在linux的网络编程中，很长的时间都在使用select来做事件触发。在linux新的内核中，有了一种替换它的机制，就是epoll。
相比于select，epoll最大的好处在于它不会随着监听fd数目的增长而降低效率。因为在内核中的select实现中，它是采用轮询来处理的，轮询的fd数目越多，自然耗时越多(In Linux network programming, long time used in the select to do event trigger. In the new Linux kernel, there is a kind of mechanism of replacing it, is the epoll.
Compared to select, epoll biggest advantage is that it will not be reduced as the number of monitored fd increase efficiency. Because in the implementation of the select kernel, it is to use the polling to deal with it, the number of fd polling, the more natural the more time-consuming)