Re: [ASIO] Using boost::asio::thread_pool with sockets without boost::asio::io_context

From: Richard Hodges (hodges.r_at_[hidden])
Date: 2020-07-09 14:49:07

On Thu, 9 Jul 2020 at 16:02, Aniruddh Agarwal via Boost-users <
boost-users_at_[hidden]> wrote:

> Hello,
>
> One strategy to load-balance I/O processing across multiple
> threads is to use a single io_context object for multiplexing and
> to call io_context::run from all threads in the thread pool. This
> can be set up by, for example, defining a type which holds a
> vector of threads, all of which are calling io_context::run. My
> first question is: is this approach of calling io_context::run
> from several threads somewhat equivalent to using
> boost::asio::thread_pool as the default execution context and
> not (explicitly, at least) creating a boost::asio::io_context
> object at all?
>

It's not equivalent because in the case of one thread per io_context, the
completion handler of asynchronous operations will always be invoked on a
known thread. Therefore concurrency protection is not a concern.

If you use the executor of a thread_pool, there is no guarantee which
thread will invoke the completion handler, and no guarantee that two
completion handlers will not touch the same user object. This argues for
the use of asio::strand<asio::thread_pool::executor_type> as the executor
to be associated with the completion handler (or set as the default
executor of related io objects).

>
> Somewhat more specifically, is it safe to define TCP
> sockets/acceptors/etc. with execution contexts set to a
> boost::asio::thread_pool instead of io_context, and will that
> work as expected (i.e., as a single io_context object scheduling
> tasks out to several threads calling io_context::run would)?
>

It will work. You will need to add concurrency protection, and it will be
less efficient. From your line of questioning I don't think it will work as
you are expecting or hoping.

Is there a use case or is this a question out of curiosity?

>
> -Ani
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-- 
Richard Hodges
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