Question

I’m trying to use boost::asio for the first time to write a process that connects to N servers reads data from them.

My question regards the way in which asynchronicity works. My design goal is to connect to all servers in parallel, and also read data from every server in parallel. This should be done with async_connect and async_read, and calling io_service::run() N times, then reading the results. And the question is: is it enough to call io_service::run() from a single thread, sequentially, N times, in order to achieve parallelism?

Note that this is a matter of the implementation of asio: specifically, when calling connect_async and write_async, does the call signal the OS to begin connecting/reading before returning, or does it simply delegate a synchronous connect/read task to the worker thread and returns immediately? – case in which calling io_service::run() from a single thread means serial execution of tasks.

My guess is the former, of course, but I need someone to please confirm. I find it off that the documentation for async stuff (http://think-async.com/Asio/boost_asio_1_3_1/doc/html/boost_asio/overview/core/basics.html) doesn’t mention when the async_xxx calls return, which would clarify my question.

Answer 1

The heart of asio is an event loop, which begins with the call to io_service::run(), which is a blocking call. When you call async_connect, you queue up the connect operation in the io_services event queue. To achieve parallelism, you must create a thread pool and have each thread call run() on the same io_service instance.