select IO复用机制：
http://www.cnblogs.com/hjslovewcl/archive/2011/03/16/2314330.html
http://blog.csdn.net/lingfengtengfei/article/details/12392449
http://blog.csdn.net/tianmohust/article/details/6595998
http://blog.csdn.net/poechant/article/details/7627894