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 15:12:25

On Thu, 9 Jul 2020 at 18:01, Aniruddh Agarwal via Boost-users <
boost-users_at_[hidden]> wrote:

> On Jul 9, 2020, at 10:49 AM, Richard Hodges via Boost-users <
> boost-users_at_[hidden]> wrote:
> > 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).
>
> It seems that my original email was a bit misleading and implied that I
> was comparing (many threads, many asio::io_context objects) with
> asio::thread_pool. In fact, I’m trying to compare (many threads, one
> asio::io_context object) with asio::thread_pool. From what you’ve said
> above, it looks like (many threads, one asio::io_context object) and
> asio::thread_pool are logically equivalent.
>

>From the soon to be released boost 1.74 docs, the executor_type of io_context
and thread_pool are the same type: typedef basic_executor_type<
std::allocator< void >, 0 > executor_type; , so it seems that it is
intended that they will have the same behaviour.

>
> Can you speak to any performance differences between these (or point to
> reference material which touches on this)?
>

I cannot point you to material offhand, but you have reminded me that I
read this *somewhere*. I seem to remember something about there needing to
be something that's calling epoll/select/pselect in a loop. But now you've
got me wondering if I am mistaken. Of course if your worker thread is just
making a blocking call to run(), that's where the io loop will happen. Your
handlers will be executed within the context of the io_context::run which
will happen on a thread associated with the thread_pool.

> > Is there a use case or is this a question out of curiosity?
>
> Mostly out of curiosity. I’ve found myself repeatedly writing code where a
> single asio::io_context is dishing out tasks to many threads, and am
> wondering if asio::thread_pool is a drop-in replacement for manually
> setting up a thread pool and calling asio::io_context::run from each thread.
>

I suppose this could save you a few lines of code.

>
>  -Ani
>
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-- 
Richard Hodges
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