C++ – 实现一个小但美的线程池
1 线程池
1.1 线程池需要实现的功能
线程池需要支持以下几个基本功能：
- 核心线程数(core_threads)：线程池中拥有的最少线程个数，初始化时就会创建好的线程，常驻于线程池
- 最大线程个数(max_threads)：线程池中拥有的最大线程个数，max_threads>=core_threads，当任务的个数太多线程池执行不过来时，内部就会创建更多的线程用于执行更多的任务，内部线程数不会超过max_threads，多创建出来的线程在一段时间内没有执行任务则会自动被回收掉，最终线程个数保持在核心线程数
- 超时时间(time_out)：如上所述，多创建出来的线程在time_out时间内没有执行任务就会被回收
- 可获取当前线程池中线程的总个数
- 可获取当前线程池中空闲线程的个数
- 开启线程池功能的开关
- 关闭线程池功能的开关，可以选择是否立即关闭，立即关闭线程池时，当前线程池里缓存的任务不会被执行
1.2 线程池中的主要数据结构
线程池中一般需要以下数据结构：
- 链表或者数组：存储线程池中的线程
- 队列：存储需要放入到线程池中执行的任务
- 条件变量：当有任务需要执行时，用于通知正在等待的线程从任务队列中取出任务，然后执行任务
2 C++中实现线程池
2.1 完整代码
C++线程池完整thread_pool.h代码如下
#ifndef __THREAD_POOL__
#define __THREAD_POOL__
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <functional>
#include <future>
#include <iostream>
#include <list>
#include <memory>
#include <mutex>
#include <queue>
#include <thread>
#include <tuple>
#include <utility>
#include <vector>
using std::cout;
using std::endl;
namespace wzq {
class ThreadPool {
public:
using PoolSeconds = std::chrono::seconds;
/** 线程池的配置
* core_threads: 核心线程个数，线程池中最少拥有的线程个数，初始化就会创建好的线程，常驻于线程池
*
* max_threads: >=core_threads，当任务的个数太多线程池执行不过来时，
* 内部就会创建更多的线程用于执行更多的任务，内部线程数不会超过max_threads
*
* max_task_size: 内部允许存储的最大任务个数，暂时没有使用
*
* time_out: Cache线程的超时时间，Cache线程指的是max_threads-core_threads的线程,
* 当time_out时间内没有执行任务，此线程就会被自动回收
*/
struct ThreadPoolConfig {
int core_threads;
int max_threads;
int max_task_size;
PoolSeconds time_out;
};
/**
* 线程的状态：有等待、运行、停止
*/
enum class ThreadState { kInit = 0, kWaiting = 1, kRunning = 2, kStop = 3 };
/**
* 线程的种类标识：标志该线程是核心线程还是Cache线程，Cache是内部为了执行更多任务临时创建出来的
*/
enum class ThreadFlag { kInit = 0, kCore = 1, kCache = 2 };
using ThreadPtr = std::shared_ptr<std::thread>;
using ThreadId = std::atomic<int>;
using ThreadStateAtomic = std::atomic<ThreadState>;
using ThreadFlagAtomic = std::atomic<ThreadFlag>;
/**
* 线程池中线程存在的基本单位，每个线程都有个自定义的ID，有线程种类标识和状态
*/
struct ThreadWrapper {
ThreadPtr ptr;
ThreadId id;
ThreadFlagAtomic flag;
ThreadStateAtomic state;
ThreadWrapper() {
ptr = nullptr;
id = 0;
state.store(ThreadState::kInit);
}
};
using ThreadWrapperPtr = std::shared_ptr<ThreadWrapper>;
using ThreadPoolLock = std::unique_lock<std::mutex>;
ThreadPool(ThreadPoolConfig config) : config_(config) {
this->total_function_num_.store(0);
this->waiting_thread_num_.store(0);
this->thread_id_.store(0);
this->is_shutdown_.store(false);
this->is_shutdown_now_.store(false);
if (IsValidConfig(config_)) {
is_available_.store(true);
} else {
is_available_.store(false);
}
}
~ThreadPool() { ShutDown(); }
bool Reset(ThreadPoolConfig config) {
if (!IsValidConfig(config)) {
return false;
}
if (config_.core_threads != config.core_threads) {
return false;
}
config_ = config;
return true;
}
// 开启线程池功能
bool Start() {
if (!IsAvailable()) {
return false;
}
int core_thread_num = config_.core_threads;
cout << "Init thread num " << core_thread_num << endl;
while (core_thread_num-- > 0) {
AddThread(GetNextThreadId());
}
cout << "Init thread end" << endl;
return true;
}
// 获取正在处于等待状态的线程的个数
int GetWaitingThreadSize() { return this->waiting_thread_num_.load(); }
// 获取线程池中当前线程的总个数
int GetTotalThreadSize() { return this->worker_threads_.size(); }
// 放在线程池中执行函数
template <typename F, typename... Args>
