Readiness-based event queues

epoll and kqueue are known as readiness-based event queues, which means they let you know when an action is ready to be performed. An example of this is a socket that is ready to be read from.

To give an idea about how this works in practice, we can take a look at what happens when we read data from a socket using epoll/kqueue:

1. We create an event queue by calling the syscall epoll_create or kqueue.
2. We ask the OS for a file descriptor representing a network socket.
3. Through another syscall, we register an interest in Read events on this socket. It’s important that we also inform the OS that we’ll be expecting to receive a notification when the event is ready in the event queue we created in step 1.
4. Next, we call epoll_wait or kevent to wait for an event. This will block (suspend) the thread it’s called on.
5. When the event is ready, our thread is unblocked (resumed) and we return from our wait call with data about...