Correct me if I'm wrong, but don't modern web servers often use a handful of threads, and use epoll for each thread? If so, could this be a Part VIII?
https://nginx.org/en/docs/ngx_core_module.html#worker_proces...
The fact that you can run Apache in a process pool model doesn't mean you should. That mode was mostly kept to support CGI scripts and old style mod_php.