[bluetomcat]

It's a subscription-based kernel API. Instead of passing roughly the same set of descriptors in each successive iteration of the event loop, you tell the kernel once that you are interested in a given fd. It can then append to the "ready list" as each of the descriptors becomes available, even when you're not waiting on epoll_wait.

With the older select() and poll() interfaces, you have to pass the whole set every time you wait. The kernel has to construct individual wait queues for each fd. Epoll is just a more convenient and efficient API, allowing an efficient implementation on the side of the kernel.