Python Event::wait with timeout 给出延迟

Question

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

Event::wait 与未触发的超时一起使用时会导致延迟，因为事件已及时设置。我不明白为什么。

谁能解释一下？

下面的程序展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，有两个截然不同的代码路径。没有超时，我们只是永远等待一个锁，当我们得到锁时，我们立即返回。

但是，有了超时，您不能简单地永远等待锁，低级锁不提供超时实现。所以代码休眠的时间呈指数级增长，每次休眠后检查是否可以获取锁。代码中的相关注释:

# 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).

因此，在短时间内无法通知条件/事件的平均情况下，您会看到 25 毫秒的延迟(随机传入事件平均会在最大休眠时间 50 毫秒之前到达一半) sleep 结束)。