#715: Parents probably not reclaimed due to too much caching
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Reporter: robertwb |
Owner: somebody
Type: defect |
Status: needs_review
Priority: major |
Milestone: sage-pending
Component: coercion |
Resolution:
Keywords: weak cache coercion Cernay2012 | Work
issues:
Report Upstream: N/A |
Reviewers: Jean-Pierre Flori, Simon King
Authors: Simon King, Jean-Pierre Flori | Merged
in:
Dependencies: #9138, #11900, #11599, to be merged with #11521 |
Stopgaps:
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Comment (by nbruin):
Excellent! Thank you for the great work. This is incredibly important for
so
many parts of sage.
> Previously, there was a global dictionary, that was shared by all
> `TripleDicts`. That probably was a bad idea, for the reasons you pointed
out.
> Now, the references are stored in a dictionary that is an attribute of
each
> `TripleDict`.
Excellent! I agree with your assessment. I think this addresses all my
concerns.
I think this is a useful data structure in general, so can we formalize
its
behaviour in the documentation? (rewrite as you see fit)
{{{
TripleDict is a structure like WeakKeyDictionary, optimized for lookup
speed.
Keys consist of a triple (k1,k2,k3) and are looked up by identity rather
than
equality. The keys are stored by weakrefs if possible. If any one of the
components k1,k2,k3 gets garbage collected, then the entry is removed from
the
TripleDict. Key components that do not allow for weakrefs are stored via a
normal refcounted reference. That means that any entry stored using a
triple
(k1,k2,k3) with none of the k1,k2,k3 weakreffable behaves as an entry in a
normal dictionary, so its existence in TripleDict prevents it from being
garbage
collected.
}}}
> Another advantage: If the `TripleDict` is deallocated, then the strong
> references associated with the `TripleDict` will vanish as well, which
wouldn't
> have been the case with the old code.
AND if an entry gets deleted/garbage collected due to a weakreffed key
component
disappearing, we also deref any strongly reffed key components! So I think
we
never behave worse than a normal dict in terms of keeping objects alive.
> Currently, there is only one bad situation I can think of: Let P be an
object
> that can not be weak-refed, has a `TripleDict` T as an attribute, is
used as a
> key in T, and has a `__del__` method. Then the reference cycle
> P->T->T._refcache->P will keep P alive.
I think we're safe for that. There are very few `__del__` definitions in
the
sage library and they're all associated with interface-type objects. And
those
are plain python classes anyway, so they are weakreffable.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/715#comment:206>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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