C++11 并发指南九(综合运用: C++11 多线程下生产者消费者模型详解)
前面八章介绍了 C++11 并发编程的基础(抱歉哈，第五章-第八章还在草稿中)，本文将综合运用 C++11 中的新的基础设施(主要是多线程、锁、条件变量)来阐述一个经典问题——生产者消费者模型，并给出完整的解决方案。
生产者消费者问题是多线程并发中一个非常经典的问题，相信学过操作系统课程的同学都清楚这个问题的根源。本文将就四种情况分析并介绍生产者和消费者问题，它们分别是：单生产者-单消费者模型，单生产者-多消费者模型，多生产者-单消费者模型，多生产者-多消费者模型，我会给出四种情况下的 C++11 并发解决方案，如果文中出现了错误或者你对代码有异议，欢迎交流 ;-)。
单生产者-单消费者模型
顾名思义，单生产者-单消费者模型中只有一个生产者和一个消费者，生产者不停地往产品库中放入产品，消费者则从产品库中取走产品，产品库容积有限制，只能容纳一定数目的产品，如果生产者生产产品的速度过快，则需要等待消费者取走产品之后，产品库不为空才能继续往产品库中放置新的产品，相反，如果消费者取走产品的速度过快，则可能面临产品库中没有产品可使用的情况，此时需要等待生产者放入一个产品后，消费者才能继续工作。C++11实现单生产者单消费者模型的代码如下：
#include <unistd.h> #include <cstdlib> #include <condition_variable> #include <iostream> #include <mutex> #include <thread> static const int kItemRepositorySize = 10; // Item buffer size. static const int kItemsToProduce = 1000; // How many items we plan to produce. struct ItemRepository { int item_buffer[kItemRepositorySize]; // 产品缓冲区, 配合 read_position 和 write_position 模型环形队列. size_t read_position; // 消费者读取产品位置. size_t write_position; // 生产者写入产品位置. std::mutex mtx; // 互斥量,保护产品缓冲区 std::condition_variable repo_not_full; // 条件变量, 指示产品缓冲区不为满. std::condition_variable repo_not_empty; // 条件变量, 指示产品缓冲区不为空. } gItemRepository; // 产品库全局变量, 生产者和消费者操作该变量. typedef struct ItemRepository ItemRepository; void ProduceItem(ItemRepository *ir, int item) { std::unique_lock<std::mutex> lock(ir->mtx); while(((ir->write_position + 1) % kItemRepositorySize) == ir->read_position) { // item buffer is full, just wait here. std::cout << "Producer is waiting for an empty slot...\n"; (ir->repo_not_full).wait(lock); // 生产者等待"产品库缓冲区不为满"这一条件发生. } (ir->item_buffer)[ir->write_position] = item; // 写入产品. (ir->write_position)++; // 写入位置后移. if (ir->write_position == kItemRepositorySize) // 写入位置若是在队列最后则重新设置为初始位置. ir->write_position = 0; (ir->repo_not_empty).notify_all(); // 通知消费者产品库不为空. lock.unlock(); // 解锁. } int ConsumeItem(ItemRepository *ir) { int data; std::unique_lock<std::mutex> lock(ir->mtx); // item buffer is empty, just wait here. while(ir->write_position == ir->read_position) { std::cout << "Consumer is waiting for items...\n"; (ir->repo_not_empty).wait(lock); // 消费者等待"产品库缓冲区不为空"这一条件发生. } data = (ir->item_buffer)[ir->read_position]; // 读取某一产品 (ir->read_position)++; // 读取位置后移 if (ir->read_position >= kItemRepositorySize) // 读取位置若移到最后，则重新置位. ir->read_position = 0; (ir->repo_not_full).notify_all(); // 通知消费者产品库不为满. lock.unlock(); // 解锁. return data; // 返回产品. } void ProducerTask() // 生产者任务 { for (int i = 1; i <= kItemsToProduce; ++i) { // sleep(1); std::cout << "Produce the " << i << "^th item..." << std::endl; ProduceItem(&gItemRepository, i); // 循环生产 kItemsToProduce 个产品. } } void ConsumerTask() // 消费者任务 { static int cnt = 0; while(1) { sleep(1); int item = ConsumeItem(&gItemRepository); // 消费一个产品. std::cout << "Consume the " << item << "^th item" << std::endl; if (++cnt == kItemsToProduce) break; // 如果产品消费个数为 kItemsToProduce, 则退出. } } void InitItemRepository(ItemRepository *ir) { ir->write_position = 0; // 初始化产品写入位置. ir->read_position = 0; // 初始化产品读取位置. } int main() { InitItemRepository(&gItemRepository); std::thread producer(ProducerTask); // 创建生产者线程. std::thread consumer(ConsumerTask); // 创建消费之线程. producer.join(); consumer.join(); }
单生产者-多消费者模型
