[boost] Asio multithreaded http server

Asio samples lacks a multithreaded server implementation: where can I find an example/tutorial about that? I need to write a multithreaded http server: I'm running multiple calls to io_service::run from a pool of threads but it looks that if my session object takes a lot of time to generate the http response the server block any accept operation until the session has finished... plz help, thanks