Python gevent协程切换实现详解

一、背景

大家都知道gevent的机制是单线程+协程机制，当遇到可能会阻塞的操作时，就切换到可运行的协程中继续运行，以此来实现提交系统运行效率的目标，但是具体是怎么实现的呢？让我们直接从代码中看一下吧。

二、切换机制

让我们从socket的send、recv方法入手：

def recv(self, *args): while 1: try: return self._sock.recv(*args) except error as ex: if ex.args[0] != EWOULDBLOCK or self.timeout == 0.0:raise # QQQ without clearing exc_info test__refcount.test_clean_exit fails sys.exc_clear() self._wait(self._read_event)

这里会开启一个死循环，在循环中调用self._sock.recv()方法，并捕获异常，当错误是EWOULDBLOCK时，则调用self._wait(self._read_event)方法，该方法其实是：_wait = _wait_on_socket，_wait_on_socket方法的定义在文件：_hub_primitives.py中，如下：

# Suitable to be bound as an instance methoddef wait_on_socket(socket, watcher, timeout_exc=None): if socket is None or watcher is None: # test__hub TestCloseSocketWhilePolling, on Python 2; Python 3 # catches the EBADF differently. raise ConcurrentObjectUseError('The socket has already been closed by another greenlet') _primitive_wait(watcher, socket.timeout, timeout_exc if timeout_exc is not None else _NONE, socket.hub)

该方法其实是调用了函数：_primitive_wait()，其仍然在文件：_hub_primitives.py中定义，如下：

def _primitive_wait(watcher, timeout, timeout_exc, hub): if watcher.callback is not None: raise ConcurrentObjectUseError(’This socket is already used by another greenlet: %r’ % (watcher.callback, )) if hub is None: hub = get_hub() if timeout is None: hub.wait(watcher) return timeout = Timeout._start_new_or_dummy( timeout, (timeout_exc if timeout_exc is not _NONE or timeout is None else _timeout_error(’timed out’))) with timeout: hub.wait(watcher)

这里其实是调用了hub.wait()函数，该函数的定义在文件_hub.py中，如下：

class WaitOperationsGreenlet(SwitchOutGreenletWithLoop): # pylint:disable=undefined-variable def wait(self, watcher): ''' Wait until the *watcher* (which must not be started) is ready. The current greenlet will be unscheduled during this time. ''' waiter = Waiter(self) # pylint:disable=undefined-variable watcher.start(waiter.switch, waiter) try: result = waiter.get() if result is not waiter:raise InvalidSwitchError( ’Invalid switch into %s: got %r (expected %r; waiting on %r with %r)’ % ( getcurrent(), # pylint:disable=undefined-variable result, waiter, self, watcher )) finally: watcher.stop()

watcher.stop()

该类WaitOperationsGreenlet是Hub的基类，其方法wait中的逻辑是：生成一个Waiter对象，并调用watcher.start(waiter.switch, waiter)方法，watcher是最开始recv方法中使用的self._read_event，watcher是gevent的底层事件框架libev中的概念；同时还有一个waiter对象，它类似与python中的future概念，该对象有一个switch()方法以及get()方法，当没有得到结果没有准备好时，调用waiter.get()方法回导致协程被挂起；get()函数的定义如下：

def get(self): '''If a value/an exception is stored, return/raise it. Otherwise until switch() or throw() is called.''' if self._exception is not _NONE: if self._exception is None: return self.value getcurrent().throw(*self._exception) # pylint:disable=undefined-variable else: if self.greenlet is not None: raise ConcurrentObjectUseError(’This Waiter is already used by %r’ % (self.greenlet, )) self.greenlet = getcurrent() # pylint:disable=undefined-variable try: return self.hub.switch() finally: self.greenlet = None

在get()中最关键的是self.hub.switch()函数，该函数将执行权转移到hub，并继续运行，至此已经分析完了当在worker协程中从网络获取数据遇到阻塞时，如何避免阻塞并切换到hub中的实现，至于何时再切换会worker协程，我们后续再继续分析。

总结

要记得gevent中一个重要的概念，协程切换不是调用而是执行权的转移，从可能会阻塞的协程切换到hub，并由hub在合适的时机切换到另一个可以继续运行的协程继续执行；gevent通过这种形式实现了提高io密集型应用吞吐率的目标。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持好吧啦网。