与单生产者和单消费者模型不同的是，单生产者-多消费者模型中可以允许多个消费者同时从产品库中取走产品。所以除了保护产品库在多个读写线程下互斥之外，还需要维护消费者取走产品的计数器，代码如下:
#include <unistd.h> #include <cstdlib> #include <condition_variable> #include <iostream> #include <mutex> #include <thread> static const int kItemRepositorySize = 4; // Item buffer size. static const int kItemsToProduce = 10; // How many items we plan to produce. struct ItemRepository { int item_buffer[kItemRepositorySize]; size_t read_position; size_t write_position; size_t item_counter; std::mutex mtx; std::mutex item_counter_mtx; std::condition_variable repo_not_full; std::condition_variable repo_not_empty; } gItemRepository; typedef struct ItemRepository ItemRepository; void ProduceItem(ItemRepository *ir, int item) { std::unique_lock<std::mutex> lock(ir->mtx); while(((ir->write_position + 1) % kItemRepositorySize) == ir->read_position) { // item buffer is full, just wait here. std::cout << "Producer is waiting for an empty slot...\n"; (ir->repo_not_full).wait(lock); } (ir->item_buffer)[ir->write_position] = item; (ir->write_position)++; if (ir->write_position == kItemRepositorySize) ir->write_position = 0; (ir->repo_not_empty).notify_all(); lock.unlock(); } int ConsumeItem(ItemRepository *ir) { int data; std::unique_lock<std::mutex> lock(ir->mtx); // item buffer is empty, just wait here. while(ir->write_position == ir->read_position) { std::cout << "Consumer is waiting for items...\n"; (ir->repo_not_empty).wait(lock); } data = (ir->item_buffer)[ir->read_position]; (ir->read_position)++; if (ir->read_position >= kItemRepositorySize) ir->read_position = 0; (ir->repo_not_full).notify_all(); lock.unlock(); return data; } void ProducerTask() { for (int i = 1; i <= kItemsToProduce; ++i) { // sleep(1); std::cout << "Producer thread " << std::this_thread::get_id() << " producing the " << i << "^th item..." << std::endl; ProduceItem(&gItemRepository, i); } std::cout << "Producer thread " << std::this_thread::get_id() << " is exiting..." << std::endl; } void ConsumerTask() { bool ready_to_exit = false; while(1) { sleep(1); std::unique_lock<std::mutex> lock(gItemRepository.item_counter_mtx); if (gItemRepository.item_counter < kItemsToProduce) { int item = ConsumeItem(&gItemRepository); ++(gItemRepository.item_counter); std::cout << "Consumer thread " << std::this_thread::get_id() << " is consuming the " << item << "^th item" << std::endl; } else ready_to_exit = true; lock.unlock(); if (ready_to_exit == true) break; } std::cout << "Consumer thread " << std::this_thread::get_id() << " is exiting..." << std::endl; } void InitItemRepository(ItemRepository *ir) { ir->write_position = 0; ir->read_position = 0; ir->item_counter = 0; } int main() { InitItemRepository(&gItemRepository); std::thread producer(ProducerTask); std::thread consumer1(ConsumerTask); std::thread consumer2(ConsumerTask); std::thread consumer3(ConsumerTask); std::thread consumer4(ConsumerTask); producer.join(); consumer1.join(); consumer2.join(); consumer3.join(); consumer4.join(); }
多生产者-单消费者模型
与单生产者和单消费者模型不同的是，多生产者-单消费者模型中可以允许多个生产者同时向产品库中放入产品。所以除了保护产品库在多个读写线程下互斥之外，还需要维护生产者放入产品的计数器，代码如下:
