1 通俗讲解 异步，非阻塞和 IO 复用
​​​ https://www.zybuluo.com/phper/note/595507 ​​​
2 知乎问答
https://www.zhihu.com/question/20122137/answer/14049112
3 Linux IO模式及 select、poll、epoll详解
http://www.hollischuang.com/archives/1830