[Seaside] Re: Why isn't this a weak dictionary of some ilk?
philippe.marschall at gmail.com
Wed Apr 22 20:21:48 UTC 2009
2009/4/22 Igor Stasenko <siguctua at gmail.com>:
> 2009/4/22 Lukas Renggli <renggli at gmail.com>:
>>> Lukas, all these topics is about a weak array finalization process,
>>> which is just made for a convenience and on top of the fact, that VM
>>> supports weak references.
>> I understand that. And I remember that the suggestion was to implement
>> our own finalization code. The reasons why we didn't do so were the
>> - It was pretty easy to get rid of the requirement for weak data
>> structures. The wasted memory was not significant enough to make it
>> worth worrying about a special solution.
>> - Introducing custom finalization code is highly non-portable across.
>> At the time of Seaside 2.7 and 2.8, there were no easy possibilities
>> to factor such code out.
>> - Last but not least, I might be wrong on this, we had the impression
>> that this was a Squeak only problem. I've never heard of similar
>> problems with other Smalltalk. The finalization of session state just
>> works out of the box.
>> Maybe the real root of the problem is not even the weak finalization,
>> but the way Squeak handles and rehashes large Dictionaries?
> Yes, rehashing is sloow and in linear dependency from a number of elements.
> To avoid rehashing, one could use a binary tree. But it having own
> cost: access to specific element takes O(log n) time, while current
> hash-based implementations is O(1) for best case, and, technically,
> O(n) for the worst case.
> If you remember, i implemented a separate set of Dictionaries
> (inluding Weak ones) for Magma, which were optimized for handling a
> large data sets at the cost of additional memory per key/value pair.
> It is also, have a different worst case access time - O(k), where k is
> number of elements which having same hash value under given key.
> As for finalization - its obvious, that you need to keep the amount of
> weak objects, which require finalization, small. Because to determine
> which object dies, you have to iterate over every item in collection -
> which is strictly linear.
> That's why putting too much objects into WeakRegistry makes squeak
> very slow, because it has to iterate over this collection over and
> over at _each_ garbage collection cycle, to determine which objects
> became garbage.
> Thus, without efficient support from VM side, a finalization scheme is
> doomed to be slow.
That's why we don't use it. Seriously we are talking about images
being blocked for 10 min to 20 min because of weak array finalization.
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