1. No experience on livev, only libevent.
2. libevent is an abstraction layer providing a single API for many event based subsystems. So you can write ANSI/POSIX C code with it portable and at the maximum available event i/o layer.
I doubt any implementation on NPTL or any threading mechanism can get close in performance to this. The overhead of managing threads is big.
About multiple cores, I don't think it is necessary. The CPU work should be minimal. If there is something else you need to do as backend, maybe it can be on a separate process and use fast inter process communication, say sharing memory (rfork is a good start, for example.)
I doubt any implementation on NPTL or any threading mechanism can get close in performance to this. The overhead of managing threads is big.
About multiple cores, I don't think it is necessary. The CPU work should be minimal. If there is something else you need to do as backend, maybe it can be on a separate process and use fast inter process communication, say sharing memory (rfork is a good start, for example.)