Use deque instead of list the threading.Condition waiter queue #61587

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1. rhettinger commented on Mar 8, 2013

   

   rhettinger

   on Mar 8, 2013

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   Condition variables implement a FIFO queue for waiting threads. The current implementation uses a regular Python list but could use a deque instead.

   A wait() call appends a new waiter. A notify() call removes the oldest waiter; this is an O(n) operation on list but only an O(1) operation on deques. A notify_all() call is O(n**2) for a list but only O(n) for a deque.

   If there is interest in this patch, I can add slicing support to collections.deque so that this patch won't need itertools.islice()
2. rhettinger

   added

   performancePerformance or resource usagePerformance or resource usage

   on Mar 8, 2013
3. pitrou commented on Mar 9, 2013

   

   pitrou

   on Mar 9, 2013

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   I don't think you need slicing if you rewrite the patch in another way, e.g.:

   for i in range(n):
       try:
           waiter = __waiters.popleft()
       except IndexError:
           break
       waiter.release()

   I think this is safe, since notify() must be called with the lock held: another thread shouldn't be able to mutate the waiters list in the meantime.

   As for notify_all(), it could be optimized to swap the internal list with an empty one: there's no need to pop the waiters one by one.
4. rhettinger commented on Mar 10, 2013

   

   rhettinger

   on Mar 10, 2013

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   Tim, do you remember why Condition.notify() went to great lengths to act as if the lock could be released after the check for self._is_owned()?

   It loops over its own a copy of __waiters, and the __waiters.remove(waiter) code is wrapped in a try/except to detect a situation where __waiters mutated during the release-loop. I'm presuming that defensive programming was put there for a reason.
5. pitrou commented on Mar 10, 2013

   

   pitrou

   on Mar 10, 2013

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   Actually, wait() calls self._waiters.remove() without holding the lock. But I think it could easily do so after taking the lock (since it takes it anyway before returning).

   Also, _waiters should better be a set, since wait() needs the associative behaviour when unregistering a waiter.

   notify() would then look like:

   for i in range(n):
       try:
           waiter = self._waiters.pop()
       except KeyError:
           break
       waiter.release()

   and wait() would look like:

   waiter = _allocate_lock()
   waiter.acquire()
   self._waiters.add(waiter)
   self._release_save()
   try:
       return waiter.acquire(timeout)
   finally:
       self._acquire_restore()
       self._waiters.discard(waiter)
6. pitrou commented on Mar 10, 2013

   

   pitrou

   on Mar 10, 2013

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   That said, I seem to remember a discussion of Condition's fairness.
   Right now, waiters are notified in the order of wait() calls. This wouldn't be the case anymore if using a set instead of a list or deque.

   Also, I can't remember a situation where I made an intensive use of a Condition (say, hundreds of calls per second), as opposed to Lock and RLock which can be heavily invoked to protect the integrity of critical data structures.
7. rhettinger commented on Mar 10, 2013

   

   rhettinger

   on Mar 10, 2013

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   Thanks Antoine. Since the calls are made without a lock, I'll go for a minimal patch and keep the existing fairness logic.

   Adding Guido to the nosy list since this is his code.

   FWIW, the heaviest load for condition variables likely arises with use of the Queue module which implements substantially all of its logic around three condition variables and a single lock.
8. rhettinger

   self-assigned this

   on Mar 10, 2013
9. python-dev commented on Mar 11, 2013

   

   python-devmannequin

   on Mar 11, 2013

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   New changeset 0f86b51f8f8b by Raymond Hettinger in branch 'default':
   Issue bpo-17385: Fix quadratic behavior in threading.Condition
   http://hg.python.org/cpython/rev/0f86b51f8f8b
10. rhettinger

    closed this as completedon Mar 11, 2013
11. gvanrossum commented on Mar 11, 2013

    

    gvanrossum

    on Mar 11, 2013

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    Looks fine. I'd say that it would be great to add slicing to deque!

    There's one oddity in the code, but it was there before the patch -- the local variable name __waiters is pretty silly. It appears to be a micro-optimization to avoid using self._waiters more than once; even if that is worth it (I kind of doubt it), the __private name is wrong and misguided.
12. ezio-melotti

    transferred this issue fromon Apr 10, 2022