(1) Select, the poll implementation needs to poll all fd collections continuously by itself until the device is ready, possibly alternating between sleep and wake up several times. Epoll also calls epoll_WAIT to poll the ready list, and it may sleep and wake up several times, but when the device is ready, epoll calls the callback function, puts the ready FD into the ready list, and wakes up the process that went to sleep in epoll_wait. Although both sleep and alternate, select and Poll traverse the entire FD collection while awake, while EPoll is awake to determine if the ready list is empty, which saves a lot of CPU time. This is the performance benefit of the callback mechanism.
(2) poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait: poll_wait Note that the wait queue here is not a device wait queue, just a wait queue defined internally by epoll. It also saves a lot of money.