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epoll nonblocking-IO

提出问题：在使用epoll等IO multiplexing system call的时候，是否需要将file descriptor设置为nonblocking？下面是对这个问题的详细讨论；

stackoverflow Why having to use non-blocking fd in a edge triggered epoll function?

A

NOTE:

解释得非常到位

The idea is to try to completely drain(喝光、耗尽) the file descriptor when you have an edge-triggered notification that there is data to be had. So, once epoll() returns, you loop over the read() or write() until it returns -EAGAIN when there is no more data to be had.

If the fd was opened blocking, then this last read() or write() would also block, and you wouldn't have the chance to go back to the epoll() call to wait on the entire set of fds. When opened nonblocking, the last read()/write() does return, and you do have the chance to go back to polling.

NOTE: 上面这段话中所描述的情况其实就是在man EPOLL(7) 中所说的：

An application that employs the EPOLLET flag should use nonblocking file descriptors to avoid having a blocking read or write starve a task that is handling multiple file descriptors.

This is not so much of a concern when using epoll() in a level-triggered fashion, since in this case epoll() will return immediately if there is any data to be had. So a (pseudocode) loop such as:

while (1) {
  epoll();
  do_read_write();
}

NOTE:

在这种情况下，只要epoll();返回就可以进行read、write，因为此时一定是由数据的；

eklitzke Blocking I/O, Nonblocking I/O, And Epoll

redis usage

在redis的listenToPort中，将用于listen的socket设置成了nonblocking。

在redis的client *createClient(int fd)中，将用于于client进行通信的socket设置成了nonblocking。