasks.size () >= max_queue_size)
{
if (block)
{
// wait for the queue to empty or be stopped
condition_producers.wait(lock,
[this]
{
return tasks.size () < max_queue_size
|| stop;
});
}
else
{
return ThreadPool::make_exception_future<return_type<F, Args...>> (
std::make_exception_ptr (would_block("queue full")));
}
}
// don't allow enqueueing after stopping the pool
if (stop)
throw std::runtime_error("enqueue on stopped ThreadPool");
tasks.emplace([task](){ (*task)(); });
std::atomic_fetch_add_explicit(&in_flight,
std::size_t(1),
std::memory_order_relaxed);
condition_consumers.notify_one();
return res;
}
// the destructor joins all threads
inline ThreadPool::~ThreadPool()
{
std::unique_lock<std::mutex> lock(queue_mutex);
stop = true;
pool_size = 0;
condition_consumers.notify_all();
condition_producers.notify_all();
condition_consumers.wait(lock, [this]{ return this->workers.empty(); });
assert(in_flight == 0);
}
inline void ThreadPool::wait_until_empty()
{
std::unique_lock<std::mutex> lock(this->queue_mutex);
this->condition_producers.wait(lock,
[this]{ return this->tasks.empty(); });
}
inline void ThreadPool::wait_until_nothing_in_flight()
{
std::unique_lock<std::mutex> lock(this->in_flight_mutex);
this->in_flight_condition.wait(lock,
[this]{ return this->in_flight == 0; });
}
inline void ThreadPool::set_queue_size_limit(std::size_t limit)
{
std::unique_lock<std::mutex> lock(this->queue_mutex);
if (stop)
return;
std::size_t const old_limit = max_queue_size;
max_queue_size = (std::max)(limit, std::size_t(1));
if (old_limit < max_queue_size)
condition_producers.notify_all();
}
inline void ThreadPool::set_pool_size(std::size_t limit)
{
if (limit < 1)
limit = 1;
std::unique_lock<std::mutex> lock(this->queue_mutex);
if (stop)
return;
std::size_t const old_size = pool_size;
assert(this->workers.size() >= old_size);
pool_size = limit;
if (pool_size > old_size)
{
// create new worker threads
// it is possible that some of these are still running because
// they have not stopped yet after a pool size reduction, such
// workers will just keep running
for (std::size_t i = old_size; i != pool_size; ++i)
start_worker(i, lock);
}
else if (pool_size < old_size)
// notify all worker threads to start downsizing
this->condition_consumers.notify_all();
}
inline void ThreadPool::start_worker(
std::size_t worker_number, std::unique_lock<std::mutex> const &lock)
{
assert(lock.owns_lock() && lock.mutex() == &this->queue_mutex);
assert(worker_number <= this->workers.size());
auto worker_func =
[this, worker_number]
{
for(;;)
{
std::function<void()> task;
bool notify;
{
std::unique_lock<std::mutex> lock(this->queue_mutex);
this->condition_consumers.wait(lock,
[this, worker_number]{
return this->stop || !this->tasks.empty()
|| pool_size < worker_number + 1; });
// deal with downsizing of thread pool or shutdown
if ((this->stop && this->tasks.empty())
|| (!this->stop && pool_size < worker_number + 1))
{
// detach this worker, effectively marking it stopped
this->workers[worker_number].detach();
// downsize the workers vector as much as possible
while (this->workers.size() > pool_size
&& !this->workers.back().joinable())
this->workers.pop_back();
// if this is was last worker, notify the destructor
if (this->workers.empty())
this->condition_consumers.notify_all();
return;
}
else if (!this->tasks.empty())
{
task = std::move(this->tasks.front());
this->tasks.pop();
notify = this->tasks.size() + 1 == max_queue_size
|| this->tasks.empty();
}
else
continue;
}
handle_in_flight_decrement guard(*this);
if (notify)
{
std::unique_lock<std::mutex> lock(this->queue_mutex);
condition_producers.notify_all();
}
task();
}
};
if (worker_number < this->workers.size()) {
std::thread & worker = this->workers[worker_number];
// start only if not already running
if (!worker.joinable()) {
worker = std::thread(worker_func);
}
} else
this->workers.push_back(std::thread(worker_func));
}
template <typename T>
inline std::future<T> ThreadPool::make_exception_future (std::exception_ptr ex_ptr)
{
std::promise<T> p;
p.set_exception (ex_ptr);
return p.get_future ();
}
} // namespace progschj
#endif // THREAD_POOL_H_7ea1ee6b_4f17_4c09_b76b_3d44e102400c