1、水平触发 VS 边缘触发

水平触发(level-triggered，也被称为条件触发)LT: 只要满足条件，就触发一个事件(只要有数据没有被获取，内核就不断通知你)

边缘触发(edge-triggered)ET: 每当状态变化时，触发一个事件(仅当状态由not ready 到 ready时触发一个事件)
Eedge-triggered readiness notification means you give the kernel a file descriptor, and later, when that descriptor transitions from not ready to ready, the kernel notifies you somehow. It then assumes you know the file descriptor is ready, and will not send any more readiness notifications of that type for that file descriptor until you do something that causes the file descriptor to no longer be ready (e.g. until you receive the EWOULDBLOCK error on a send, recv, or accept call, or a send or recv transfers less than the requested number of bytes). epoll is the recommended edge-triggered poll replacement for the 2.6 Linux kernel.

举个读socket的例子，假定经过长时间的沉默后，现在来了100个字节，这时无论边缘触发和条件触发都会产生一个read ready notification通知应用程序可读。应用程序读了50个字节，然后重新调用api等待io事件。这时水平触发的api会因为还有50个字节可读从 而立即返回用户一个read ready notification。而边缘触发的api会因为可读这个状态没有发生变化而陷入长期等待。 因此在使用边缘触发的api时，要注意每次都要读到socket返回EWOULDBLOCK为止，否则这个socket就算废了。而使用条件触发的api 时，如果应用程序不需要写就不要关注socket可写的事件，否则就会无限次的立即返回一个write ready notification。大家常用的select就是属于水平触发这一类，长期关注socket写事件会出现CPU 100%的毛病。