今天看了ASIO的介绍，不太明白asio在POSIX上如何用reactor模拟proactor。所以稍微看了下源代码，此文当作笔记。
ASIO Proactor:
Proactor design pattern (adapted from [POSA2])
— Asynchronous Operation
Defines an operation that is executed asynchronously, such as an asynchronous read or write on a socket.
— Asynchronous Operation Processor
Executes asynchronous operations and queues events on a completion event queue when operations complete. From a high-level point of view, services like
stream_socket_serviceare asynchronous operation processors.
— Completion Event Queue
Buffers completion events until they are dequeued by an asynchronous event demultiplexer.
— Completion Handler
Processes the result of an asynchronous operation. These are function objects, often created using
boost::bind.
— Asynchronous Event Demultiplexer
Blocks waiting for events to occur on the completion event queue, and returns a completed event to its caller.
— Proactor
Calls the asynchronous event demultiplexer to dequeue events, and dispatches the completion handler (i.e. invokes the function object) associated with the event. This abstraction is represented by the
io_serviceclass.
— Initiator
Application-specific code that starts asynchronous operations. The initiator interacts with an asynchronous operation processor via a high-level interface such as
basic_stream_socket, which in turn delegates to a service likestream_socket_service.
Implementation Using Reactor
On many platforms, Asio implements the Proactor design pattern in terms of a Reactor, such as select, epoll or kqueue. This implementation approach corresponds to the Proactor design pattern as follows:
— Asynchronous Operation Processor
A reactor implemented using
select,epollorkqueue. When the reactor indicates that the resource is ready to perform the operation, the processor executes the asynchronous operation and enqueues the associated completion handler on the completion event queue.
— Completion Event Queue
A linked list of completion handlers (i.e. function objects).
— Asynchronous Event Demultiplexer
This is implemented by waiting on an event or condition variable until a completion handler is available in the completion event queue.
1. Initiator使用Asynchronous Operation Processor发起异步I/O操作2. 保存每个异步I/O操作的参数，包括回调函数的地址，并将其放入Completion Event Queue3. Proactor调用Asynchronous Event Demultiplexer检测完成事件。4. 当检测到I/O操作完成事件，从Completion Event Queue中取出对应的异步I/O操作，并且分派到相应的Completion Handler。5. Completion Handler调用回调函数。
reactor模拟的实现:
task_io_service<reactor> io_service_impl。
// Run the event loop until interrupted or no more work. size_t run(boost::system::error_code& ec) { ec = boost::system::error_code(); if (outstanding_work_ == 0) { stop(); return 0; } typename call_stack<task_io_service>::context ctx(this); idle_thread_info this_idle_thread; this_idle_thread.next = 0; boost::asio::detail::mutex::scoped_lock lock(mutex_); size_t n = 0; for (; do_one(lock, &this_idle_thread); lock.lock()) if (n != (std::numeric_limits<size_t>::max)()) ++n; return n; }
1. this_idle_thread是个保存idle线程的链表。
2. 如果没有异步操作要处理，那么就加入到idle线程的链表中去，一直在那边阻塞等下去。
size_t do_one(boost::asio::detail::mutex::scoped_lock& lock, idle_thread_info* this_idle_thread) { ...... else if (this_idle_thread) { // Nothing to run right now, so just wait for work to do. this_idle_thread->next = first_idle_thread_; first_idle_thread_ = this_idle_thread; this_idle_thread->wakeup_event.clear(lock); this_idle_thread->wakeup_event.wait(lock); } else { return 0; } } return 0; }
socket_service:
类型:
datagram_socket_service
raw_socket_service
socket_acceptor_service
stream_socket_service
当构造socket_service的时候间接会调用init_task():
// Initialise the task, if required. void init_task() { boost::asio::detail::mutex::scoped_lock lock(mutex_); if (!shutdown_ && !task_) { task_ = &use_service<Task>(this->get_io_service()); op_queue_.push(&task_operation_); wake_one_thread_and_unlock(lock); } }
1. 这边的task_operation就是用来表示异步操作即将开始的，而后面会发现op_queue中不但出存放异步操作即将开始的指针，还会存放完成队列指针。
2. 唤醒一个当前的idle线程。
这个时候idle线程就会继续循环，下面我们来看完整的do_one()函数:
size_t do_one(boost::asio::detail::mutex::scoped_lock& lock, idle_thread_info* this_idle_thread) { bool polling = !this_idle_thread; bool task_has_run = false; while (!stopped_) { if (!op_queue_.empty()) // 操作队列不为空 { // Prepare to execute first handler from queue. operation* o = op_queue_.front(); op_queue_.pop(); bool more_handlers = (!op_queue_.empty()); if (o == &task_operation_) // 如果是异步操作即将开始，即socket_service创建完成 { task_interrupted_ = more_handlers || polling; // If the task has already run and we're polling then we're done. if (task_has_run && polling) { task_interrupted_ = true; op_queue_.push(&task_operation_); return 0; } task_has_run = true; if (!more_handlers || !wake_one_idle_thread_and_unlock(lock)) lock.unlock(); op_queue<operation> completed_ops; task_cleanup c = { this, &lock, &completed_ops }; // 当所在的block结束的时候，即下面的通过reactor去等待并且执行操作，去将任务加入到完成队列中，还会添加一个task_operation_ (void)c; // Run the task. May throw an exception. Only block if the operation // queue is empty and we're not polling, otherwise we want to return // as soon as possible. task_->run(!more_handlers && !polling, completed_ops); // 通过reactor去等待并且执行操作 } else { if (more_handlers) // 如果操作队列中还有需要处理的任务 wake_one_thread_and_unlock(lock); // 唤醒idle线程 else lock.unlock(); // Ensure the count of outstanding work is decremented on block exit. work_finished_on_block_exit on_exit = { this }; (void)on_exit; // Complete the operation. May throw an exception. o->complete(*this); // deletes the operation object //异步操作完成回调 return 1; } } else if (this_idle_thread) // 加入到idle线程链表中，并且wait阻塞。 { // Nothing to run right now, so just wait for work to do. this_idle_thread->next = first_idle_thread_; first_idle_thread_ = this_idle_thread; this_idle_thread->wakeup_event.clear(lock); this_idle_thread->wakeup_event.wait(lock); } else { return 0; } } return 0; }
struct task_cleanup { ~task_cleanup() { 添加完成操作指针到操作队列，并且重新插入task_operation_ lock_->lock(); task_io_service_->task_interrupted_ = true; task_io_service_->op_queue_.push(*ops_); task_io_service_->op_queue_.push(&task_io_service_->task_operation_); } task_io_service* task_io_service_; boost::asio::detail::mutex::scoped_lock* lock_; op_queue<operation>* ops_; };
这边还会有很多细节，等到啥时有空的时候来个深入分析。