auto Run(F &&f, Args &&... args) -> std::shared_ptr<std::future<std::result_of_t<F(Args...)>>> {
if (this->is_shutdown_.load() || this->is_shutdown_now_.load() || !IsAvailable()) {
return nullptr;
}
if (GetWaitingThreadSize() == 0 && GetTotalThreadSize() < config_.max_threads) {
AddThread(GetNextThreadId(), ThreadFlag::kCache);
}
using return_type = std::result_of_t<F(Args...)>;
auto task = std::make_shared<std::packaged_task<return_type()>>(
std::bind(std::forward<F>(f), std::forward<Args>(args)...));
total_function_num_++;
std::future<return_type> res = task->get_future();
{
ThreadPoolLock lock(this->task_mutex_);
this->tasks_.emplace([task]() { (*task)(); });
}
this->task_cv_.notify_one();
return std::make_shared<std::future<std::result_of_t<F(Args...)>>>(std::move(res));
}
// 获取当前线程池已经执行过的函数个数
int GetRunnedFuncNum() { return total_function_num_.load(); }
// 关掉线程池，内部还没有执行的任务会继续执行
void ShutDown() {
ShutDown(false);
cout << "shutdown" << endl;
}
// 执行关掉线程池，内部还没有执行的任务直接取消，不会再执行
void ShutDownNow() {
ShutDown(true);
cout << "shutdown now" << endl;
}
// 当前线程池是否可用
bool IsAvailable() { return is_available_.load(); }
private:
void ShutDown(bool is_now) {
if (is_available_.load()) {
if (is_now) {
this->is_shutdown_now_.store(true);
} else {
this->is_shutdown_.store(true);
}
this->task_cv_.notify_all();
is_available_.store(false);
}
}
void AddThread(int id) { AddThread(id, ThreadFlag::kCore); }
void AddThread(int id, ThreadFlag thread_flag) {
cout << "AddThread " << id << " flag " << static_cast<int>(thread_flag) << endl;
ThreadWrapperPtr thread_ptr = std::make_shared<ThreadWrapper>();
thread_ptr->id.store(id);
thread_ptr->flag.store(thread_flag);
auto func = [this, thread_ptr]() {
for (;;) {
std::function<void()> task;
{
ThreadPoolLock lock(this->task_mutex_);
if (thread_ptr->state.load() == ThreadState::kStop) {
break;
}
cout << "thread id " << thread_ptr->id.load() << " running start" << endl;
thread_ptr->state.store(ThreadState::kWaiting);
++this->waiting_thread_num_;
bool is_timeout = false;
if (thread_ptr->flag.load() == ThreadFlag::kCore) {
this->task_cv_.wait(lock, [this, thread_ptr] {
return (this->is_shutdown_ || this->is_shutdown_now_ || !this->tasks_.empty() ||
thread_ptr->state.load() == ThreadState::kStop);
});
} else {
this->task_cv_.wait_for(lock, this->config_.time_out, [this, thread_ptr] {
return (this->is_shutdown_ || this->is_shutdown_now_ || !this->tasks_.empty() ||
thread_ptr->state.load() == ThreadState::kStop);
});
is_timeout = !(this->is_shutdown_ || this->is_shutdown_now_ || !this->tasks_.empty() ||
thread_ptr->state.load() == ThreadState::kStop);
}
--this->waiting_thread_num_;
cout << "thread id " << thread_ptr->id.load() << " running wait end" << endl;
if (is_timeout) {
thread_ptr->state.store(ThreadState::kStop);
}
if (thread_ptr->state.load() == ThreadState::kStop) {
cout << "thread id " << thread_ptr->id.load() << " state stop" << endl;
break;
}
if (this->is_shutdown_ && this->tasks_.empty()) {
cout << "thread id " << thread_ptr->id.load() << " shutdown" << endl;
break;
}
if (this->is_shutdown_now_) {
cout << "thread id " << thread_ptr->id.load() << " shutdown now" << endl;
break;
}
thread_ptr->state.store(ThreadState::kRunning);
task = std::move(this->tasks_.front());
this->tasks_.pop();
}
task();
}
