Some time ago, I was forced to replace the Select mechanism in a bunch of code that no one maintained with Epoll. I take this opportunity to sort out the principles of Select, Poll and Epoll.

I/O multiplexing:

I/O multiplexing can monitor multiple descriptors through a mechanism. Once a descriptor is ready (generally read ready or write ready), it can notify the program to perform corresponding read and write operations. However, select, poll, and epoll are all synchronous I/Os in essence, because they are all responsible for reading and writing after the read-write event is ready, that is, the process of reading and writing is blocked, while asynchronous I/O is not responsible for reading and writing. The implementation of asynchronous I/O is responsible for copying data from the kernel to the user space.

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