#include <unistd.h> #include <cstdlib> #include <condition_variable> #include <iostream> #include <mutex> #include <thread> static const int kItemRepositorySize = 4; // Item buffer size. static const int kItemsToProduce = 10; // How many items we plan to produce. struct ItemRepository { int item_buffer[kItemRepositorySize]; size_t read_position; size_t write_position; size_t item_counter; std::mutex mtx; std::mutex item_counter_mtx; std::condition_variable repo_not_full; std::condition_variable repo_not_empty; } gItemRepository; typedef struct ItemRepository ItemRepository; void ProduceItem(ItemRepository *ir, int item) { std::unique_lock<std::mutex> lock(ir->mtx); while(((ir->write_position + 1) % kItemRepositorySize) == ir->read_position) { // item buffer is full, just wait here. std::cout << "Producer is waiting for an empty slot...\n"; (ir->repo_not_full).wait(lock); } (ir->item_buffer)[ir->write_position] = item; (ir->write_position)++; if (ir->write_position == kItemRepositorySize) ir->write_position = 0; (ir->repo_not_empty).notify_all(); lock.unlock(); } int ConsumeItem(ItemRepository *ir) { int data; std::unique_lock<std::mutex> lock(ir->mtx); // item buffer is empty, just wait here. while(ir->write_position == ir->read_position) { std::cout << "Consumer is waiting for items...\n"; (ir->repo_not_empty).wait(lock); } data = (ir->item_buffer)[ir->read_position]; (ir->read_position)++; if (ir->read_position >= kItemRepositorySize) ir->read_position = 0; (ir->repo_not_full).notify_all(); lock.unlock(); return data; } void ProducerTask() { bool ready_to_exit = false; while(1) { sleep(1); std::unique_lock<std::mutex> lock(gItemRepository.item_counter_mtx); if (gItemRepository.item_counter < kItemsToProduce) { ++(gItemRepository.item_counter); ProduceItem(&gItemRepository, gItemRepository.item_counter); std::cout << "Producer thread " << std::this_thread::get_id() << " is producing the " << gItemRepository.item_counter << "^th item" << std::endl; } else ready_to_exit = true; lock.unlock(); if (ready_to_exit == true) break; } std::cout << "Producer thread " << std::this_thread::get_id() << " is exiting..." << std::endl; } void ConsumerTask() { static int item_consumed = 0; while(1) { sleep(1); ++item_consumed; if (item_consumed <= kItemsToProduce) { int item = ConsumeItem(&gItemRepository); std::cout << "Consumer thread " << std::this_thread::get_id() << " is consuming the " << item << "^th item" << std::endl; } else break; } std::cout << "Consumer thread " << std::this_thread::get_id() << " is exiting..." << std::endl; } void InitItemRepository(ItemRepository *ir) { ir->write_position = 0; ir->read_position = 0; ir->item_counter = 0; } int main() { InitItemRepository(&gItemRepository); std::thread producer1(ProducerTask); std::thread producer2(ProducerTask); std::thread producer3(ProducerTask); std::thread producer4(ProducerTask); std::thread consumer(ConsumerTask); producer1.join(); producer2.join(); producer3.join(); producer4.join(); consumer.join(); }
多生产者-多消费者模型
该模型可以说是前面两种模型的综合，程序需要维护两个计数器，分别是生产者已生产产品的数目和消费者已取走产品的数目。另外也需要保护产品库在多个生产者和多个消费者互斥地访问。
代码如下：
