I/O多路复用（multiplexing）的本质是通过一种机制（系统内核缓冲I/O数据），让单个进程可以监视多个文件描述符，一旦某个描述符就绪（一般是读就绪或写就绪），能够通知程序进行相应的读写操作。

select、poll 和 epoll 都提供的 IO 复用方式。

（1）select，poll实现需要自己不断轮询所有fd集合，直到设备就绪，期间可能要睡眠和唤醒多次交替。而epoll其实也需要调用 epoll_wait不断轮询就绪链表，期间也可能多次睡眠和唤醒交替，但是它是设备就绪时，调用回调函数，把就绪fd放入就绪链表中，并唤醒在 epoll_wait中进入睡眠的进程。虽然都要睡眠和交替，但是select和poll在“醒着”的时候要遍历整个fd集合，而epoll在“醒着”的 时候只要判断一下就绪链表是否为空就行了，这节省了大量的CPU时间，这就是回调机制带来的性能提升。

（2）select，poll每次调用都要把fd集合从用户态往内核态拷贝一次，并且要把current往设备等待队列中挂一次，而epoll只要 一次拷贝，而且把current往等待队列上挂也只挂一次（在epoll_wait的开始，注意这里的等待队列并不是设备等待队列，只是一个epoll内 部定义的等待队列），这也能节省不少的开销。
selectors实现IO多路复用

#服务端
#!/usr/bin/env python
# -*- coding: utf-8 -*-

import select, socket

response = b"hello world"

#创建一个server socket
serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
serversocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
serversocket.bind(('localhost', 8080))
serversocket.listen(1)
serversocket.setblocking(0)

inputs = [serversocket, ]

while True:
    rlist, wlist, xlist = select.select(inputs, [], [])
    for sock in rlist:
        ＃ server socket读就绪
        if sock == serversocket:
            con, addr = serversocket.accept()
            ＃将这个connection添加到读就绪中
            inputs.append(con)
        else:
            data = sock.recv(1024)
            if data:
                sock.send(response)
                ＃从读就绪的list中删除
                inputs.remove(sock)
                sock.close()
#客户端
from socket import *
c=socket(AF_INET,SOCK_STREAM)
c.connect(('127.0.0.1',8080))

while True:
    msg=input('>>: ')
    if not msg:continue
    c.send(msg.encode('utf-8'))
    data=c.recv(1024)
    print(data.decode('utf-8'))

poll实现IO多路复用

server端：

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import select, socket

response = b"hello world"

serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
serversocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
serversocket.bind(('192.168.199.197', 8080))
serversocket.listen(1)
serversocket.setblocking(0)

#
poll = select.poll()
poll.register(serversocket.fileno(), select.POLLIN)

connections = {}
while True:
    for fd, event in poll.poll():
        if event == select.POLLIN:
            if fd == serversocket.fileno():
                con, addr = serversocket.accept()
                poll.register(con.fileno(), select.POLLIN)
                connections[con.fileno()] = con
            else:
                con = connections[fd]
                data = con.recv(1024)
                if data:
                    poll.modify(con.fileno(), select.POLLOUT)
        elif event == select.POLLOUT:
            con = connections[fd]
            con.send(response)
            poll.unregister(con.fileno())
            con.close()

 epoll实现IO多路复用

import socket, select

EOL1 = b'\n\n'
EOL2 = b'\n\r\n'
response  = b'HTTP/1.0 200 OK\r\nDate: Mon, 1 Jan 1996 01:01:01 GMT\r\n'
response += b'Content-Type: text/plain\r\nContent-Length: 13\r\n\r\n'
response += b'Hello, world!'

serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
serversocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
serversocket.bind(('0.0.0.0', 8080))
serversocket.listen(1)
serversocket.setblocking(0)

epoll = select.epoll()
epoll.register(serversocket.fileno(), select.EPOLLIN)

try:
   connections = {}; requests = {}; responses = {}
   while True:
      events = epoll.poll(1)
      for fileno, event in events:
         if fileno == serversocket.fileno():
            connection, address = serversocket.accept()
            connection.setblocking(0)
            epoll.register(connection.fileno(), select.EPOLLIN)
            connections[connection.fileno()] = connection
            requests[connection.fileno()] = b''
            responses[connection.fileno()] = response
         elif event & select.EPOLLIN:
            requests[fileno] += connections[fileno].recv(1024)
            if EOL1 in requests[fileno] or EOL2 in requests[fileno]:
               epoll.modify(fileno, select.EPOLLOUT)
               print('-'*40 + '\n' + requests[fileno].decode()[:-2])
         elif event & select.EPOLLOUT:
            byteswritten = connections[fileno].send(responses[fileno])
            responses[fileno] = responses[fileno][byteswritten:]
            if len(responses[fileno]) == 0:
               epoll.modify(fileno, 0)
               connections[fileno].shutdown(socket.SHUT_RDWR)
         elif event & select.EPOLLHUP:
            epoll.unregister(fileno)
            connections[fileno].close()
            del connections[fileno]
finally:
   epoll.unregister(serversocket.fileno())
   epoll.close()
   serversocket.close()

详情见：https://www.cnblogs.com/9527chu/p/5661932.html