How does non-blocking IO work under the hood?

IOCP is not an event loop. It is true async, non-blocking I/O; completion-oriented; the I/O manager informs the thread the operation is completed.

epoll/kqueue are readiness-oriented, which continuously asks if the operation is completed via a single thread.

https://speakerdeck.com/trent/pyparallel-how-we-removed-the-...