Are multiple ASIO io_services a good thing?

Question

I\'ve begun using Boost.ASIO for some simple network programming, my understanding of the library is not a great deal, so please bear with me and my newbie question.

At

Answer 1

This discussion may be enlightening:

http://thread.gmane.org/gmane.comp.lib.boost.asio.user/1300

I don't have the code right here, but why would you use multiple io_services? I thought it used one io_service and multiple threads executing run on that one io_service.

IIUC, each io_service owns a select/epoll/whatever queue, so having multiple io_services is akin to having multiple independent select/epoll loops. In some situations, eg. large numbers of sockets and multiple CPUs, this might help.

Something I'm less sure about is with multiple threads all running io_service::run (with the same io_service). I think this just means the handlers are run concurrently, while the select/epoll/etc. loop is 'shared'. I think this is best for when your handlers are relatively long-running operations.

Answer 2

We use multiple io_service's because some of the components in our application need to run all their worker threads at certain fixed priorities, different for each component. Thus each component is given its own io_service, and each component has its own pool of threads executing run().

Other designs I could think of would be if a different number of threads in the pool is required for each IO, or, more relevant to your case, is if the pool cannot be shared because, for example, if your network IO can take out every thread and leave your serial IO waiting.

Answer 3

IIRC, during Michael Caisse's Boostcon ASIO talk (which is worth watching anyway), I believe this question is explicitly asked by an audience member and ok'd as a potential solution. I take from that that it's not wrong per se, and can be used that way according to your design.