Python的事件::等待与超时延迟给出

Question

在Windows 7上运行的Python 2.6和2.7和Server 2012中

事件::因为与事件在时间设置未触发超时使用时的等待造成的延迟。我不明白为什么。

有人可以解释一下吗？

下面的程序显示了这一点并给出了可能的解释;

'''Shows that using a timeout in Event::wait (same for Queue::wait) causes a 
delay. This is perhaps caused by a polling loop inside the wait implementation. 
This polling loop sleeps some time depending on the timeout. 
Probably wait timeout > 1ms => sleep = 1ms 
A wait with timeout can take at least this sleep time even though the event is 
set or queue filled much faster.''' 
import threading 

event1 = threading.Event() 
event2 = threading.Event() 

def receiver(): 
    '''wait 4 event2, clear event2 and set event1.''' 
    while True: 
    event2.wait() 
    event2.clear() 
    event1.set() 

receiver_thread = threading.Thread(target = receiver) 
receiver_thread.start() 

def do_transaction(timeout): 
    '''Performs a transaction; clear event1, set event2 and wait for thread to set event1.''' 
    event1.clear() 
    event2.set() 
    event1.wait(timeout = timeout) 

while True: 
    # With timeout None this runs fast and CPU bound. 
    # With timeout set to some value this runs slow and not CPU bound. 
    do_transaction(timeout = 10.0) 

Answer 1

查看threading.Condition类的wait（）方法的源代码，有两种截然不同的代码路径。如果没有超时，我们会永远等待锁，当我们获得锁时，我们会立即返回。

但是，由于超时，您不能简单地等待锁定，并且低级锁定不会实现超时。因此，如果可以获取锁，每次睡眠检查之后，代码将会处于指数级更长的时间周期。从代码中的相关评论：

# Balancing act: We can't afford a pure busy loop, so we 
# have to sleep; but if we sleep the whole timeout time, 
# we'll be unresponsive. The scheme here sleeps very 
# little at first, longer as time goes on, but never longer 
# than 20 times per second (or the timeout time remaining). 

所以在平均场景条件/事件不能是不是很短的时间内通知，你会看到一个25毫秒的延迟（随机进入事件平均将抵达在睡眠结束前剩余50ms的最大睡眠时间的一半）。