> this means that the relevant kernel thread woke up from epoll_wait() or something similar. With blocking I/O, you call read(), and the kernel wakes up your th

> this means that the relevant kernel thread woke up from epoll_wait() or something similar. With blocking I/O, you call read(), and the kernel wakes up your thread when the read() completes. With non-blocking I/O, you call read(), get EAGAIN, call epoll_wait(), the kernel wakes up your thread when data is ready, and then you call read() a second time. This is true for readiness-based IO (which, admittedly, most current async IO loops are using), but completion-based IO (such as IOCP or io_uring) don't suffer from this problem: you just add your IO operation to the queue, and do a single syscall that will return once one of the operation in your queue is completed AFAIK.