Multiple threads and boost's io_service

Question

Boost's documentation says: it is important to give the io_service some work to do before calling boost::asio::io_service::run() . But what happen if I give some work to do and my io_service object run method is running onto multiple threads? Should I give 1 work per thread, to prevent others to finish? Or I may start io's run on many threads and give only 1 work to do. I wish to mention, the word 'work' in my question DOES NOT refer to io_service::work::work .

Answer 1

The io_service 's work state is not determined by the amount of threads processing the io_service . For example, if an io_service has work, all threads processing the io_service via io_service::run() will remain blocked processing the event loop, even if the amount of threads is greater than the amount of posted work. Therefore, it is safe to add a single work operation to an io_service , then have many threads process the io_service .

Overall, unless concurrency is specifically hinted in the io_service constructor, the io_service does not make a distinction between its event loop being processed by a single thread or multiple threads. As noted in the threads overview , an io_service will treat all threads that have joined its pool as being equivalent, distributing work across threads in an arbitrary fashion.