一、背景

大家都知道gevent的機制是單線程+協程機制，當遇到可能會阻塞的操作時，就切換到可運行的協程中繼續運行，以此來實現提交系統運行效率的目標，但是具體是怎么實現的呢？讓我們直接從代碼中看一下吧。

二、切換機制

讓我們從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)

這里會開啟一個死循環，在循環中調用self._sock.recv()方法，并捕獲異常，當錯誤是EWOULDBLOCK時，則調用self._wait(self._read_event)方法，該方法其實是：_wait = _wait_on_socket，_wait_on_socket方法的定義在文件：_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)

該方法其實是調用了函數：_primitive_wait()，其仍然在文件：_hub_primitives.py中定義，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)

這里其實是調用了hub.wait()函數，該函數的定義在文件_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()

該類WaitOperationsGreenlet是Hub的基類，其方法wait中的邏輯是：生成一個Waiter對象，并調用watcher.start(waiter.switch, waiter)方法，watcher是最開始recv方法中使用的self._read_event，watcher是gevent的底層事件框架libev中的概念；同時還有一個waiter對象，它類似與python中的future概念，該對象有一個switch()方法以及get()方法，當沒有得到結果沒有準備好時，調用waiter.get()方法回導致協程被掛起；get()函數的定義如下：

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()中最關鍵的是self.hub.switch()函數，該函數將執行權轉移到hub，并繼續運行，至此已經分析完了當在worker協程中從網絡獲取數據遇到阻塞時，如何避免阻塞并切換到hub中的實現，至于何時再切換會worker協程，我們后續再繼續分析。

總結

要記得gevent中一個重要的概念，協程切換不是調用而是執行權的轉移，從可能會阻塞的協程切換到hub，并由hub在合適的時機切換到另一個可以繼續運行的協程繼續執行；gevent通過這種形式實現了提高io密集型應用吞吐率的目標。

以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持好吧啦網。