Parents probably not reclaimed due to too much caching

[robertwb]

Here is a small example illustrating the issue.

The memory footprint of the following piece of code grows indefinitely.

sage: K = GF(1<<55,'t') 
sage: a = K.random_element() 
sage: while 1: 
....:     E = EllipticCurve(j=a); P = E.random_point(); 2*P; del E, P;

E and P get deleted, but when 2*P is computed, the action of integers on A, the abelian group of rational points of the ellitpic curve, gets cached in the corecion model.

A key-value pair is left in coercion_model._action_maps dict:

(ZZ,A,*) : IntegerMulAction?

Moreover there is at least also references to A in the IntegerMulAction? and one in ZZ._action_hash.

So weak refs should be used in all these places if it does not slow things too much.

[mhansen]

I think this is a bit too vague for a ticket. Robert, could you be more specific or close this?

[robertwb]

The coercion model needs to use weakrefs so that parents aren't needlessly referenced when they're discarded. It is nontrivial to see where the weakrefs need to go, and how to do so without slowing the code down.

The ticket is still valid.

[jpflori]

With the piece of code in the desrciption, there is only one reference to these objects in that ZZ._hash_actions dictionary because to build it we test if A1 == A2 and not A1 is A2 as in coercion_model._action_maps, and because of the current implementation of ellitpic curves, see ​http://groups.google.com/group/sage-nt/browse_thread/thread/ec8d0ad14a819082 and ​#11474, and decause the above code use only one j-invariant, only ones gets finally stored.

However with random curves, I guess there would be all of them.

About the weakref, the idea should more be to build something like WeakKeyDictionnary? if it does not slow down coercion too much...