#14159: Don't install callbacks on values of TripleDict, MonoDict
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Reporter: nbruin | Owner: tbd
Type: PLEASE CHANGE | Status: needs_review
Priority: major | Milestone: sage-5.8
Component: PLEASE CHANGE | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: Simon King | Merged in:
Dependencies: | Stopgaps:
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Comment (by nbruin):
"Reviewer" patch to provide some doc and pointers if this ever proves to
be a problem. An example of the kind of situation where this could lead to
quadratic memory use (relative to what is required):
{{{
R=[ZZ.quo(3^n) for n in [1..100]]
for i in range(100):
for j in range(i,100):
H = Hom(R[j],R[i])
}}}
This leaves `100*101/2` entries with (eventually) dead weakrefs in the
dictionary.
I think we should leave ample documentation of this leak in the code,
including the strategy to fix it, should it become necessary.
(for posterity: The KeyedRef alternative is ''probably'' OK, because it
seems that H can never outlive its domain and codomain X and Y, so I don't
see how their id's could get reused before the callback on H has happened
or got purged. I'm just very cautious with anything that has to do with GC
and callback orders after the problems we encountered with it elsewhere.
In any case, the incantation would have to documented "DO NOT COPY THIS
WITHOUT UNDERSTANDING THE CONSEQUENCES FULLY", because it's a bad pattern
to use elsewhere, unless you prove a whole bunch of extra conditions.)
Simon, are you OK with the doc? If so we can put this to positive review
(doctests passed for me)
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/14159#comment:12>
Sage <http://www.sagemath.org>
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