#include <unistd.h> #include <cstdlib> #include <condition_variable> #include <iostream> #include <mutex> #include <thread> static const int kItemRepositorySize = 4; // Item buffer size. static const int kItemsToProduce = 10; // How many items we plan to produce. struct ItemRepository { int item_buffer[kItemRepositorySize]; size_t read_position; size_t write_position; size_t produced_item_counter; size_t consumed_item_counter; std::mutex mtx; std::mutex produced_item_counter_mtx; std::mutex consumed_item_counter_mtx; std::condition_variable repo_not_full; std::condition_variable repo_not_empty; } gItemRepository; typedef struct ItemRepository ItemRepository; void ProduceItem(ItemRepository *ir, int item) { std::unique_lock<std::mutex> lock(ir->mtx); while(((ir->write_position + 1) % kItemRepositorySize) == ir->read_position) { // item buffer is full, just wait here. std::cout << "Producer is waiting for an empty slot...\n"; (ir->repo_not_full).wait(lock); } (ir->item_buffer)[ir->write_position] = item; (ir->write_position)++; if (ir->write_position == kItemRepositorySize) ir->write_position = 0; (ir->repo_not_empty).notify_all(); lock.unlock(); } int ConsumeItem(ItemRepository *ir) { int data; std::unique_lock<std::mutex> lock(ir->mtx); // item buffer is empty, just wait here. while(ir->write_position == ir->read_position) { std::cout << "Consumer is waiting for items...\n"; (ir->repo_not_empty).wait(lock); } data = (ir->item_buffer)[ir->read_position]; (ir->read_position)++; if (ir->read_position >= kItemRepositorySize) ir->read_position = 0; (ir->repo_not_full).notify_all(); lock.unlock(); return data; } void ProducerTask() { bool ready_to_exit = false; while(1) { sleep(1); std::unique_lock<std::mutex> lock(gItemRepository.produced_item_counter_mtx); if (gItemRepository.produced_item_counter < kItemsToProduce) { ++(gItemRepository.produced_item_counter); ProduceItem(&gItemRepository, gItemRepository.produced_item_counter); std::cout << "Producer thread " << std::this_thread::get_id() << " is producing the " << gItemRepository.produced_item_counter << "^th item" << std::endl; } else ready_to_exit = true; lock.unlock(); if (ready_to_exit == true) break; } std::cout << "Producer thread " << std::this_thread::get_id() << " is exiting..." << std::endl; } void ConsumerTask() { bool ready_to_exit = false; while(1) { sleep(1); std::unique_lock<std::mutex> lock(gItemRepository.consumed_item_counter_mtx); if (gItemRepository.consumed_item_counter < kItemsToProduce) { int item = ConsumeItem(&gItemRepository); ++(gItemRepository.consumed_item_counter); std::cout << "Consumer thread " << std::this_thread::get_id() << " is consuming the " << item << "^th item" << std::endl; } else ready_to_exit = true; lock.unlock(); if (ready_to_exit == true) break; } std::cout << "Consumer thread " << std::this_thread::get_id() << " is exiting..." << std::endl; } void InitItemRepository(ItemRepository *ir) { ir->write_position = 0; ir->read_position = 0; ir->produced_item_counter = 0; ir->consumed_item_counter = 0; } int main() { InitItemRepository(&gItemRepository); std::thread producer1(ProducerTask); std::thread producer2(ProducerTask); std::thread producer3(ProducerTask); std::thread producer4(ProducerTask); std::thread consumer1(ConsumerTask); std::thread consumer2(ConsumerTask); std::thread consumer3(ConsumerTask); std::thread consumer4(ConsumerTask); producer1.join(); producer2.join(); producer3.join(); producer4.join(); consumer1.join(); consumer2.join(); consumer3.join(); consumer4.join(); }
另外，所有例子的代码(包括前面一些指南的代码均放在github上)，希望对大家学习 C++11 多线程并发有所帮助。