[Development] leak in QMetaObject?

André Somers andre at familiesomers.nl
Wed Jul 20 11:02:53 CEST 2016

Op 20/07/2016 om 10:41 schreef Olivier Goffart:
> On Mittwoch, 20. Juli 2016 10:01:26 CEST André Somers wrote:
>> Op 19/07/2016 om 18:06 schreef Olivier Goffart:
>>> It is true that we could consider trying to clean the connection list
>>> after
>>> each signal emission if it is dirty.
>>> We don't want to do it after each disconnect because this can turn a O(n)
>>> algorithm into a O(n^2)   (cleaning the list is O(n), if we have to do it
>>> for every disconnect, that n*O(n))
>> Did anyone ever measure in practice what the distribution in values for
>> n is here? My *guess* is that it won't be all that large. I expect a
>> distribution with a spike at n=1 or 2, and then a dropoff with a very
>> long tail, perhaps spiking at a bit larger number when using QML for all
>> the propertybindings going on. Big-O notation doesn't make a lot of
>> sense if the n is small, it only starts to have an impact if n grows.
> The distribution does not matter. If it can be big, quadradic complexity can
> be a blocker.
Nonsense. There is no need to pessimize the frequent cases to cater for 
avoiding a performance issue in an exceptional case. The test case 
presented is clearly not a normal usage pattern. Of course, if you can 
optimize the algorithmic complexity for the exceptional cases without 
affecting the performance for the cases where n is small, then by all 
means do just that. I also don't think it is up to Qt to try to mitigate 
wrong usage patterns like disconnecting many slots from a single signal 
in a loop.
>> I would be suprised if the impact of the list being a single-linked list
>> and thus having to skip through memory to get from node to node was not
>> bigger than any "big-O" benefits you gained from doing it this way in
>> the first place.
> Maybe changing to a vector could be indeed beneficial. Note that it's not that
> simple as the nodes are in two different linked list at the same time. And
> this is used for efficient disconnect when a reciever is destroyed.
You know much more about this than I do of course, and perhaps a linked 
list _is_ the best data structure for this case.
>> On the other hand, you'd have to consider the likelyhood of another
>> connect hapening after a disconnect. If that is nearly 1, then sure,
>> that may be a good time to clean up. But if it is closer to 0 in reality...
> It would still not be a leak since the used memory is bounded to O (maximum
> amount of connection on that object at the same time)
No, it would not be a leak. I did not claim it was. It would still be 
wasted memory though that is kept for (way) longer than needed.


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