Boost.ASIO简要分析-5 多io_service

5. 多io_service

前面那篇讲到了多线程的用法。这篇讲一下多io_service的用法,大家可参考下官方提供的HTTP Server 2（an io_service-per-CPU）这个例子。

官方提供的例子中，使用方法很简单，建立一个io_service_pool，然后对每一个io_service开一个线程去让它跑起来（毕竟，io_service::run这个函数在有任务的时候会一直工作的，只有开多个线程才做到不影响别的io_service）。当然，我们也可以为每个io_service开多个线程，这样才不会存在上一篇中说到的多线程问题。

下面以Windows平台讨论io_service，看下io_service的构造过程在Windows下的实现

win_iocp_io_service::win_iocp_io_service(

boost::asio::io_service& io_service, size_t concurrency_hint)

  : boost::asio::detail::service_base<win_iocp_io_service>(io_service),

iocp_(),

outstanding_work_(0),

stopped_(0),

stop_event_posted_(0),

shutdown_(0),

gqcs_timeout_(get_gqcs_timeout()),

dispatch_required_(0)

{

BOOST_ASIO_HANDLER_TRACKING_INIT;

iocp_.handle = ::CreateIoCompletionPort(INVALID_HANDLE_VALUE, 0, 0,

static_cast<DWORD>(concurrency_hint < DWORD(~0)

        ? concurrency_hint : DWORD(~0)));

if (!iocp_.handle)

  {

DWORD last_error = ::GetLastError();

boost::system::error_code ec(last_error,

boost::asio::error::get_system_category());

boost::asio::detail::throw_error(ec, "iocp");

  }

}

通过源码，我们可以发现每个io_service在构造的过程中，会创建一个IOCP。

创建好之后，我们就可以使用这个IOCP了，

boost::system::error_code win_iocp_io_service::register_handle(

HANDLE handle, boost::system::error_code& ec)

{

if (::CreateIoCompletionPort(handle, iocp_.handle, 0, 0) == 0)

  {

DWORD last_error = ::GetLastError();

ec = boost::system::error_code(last_error,

boost::asio::error::get_system_category());

  }

else

  {

ec = boost::system::error_code();

  }

return ec;

}

哦，看着就这么简单。

在io_service类代码中发现add_service这么个函数

/// Add a service object to the io_service.

/**

   * This function is used to add a service to the io_service.

   *

   * @param ios The io_service object that owns the service.

   *

   * @param svc The service object. On success, ownership of the service object

   * is transferred to the io_service. When the io_service object is destroyed,

   * it will destroy the service object by performing:

   * @code delete static_cast<io_service::service*>(svc) @endcode

   *

   * @throws boost::asio::service_already_exists Thrown if a service of the

   * given type is already present in the io_service.

   *

   * @throws boost::asio::invalid_service_owner Thrown if the service's owning

   * io_service is not the io_service object specified by the ios parameter.

   */

template <typename Service>

friend void add_service(io_service& ios, Service* svc);

从注释可以看到，io_service之间可以相互转移service对象。单io_service应该不会用到这个函数吧。