Linux下Boost.Asio Proactor模式实现分析
背景：
epoll的实现是基于回调的，如果fd有期望的事件发生就通过回调函数将其加入epoll就绪队列中，用户针对该队列中的文件句柄发起相应操作，如read等，此时数据真正才会开始从内核buffer写入应用buffer中，整个过程是一种同步IO。而Boost.Asio的说明文档中明确其采用Proactor模式实现了异步IO，也就是说用户在发起async_read后，可以去进行其它操作，数据将会从内核buffer写入应用buffer，数据拷贝完毕会调用用户提供的回调函数。
问题：
Boost.Asio在Linux下封装epoll这种同步接口是如何做到异步IO的呢？通过下面的分析，我们会发现，Boost.Asio在应用层上对epoll返回的就绪队列做了一层封装，实现数据拷贝，从而完成了异步IO操作。下面结合boost.Asio 1.55源码，进行分析。
分析：
Boost.Asio中最重要的一个类是io_service，io_service抽象系统I/O接口,提供异步数据传输的能力，它是应用程序和系统I/O接口的桥梁。Boost.Asio主要采用它实现了Proactor模式。io_service有一个重要的成员，io_service_impl，它在不同的系统下有不同的实现。在Windows下是基于IOCP的，在Linux下是基于task_io_service，这主要是通过预处理进行区分的：
// /asio/io_service.hpp
namespace detail {
#if defined(BOOST_ASIO_HAS_IOCP)
typedef class win_iocp_io_service io_service_impl;
class win_iocp_overlapped_ptr;
#else
typedef class task_io_service io_service_impl;
#endif
class service_registry;
} // namespace detail
io_service类如下：
// /asio/io_service.hpp
class io_service: private noncopyable
{
private:
typedef detail::io_service_impl impl_type;
...
// The service registry.
boost::asio::detail::service_registry* service_registry_;
// The implementation.
impl_type& impl_;
public:
...
// Run the io_service object's event processing loop.
BOOST_ASIO_DECL std::size_t run();
// Run the io_service object's event processing loop to execute ready handlers.
BOOST_ASIO_DECL std::size_t poll();
};
其中run()函数的具体实现如下：
std::size_t io_service::run()
{
boost::system::error_code ec;
std::size_t s = impl_.run(ec);
boost::asio::detail::throw_error(ec);
return s;
}
io_service的run()函数最终是调用了impl_(task_io_service)的run函数，对于poll也类似。
下面来分析下task_io_service：
task_io_service作为proactor模式在Linux下的具体实现，主要功能有两个：
1. 对IO是否就绪的进行扫描
2. 事件到达后对线程池的统一调度
task_io_service如下：
// /asio/detail/task_io_service.hpp
class task_io_service : public boost::asio::detail::service_base<task_io_service>
{
public:
...
typedef task_io_service_operation operation;
// Run the event loop until interrupted or no more work.
BOOST_ASIO_DECL std::size_t run(boost::system::error_code& ec);
private:
...
// The task to be run by this service.
reactor* task_;
// The count of unfinished work.
atomic_count outstanding_work_;
// The queue of handlers that are ready to be delivered.
op_queue<operation> op_queue_;
}
它有几个比较重要的成员：
1. reactor：这是一个typedef定义的同义词，它在不同平台有不同的实现：
// asio/detail/reactor_fwd.hpp #if defined(BOOST_ASIO_WINDOWS_RUNTIME) typedef class null_reactor reactor; #elif defined(BOOST_ASIO_HAS_IOCP) typedef class select_reactor reactor; #elif defined(BOOST_ASIO_HAS_EPOLL) typedef class epoll_reactor reactor; #elif defined(BOOST_ASIO_HAS_KQUEUE) typedef class kqueue_reactor reactor; #elif defined(BOOST_ASIO_HAS_DEV_POLL) typedef class dev_poll_reactor reactor; #else typedef class select_reactor reactor; #endif
平台使用的reactor类型可以通过下面的方法得到：
#include <iostream>
#include <string>
#include <boost/asio.hpp>
int main()
{
std::string output;
#if defined(BOOST_ASIO_HAS_IOCP)
output = "iocp" ;
#elif defined(BOOST_ASIO_HAS_EPOLL)
output = "epoll" ;
#elif defined(BOOST_ASIO_HAS_KQUEUE)
output = "kqueue" ;
#elif defined(BOOST_ASIO_HAS_DEV_POLL)
output = "/dev/poll" ;
#else
output = "select" ;
#endif
std::cout << output << std::endl;
}
在我的环境中使用的是epoll。通常，Linux下主要采用epoll，对应的实现类是epoll_reactor
2. op_queue<operation>：回调函数对象列表，这里面的每一个operation都会在调用了run函数的用户线程里面执行，每个操作选择一个空闲的线程，关于这点可以看下面的程序：
#include <boost/asio.hpp>
