#715: Parents probably not reclaimed due to too much caching
--------------------------------+-------------------------------------------
Reporter: robertwb | Owner: somebody
Type: defect | Status: needs_work
Priority: major | Milestone: sage-4.8
Component: coercion | Keywords: weak cache coercion
Work_issues: avoid regression | Upstream: N/A
Reviewer: | Author: Simon King
Merged: | Dependencies: #9138, #11900
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Old description:
> 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.
>
> Apply:
>
> [attachment:trac715_two_tripledicts.patch]
New description:
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.
Apply:
* [attachment:trac715_two_tripledicts.patch]
* [attachment:trac715_weak_action.patch]
--
Comment(by SimonKing):
I have attached another patch, which implements the ideas sketched above.
I think it corresponds to what you suggested ("use a weak reference from
the action to the domain").
One detail: We have to distinguish between the underlying set, the domain
and the codomain of an action. In fact, the new patch only uses a weak
reference to the underlying set, and introduces a cdef function (hence,
hopefully with little overhead) returning it.
I consider sage-5.0.prealpha0 plus
trac11780_unique_auxiliar_polyring.patch (probably not needed) plus
[attachment:trac715_two_tripledicts.patch] plus
[attachment:trac715_weak_action.patch].
At least `sage -t sage/modular/modsym/space.py` passes, but I need to run
the whole test suite.
The example from the ticket description does not leak. However, if the
j-invariant varies, it seems that for each elliptic curve one copy is
preserved:
{{{
sage: K = GF(1<<55,'t')
sage: for i in range(500):
....: a = K.random_element()
....: E = EllipticCurve(j=a)
....: P = E.random_point()
....: Q = 2*P
....:
sage: import gc
sage: gc.collect()
2124
sage: from sage.schemes.generic.homset import
SchemeHomsetModule_abelian_variety_coordinates_field
sage: LE = [x for x in gc.get_objects() if
isinstance(x,SchemeHomsetModule_abelian_variety_coordinates_field)]
sage: len(LE)
500
}}}
In any case, the original leak is fixed with the two patches. The question
is whether the second patch suffices to keep actions alive, whether it
avoids a regression, and whether all tests pass.
If everything is alright, we may still try to find out where the remaining
strong reference to an elliptic curve comes from.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/715#comment:88>
Sage <http://www.sagemath.org>
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