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Python Event :: wait with timeout gives delay

Running Python 2.6 and 2.7 on Windows 7 and Server 2012

The :: wait event causes a delay when used with a timeout that does not fire because the event is set in time. I do not understand why.

Can someone explain?

The following program shows this and gives a possible explanation;

'''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)
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1 answer

Looking at the source code of the wait () method of the threading.Condition class, there are two very different code paths. Without a timeout, we just hope to block forever, and when we get the lock, we will return immediately.

, -. , , , . :

# 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).

, , / , 25 ( 50 , ).

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