[Python-Dev] shrinking dicts

[Vladimir Marangozov]

I wrote:
> 
> The (only) solution I could came with for this pb is the appended patch.
> It is not immediately obvious, but in practice, it seems to work fine.
> (inserting a print statement after the condition, showing the dict size
>  and current usage helps in monitoring what's going on).
> 
> Any other ideas on how to deal with this? Thoughts, comments?
> 

To clarify a bit what the patch does "as is", here's a short description:

The code is triggered in PyDict_DelItem only for sizes which are > MINSIZE*4,
i.e. greater than 4*4 = 16. Therefore, resizing will occur for a min size of
32 items.

one third  32 / 3 = 10
two thirds 32 * 2/3 = 21

one sixth  32 / 6 = 5

So the shinking will happen for a dict size of 32, of which 5 items are used
(the sixth was just deleted).  After the dictresize, the size will be 16, of
which 5 items are used, i.e. one third.

The threshold is fixed by the first condition of the patch. It could be
made 64, instead of 32. This is subject to discussion...

Obviously, this is most useful for bigger dicts, not for small ones.
A threshold of 32 items seemed to me to be a reasonable compromise.

-- 
       Vladimir MARANGOZOV          | Vladimir.Marangozov@inrialpes.fr
http://sirac.inrialpes.fr/~marangoz | tel:(+33-4)76615277 fax:76615252