How do event loops work in Python?


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Slides from the talk given at FOSDEM 2013 Python Devroom about how async I/O and event loops work in Python.

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How do event loops work in Python?

  1. 1. How do event loops work in Python? Saúl Ibarra Corretgé FOSDEM 2013
  2. 2. print(self)• Hi there!• @saghul• I work on VoIP and Real Time Communications• Python and C are my tools
  3. 3. try: socketsfrom __future__ import print_functionimport socketserver = socket.socket(family=socket.AF_INET, type=socket.SOCK_STREAM)server.bind((, 1234))server.listen(10)print("Server listening on: {}".format(server.getsockname()))client, addr = server.accept()print("Client connected: {}".format(addr))while True: data = client.recv(4096) if not data: print("Client has disconnected") break client.send(data.upper())server.close()
  4. 4. except Exception:• We can only handle one client at a time!• Solutions for handling multiple clients • Threads • I/O multiplexing• Check the C10K problem if you haven’t already! •
  5. 5. try: sockets + threadsfrom __future__ import print_functionimport socketimport threaddef handle_client(client, addr): print("Client connected: {}".format(addr)) while True: data = client.recv(4096) if not data: print("Client has disconnected") break client.send(data.upper())server = socket.socket(family=socket.AF_INET, type=socket.SOCK_STREAM)server.bind((, 1234))server.listen(10)print("Server listening on: {}".format(server.getsockname()))while True: client, addr = server.accept() thread.start_new_thread(handle_client, (client, addr))
  6. 6. except Exception:• Threads have overhead • Stack size • Context switching• Synchronization• GIL? • Not for I/O!
  7. 7. I/O multiplexing• Examine and block for I/O in multiple file descriptors at the same time • Single thread• A file descriptor is ready if the corresponding I/O operation can be performed without blocking• File descriptor has to be set to be non-blocking
  8. 8. I/O multiplexing (II)1. Put file descriptors in non-blocking mode2. Add file descriptors to a I/O multiplexor3. Block for some time4. Perform blocking operations on file descriptors which are ready
  9. 9. def set_nonblockingdef set_nonblocking(fdobj): try: setblocking = fdobj.setblocking except AttributeError: try: import fcntl except ImportError: raise NotImplementedError try: fd = fdobj.fileno() except AttributeError: fd = fdobj flags = fcntl.fcntl(fd, fcntl.F_GETFL) fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK) else: setblocking(False)
  10. 10. select• Lowest common denominator I/O multiplexor• Takes a list of “readers”, “writers”, “exceptional conditions” and a timeout• Limit of FD_SETSIZE file descriptors, usually 1024• Processing takes O(number of fds)
  11. 11. epoll• “A better select”• No FD_SETSIZE limit!• Processing takes O(1) time!• Linux only
  12. 12. Other I/O multiplexors• kqueue • Like epoll but for Max OSX and BSD• poll • Like select but without a limit of file descriptors • O(number of file descriptors) processing time • Very broken on some systems (OSX 10.3-5)
  13. 13. sys.platform == “win32” • Windows supports select • 64 file descriptors per thread - WAT • Starting with Vista WSAPoll (poll) is supported • It’s broken - 2012/10/10/wsapoll-is-broken/ • IOCP is the good stuff • Based on completion, not readiness
  14. 14. edge/level triggering• Level triggering (the most common) • You get notified when the condition is present• Edge triggering • You get notified when the condition happens• epoll and kqueue support both mechanisms• IOCP is kind-of edge-triggered epoll
  15. 15. Event loop libraries epoll, IOCP on File I/O kqueue windows libev YES NO libeiolibevent YES YES NO libuv YES YES YES
  16. 16. Event loop libraries (II) 1. Update loop time 2. Process timers 3. Process idle handles 4. Process prepare handles 5. Block for I/O 6. Process check handles
  17. 17. libuv: the powerunderneath nodeJS
  18. 18. nodeJS <= 0.4.x node standard libraryJavaScript C / C++ node bindings V8 libev libeio
  19. 19. nodeJS >= 0.6.x node standard libraryJavaScript C / C++ node bindings V8 libuv
  20. 20. libuv• Originally based on libev + libeio + IOCP• Now: epoll + kqueue + event ports + IOCP + file I/O• TCP, UDP, named pipes, TTY• Nice additions: interface addresses, process title, ...• Most complete cross platform networking library• Python bindings - pyuv
  21. 21. import pyuv• Written in C, wraps everything libuv has to offer• Python >= 2.6, supports Python 3!• Works on Windows•
  22. 22. from __future__ import print_functionimport signalimport pyuvdef on_read(client, data, error): if data is None: print("Client read error: {}".format(pyuv.errno.strerror(error))) client.close() clients.remove(client) return client.write(data.upper())def on_connection(server, error): client = pyuv.TCP(server.loop) server.accept(client) clients.append(client) client.start_read(on_read) print("Client connected: {}".format(client.getpeername()))def signal_cb(handle, signum): [c.close() for c in clients] signal_h.close() server.close()clients = []loop = pyuv.Loop.default_loop()server = pyuv.TCP(loop)server.bind(("", 1234))server.listen(on_connection)signal_h = pyuv.Signal(loop)signal_h.start(signal_cb, signal.SIGINT)
  23. 23. import pyuv• Experiments in replacing event loops of common Python networking frameworks • Twisted - • Tornado - • Gevent -
  24. 24. import pyuv• gaffer: application deployment and supervisor •• ts_analyzer: realtime analysis of multicast video streams •
  25. 25. The Python async I/O problem• Each framework uses it’s own event loop implementation• Protocols aren’t reusable• PEP-3156 to the rescue! • For Python >= 3.3• Reference implementation (Tulip)
  26. 26. import rose• PEP-3156 EventLoop implementation using pyuv• pyuv.Poll: high performance level triggered I/O (works on Windows as well!) • Uses fairy dust covered unicorns•
  27. 27. import roseimport signalfrom rose import events, protocolsclass EchoProtocol(protocols.Protocol): def connection_made(self, transport): # TODO: Transport should probably expose getsockname/getpeername print("Client connected: {}".format(transport._sock.getpeername())) self.transport = transport def data_received(self, data): self.transport.write(data.upper()) def eof_received(self): self.transport.close() def connection_lost(self, exc): print("Client closed connection")reactor = events.new_event_loop()events.set_event_loop(reactor)reactor.start_serving(EchoProtocol,, 1234)reactor.add_signal_handler(signal.SIGINT, reactor.stop)
  28. 28. saghul