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 method
def 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()
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。