cout << "thread id " << thread_ptr->id.load() << " running end" << endl;
};
thread_ptr->ptr = std::make_shared<std::thread>(std::move(func));
if (thread_ptr->ptr->joinable()) {
thread_ptr->ptr->detach();
}
this->worker_threads_.emplace_back(std::move(thread_ptr));
}
void Resize(int thread_num) {
if (thread_num < config_.core_threads) return;
int old_thread_num = worker_threads_.size();
cout << "old num " << old_thread_num << " resize " << thread_num << endl;
if (thread_num > old_thread_num) {
while (thread_num-- > old_thread_num) {
AddThread(GetNextThreadId());
}
} else {
int diff = old_thread_num - thread_num;
auto iter = worker_threads_.begin();
while (iter != worker_threads_.end()) {
if (diff == 0) {
break;
}
auto thread_ptr = *iter;
if (thread_ptr->flag.load() == ThreadFlag::kCache &&
thread_ptr->state.load() == ThreadState::kWaiting) { // wait
thread_ptr->state.store(ThreadState::kStop); // stop;
--diff;
iter = worker_threads_.erase(iter);
} else {
++iter;
}
}
this->task_cv_.notify_all();
}
}
int GetNextThreadId() { return this->thread_id_++; }
bool IsValidConfig(ThreadPoolConfig config) {
if (config.core_threads < 1 || config.max_threads < config.core_threads || config.time_out.count() < 1) {
return false;
}
return true;
}
private:
ThreadPoolConfig config_;
std::list<ThreadWrapperPtr> worker_threads_;
std::queue<std::function<void()>> tasks_;
std::mutex task_mutex_;
std::condition_variable task_cv_;
std::atomic<int> total_function_num_;
std::atomic<int> waiting_thread_num_;
std::atomic<int> thread_id_;
std::atomic<bool> is_shutdown_now_;
std::atomic<bool> is_shutdown_;
std::atomic<bool> is_available_;
};
} // namespace wzq
#endif
测试代码如下
#include <iostream>
#include "thread_pool.h"
int main() {
cout << "hello" << endl;
ThreadPool pool(ThreadPool::ThreadPoolConfig{4, 5, 6, std::chrono::seconds(4)});
pool.Start();
std::this_thread::sleep_for(std::chrono::seconds(4));
cout << "thread size " << pool.GetTotalThreadSize() << endl;
std::atomic<int> index;
index.store(0);
std::thread t([&]() {
for (int i = 0; i < 10; ++i) {
pool.Run([&]() {
cout << "function " << index.load() << endl;
std::this_thread::sleep_for(std::chrono::seconds(4));
index++;
});
// std::this_thread::sleep_for(std::chrono::seconds(2));
}
});
t.detach();
cout << "=================" << endl;
std::this_thread::sleep_for(std::chrono::seconds(4));
pool.Reset(ThreadPool::ThreadPoolConfig{4, 4, 6, std::chrono::seconds(4)});
std::this_thread::sleep_for(std::chrono::seconds(4));
cout << "thread size " << pool.GetTotalThreadSize() << endl;
cout << "waiting size " << pool.GetWaitingThreadSize() << endl;
cout << "---------------" << endl;
pool.ShutDownNow();
getchar();
cout << "world" << endl;
return 0;
}
2.2 如何将任务放入线程池中执行
将任务使用std::bind封装成std::function放入任务队列中，任务较多时内部还会判断是否有空闲线程，如果没有空闲线程，会自动创建出最多(max_threads-core_threads)个Cache线程用于执行任务。
2.3 如何为线程池添加线程
具体代码在AddThread函数中，默认会创建Core线程，也可以选择创建Cache线程，线程内部会有一个死循环，不停的等待任务，有任务到来时就会执行，同时内部会判断是否是Cache线程，如果是Cache线程，timeout时间内没有任务执行就会自动退出循环，线程结束。
另外还会检查is_shutdown和isshutdown_now标志，根据两个标志位是否为true来判断是否结束线程。
参考
本文作者：StubbornHuang
版权声明：本文为站长原创文章，如果转载请注明原文链接！
原文标题：C++ – 实现一个小但美的线程池
原文链接：https://www.stubbornhuang.com/2949/
发布于：2024年01月11日 17:12:01
修改于：2024年01月11日 17:12:49
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