On 17/12/2013 09:18, Quoth maks:

In what cases using "io_service(std::size_t concurrency_hint)" ctor is more effective? 1. Use multiple threads - multiple io_service 2. Use multiple threads on one io_service object

Multiple io_services are completely independent. The parameter is a hint for permissible concurrency for a single io_service being run by multiple threads. What behaviour this actually affects depends on the platform -- for example, on Windows this is passed to the OS when creating the underlying IOCP, which tells it how to throttle thread wakeups.