#include <boost/thread.hpp>
#include <iostream>
void handler1(const boost::system::error_code &ec)
{
std::cout << boost::this_thread::get_id() << " handler1." << std::endl;
//sleep(3);
}
void handler2(const boost::system::error_code &ec)
{
std::cout << boost::this_thread::get_id() << " handler2." << std::endl;
sleep(3);
}
boost::asio::io_service io_service;
void run()
{
io_service.run();
}
int main()
{
boost::asio::deadline_timer timer1(io_service, boost::posix_time::seconds(2));
timer1.async_wait(handler1);
boost::asio::deadline_timer timer2(io_service, boost::posix_time::seconds(2));
timer2.async_wait(handler2);
boost::thread thread1(run);
boost::thread thread2(run);
thread1.join();
thread2.join();
}
通过使用定义在boost/thread.hpp中的boost::thread类，在main()中创建了两个线程。这两个线程为同一个I/O service调用run()。这样做的好处是，一旦独立的异步操作完成，I/O service可以有效利用两个线程来执行handler方法。
上面的两个timer都是让时间停顿2s。由于有两个线程，handler1和handler2可以同时执行。如果timer2在停顿期间，timer1对应的handler1仍然在执行，那么handler2将会在第二个线程内执行。如果handler1已经结束了，那么I/O service将会自由选择线程来执行handler2。这可以通过注释掉handler1中的sleep(3)来验证：不注释，handler1和handler2总是不同的线程中执行；注释后，handler1和handler2可能在同一线程中执行，也可能不再，这要取决于整个系统当时的线程调度情况。通过调整timer的时间也可以观察到同样的现象。
task_io_service的run函数最终调用的是reactor的run函数，在Linux下是epoll_reactor的run函数，调用层次为：
// asio/detail/impl/Task_io_service.ipp
std::size_t task_io_service::run(boost::system::error_code& ec)
{
...
mutex::scoped_lock lock(mutex_);
std::size_t n = 0;
for (; do_run_one(lock, this_thread, ec); lock.lock())
if (n != (std::numeric_limits<std::size_t>::max)())
++n;
return n;
}
// asio/detail/impl/Task_io_service.ipp
std::size_t task_io_service::do_run_one(mutex::scoped_lock& lock,
task_io_service::thread_info& this_thread,
const boost::system::error_code& ec)
{
while (!stopped_)
{
if (!op_queue_.empty())
{
// 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_)
{
...
task_->run(!more_handlers, this_thread.private_op_queue);
}
else
{
...
// Complete the operation. May throw an exception. Deletes the object.
o->complete(*this, ec, task_result);
return 1;
}
}
else
{
// Nothing to run right now, so just wait for work to do.
this_thread.next = first_idle_thread_;
first_idle_thread_ = &this_thread;
this_thread.wakeup_event->clear(lock);
this_thread.wakeup_event->wait(lock);
}
}
return 0;
}
epoll_reactor的run()方法最终调用的是epoll的epoll_wait的，通过epoll_wait，将就绪的事件放入ops，等待处理。
//asio/detail/impl/epoll_reactor.ipp
void epoll_reactor::run(bool block, op_queue<operation>& ops)
{
...
// Block on the epoll descriptor.
epoll_event events[128];
int num_events = epoll_wait(epoll_fd_, events, 128, timeout);
...
descriptor_state* descriptor_data = static_cast<descriptor_state*>(ptr);
descriptor_data->set_ready_events(events[i].events);
ops.push(descriptor_data);
}
epoll_reactor类的定义如下：
// detail/epoll_reactor.hpp
class epoll_reactor : public boost::asio::detail::service_base<epoll_reactor>
{
...
private:
...
BOOST_ASIO_DECL static int do_epoll_create();
...
// The epoll file descriptor.
int epoll_fd_;
// The io_service implementation used to post completions.
io_service_impl& io_service_;
...
};
整个调用过程如下图所示：
Boost.Asio是这样使用epoll来进行事件分发的，实际的IO操作是如何和epoll联系起来的呢？继续...
以TCP为例，Boost.Asio中对TCP类的封装如下：
//asio/ip/Tcp.hpp
class tcp
{
public:
/// The type of a TCP endpoint.
typedef basic_endpoint<tcp> endpoint;
/// Construct to represent the IPv4 TCP protocol.
static tcp v4()
{
return tcp(BOOST_ASIO_OS_DEF(AF_INET));
}
...
