when epoll instead poll ?

1. You'll have to be more specific.
2. The client library is multithreaded, it's much more complicated for the broker. The single threaded broker can handle 200k clients with QoS 2 and modest message rates: https://lists.launchpad.net/mosquitto-users/msg00107.html
3. epoll is Linux specific whereas poll is available everywhere, so it's extremely unlikely that epoll will ever be supported. I am looking at event loop libraries that abstract out the poll/epoll/kqueue interfaces. Using one of those would allow epoll to be used on Linux.

I hope that helps.

Roger

thank you ,usefully ,I will do more test on them.

I second that. I would go for libevent, its a easy to use library with great multithread support, and you can even leverage the integrated SSL.

Cheers