boost::asio threadpool vs. io_service_per_cpu design

Currently I´m not sure, I try to make a high-performance server, I got a 6Core CPU, so if I would use the "io_service_per_cpu" design, I have 6 io_service´s.

I already heard that the threadpool design isn´t the best one, but I´m not sure about that.

What knowledge do you have? Someone already made a Stress test with each, or something else?