求解协程+多进程的正确使用姿势

用 asyncio 做了一个 UDP 传输性能测试工具，目前单进程服务端性能不够，流量大的时候处理不过来，服务端用的 asyncio.DatagramProtocol，怎么变成多进程的呢？试试了一下抢占式的写法，运行报错了，运行起来也只有一个进程工作，上代码

import asyncio
import time
import socket
from multiprocessing import Process

loop = asyncio.get_event_loop()
size = 0

class ServerProtocol(asyncio.DatagramProtocol):

    def connection_made(self, transport):
        self.transport = transport

    def datagram_received(self, data, addr, args=None):
        global size
        data = data.decode()
        message = data
        index = data.find('\n')
        if index > 0:
            filename = data[0:index]
            data = data[index+1::]
            size += len(data)
        task = self.WriteToFile(filename, data)
        asyncio.run_coroutine_threadsafe(task, loop)

    async def WriteToFile(self, f, data):
        await asyncio.sleep(1)
        return True

async def print_size():
    global size
    while True:
        await asyncio.sleep(1)
        print(size)

def start(sock):
    listen = loop.create_datagram_endpoint(
        ServerProtocol,
        sock = sock
    )
    transport, protocol = loop.run_until_complete(listen)

    task = print_size()
    asyncio.run_coroutine_threadsafe(task, loop)
    try:
        loop.run_forever()
    except KeyboardInterrupt:
        pass
    transport.close()
    loop.close()
    sock.close()

if __name__ == '__main__':
    print("Starting UDP server")
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    sock.bind(('0.0.0.0', 9873))
    for i in range(1):
        t = Process(target=start, args=(sock,))
        t.deamon = True
        t.start()
    start(sock)

报错信息

Exception in callback BaseSelectorEventLoop._add_reader(6, <bound method..., bufsize=0>>>)
handle: <Handle BaseSelectorEventLoop._add_reader(6, <bound method..., bufsize=0>>>)>
Traceback (most recent call last):
  File "/usr/local/python36/lib/python3.6/asyncio/selector_events.py", line 264, in _add_reader
    key = self._selector.get_key(fd)
  File "/usr/local/python36/lib/python3.6/selectors.py", line 191, in get_key
    raise KeyError("{!r} is not registered".format(fileobj)) from None
KeyError: '6 is not registered'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/python36/lib/python3.6/asyncio/events.py", line 145, in _run
    self._callback(*self._args)
  File "/usr/local/python36/lib/python3.6/asyncio/selector_events.py", line 267, in _add_reader
    (handle, None))
  File "/usr/local/python36/lib/python3.6/selectors.py", line 412, in register
    self._epoll.register(key.fd, epoll_events)
FileExistsError: [Errno 17] File exists