/// The TCP socket type.
typedef basic_stream_socket<tcp> socket;
/// The TCP acceptor type.
typedef basic_socket_acceptor<tcp> acceptor;
/// The TCP resolver type.
typedef basic_resolver<tcp> resolver;
#if !defined(BOOST_ASIO_NO_IOSTREAM)
/// The TCP iostream type.
typedef basic_socket_iostream<tcp> iostream;
#endif // !defined(BOOST_ASIO_NO_IOSTREAM)
...
private:
// Construct with a specific family.
explicit tcp(int protocol_family)
: family_(protocol_family)
{
}
int family_;
};
basic_stream_socket<tcp>类似于TCP中的socket，basic_socket_acceptor<tcp>用于监听套接字，它们均有许多异步方法，如async_receive，async_send等。basic_stream_socket和basic_socket_acceptor都是模板类，以basic_stream_socket为例，其async_receive方法如下：
// asio/Basic_stream_socket.hpp
template <typename MutableBufferSequence, typename ReadHandler>
BOOST_ASIO_INITFN_RESULT_TYPE(ReadHandler,
void (boost::system::error_code, std::size_t))
async_receive(const MutableBufferSequence& buffers,
socket_base::message_flags flags,
BOOST_ASIO_MOVE_ARG(ReadHandler) handler)
{
// If you get an error on the following line it means that your handler does
// not meet the documented type requirements for a ReadHandler.
BOOST_ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
return this->get_service().async_receive(this->get_implementation(),
buffers, flags, BOOST_ASIO_MOVE_CAST(ReadHandler)(handler));
}
get->service()的原型是什么呢？basic_stream_socket继承于basic_socket<Protocol, stream_socket_service>，而stream_socket_service类为：
// boost/asio/stream_socket_service.hpp
class stream_socket_service
#if defined(GENERATING_DOCUMENTATION)
: public boost::asio::io_service::service
#else
: public boost::asio::detail::service_base<stream_socket_service<Protocol> >
#endif
{
public:
#if defined(GENERATING_DOCUMENTATION)
/// The unique service identifier.
static boost::asio::io_service::id id;
#endif
/// The protocol type.
typedef Protocol protocol_type;
/// The endpoint type.
typedef typename Protocol::endpoint endpoint_type;
private:
// The type of the platform-specific implementation.
#if defined(BOOST_ASIO_WINDOWS_RUNTIME)
typedef detail::winrt_ssocket_service<Protocol> service_impl_type;
#elif defined(BOOST_ASIO_HAS_IOCP)
typedef detail::win_iocp_socket_service<Protocol> service_impl_type;
#else
typedef detail::reactive_socket_service<Protocol> service_impl_type;
#endif
}
从上面可以看出，Linux下，真正干活的类是reactive_socket_service，reactive_socket_service又继承自reactive_socket_service_base，该类如下：
// boost/asio/detail/Reactive_socket_service_base.hpp
class reactive_socket_service_base
{
public:
// The native type of a socket.
typedef socket_type native_handle_type;
// The implementation type of the socket.
struct base_implementation_type
{
// The native socket representation.
socket_type socket_;
// The current state of the socket.
socket_ops::state_type state_;
// Per-descriptor data used by the reactor.
reactor::per_descriptor_data reactor_data_;
};
protected:
// The selector that performs event demultiplexing for the service.
reactor& reactor_;
}
基本的继承关系图如下：
// boost/asio/detail/impl/Reactive_socket_service_base.ipp
// Start an asynchronous receive. The buffer for the data being received
// must be valid for the lifetime of the asynchronous operation.
template <typename MutableBufferSequence, typename Handler>
void async_receive(base_implementation_type& impl,
const MutableBufferSequence& buffers,
socket_base::message_flags flags, Handler& handler)
{
bool is_continuation =
boost_asio_handler_cont_helpers::is_continuation(handler);
// Allocate and construct an operation to wrap the handler.
typedef reactive_socket_recv_op<MutableBufferSequence, Handler> op;
typename op::ptr p = { boost::asio::detail::addressof(handler),
boost_asio_handler_alloc_helpers::allocate(
sizeof(op), handler), 0 };
p.p = new (p.v) op(impl.socket_, impl.state_, buffers, flags, handler);
BOOST_ASIO_HANDLER_CREATION((p.p, "socket", &impl, "async_receive"));
start_op(impl,
(flags & socket_base::message_out_of_band)
? reactor::except_op : reactor::read_op,
p.p, is_continuation,
(flags & socket_base::message_out_of_band) == 0,
((impl.state_ & socket_ops::stream_oriented)
&& buffer_sequence_adapter<boost::asio::mutable_buffer,
MutableBufferSequence>::all_empty(buffers)));
p.v = p.p = 0;
}
reactive_socket_recv_op对象op封装用户回调函数，设置事件状态；start_op调用epoll_reactor的start_op将read_op操作注册到epoll的文件描述符中。在这两个过程中，op起到了桥梁作用，一方面通过epoll检查对应描述符事件是否就绪，另一方面在就绪后进行数据IO操作，并触发用户注册的回调函数。这样就完成了整个异步IO过程。
结论：
Boost.Asio通过对用户操作、回调函数、epoll的封装，完成了异步IO，从而实现了Proactor模式。
【参考】
1.http://www.cnblogs.com/zhiranok/archive/2011/10/07/boost-asio-proactor.html
2.http://www.cnblogs.com/hello-leo/archive/2011/04/12/2013958.html