#11521: Use weak references to cache homsets
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Reporter: jpflori | Owner: robertwb
Type: defect | Status:
positive_review
Priority: major | Milestone: sage-5.4
Component: coercion | Resolution:
Keywords: sd35 | Work issues:
Report Upstream: N/A | Reviewers: Jean-Pierre
Flori, Nils Bruin
Authors: Simon King | Merged in:
Dependencies: #12969; to be merged with #715 | Stopgaps:
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Changes (by nbruin):
* status: needs_work => positive_review
Comment:
OK, I retract my concern for ''this patch''. No coercions are stored here!
In particular, any `H` occurring here is a homset, not a map. This cache
is simply ensuring that `Hom(X,Y)` is unique. So indeed, this cache should
not be preventing `H` from being collected. The `KeyedRef` is entirely in
place.
I `was` looking at #12313 when this struck me and I think the discussion
above has relevance to that case. Once a coercion `h: X -> Y` is
discovered, a strong reference to `h` is stored in
`self._coerce_from_hash`. Since `h` stores a strong reference to `X` we
now have ensured that the life time of `X` is bounded below by the life
time of `Y`. So the weak caching there only helps for `X` that do not
coerce into `Y`.
How eagerly does the system use the `self._convert_from_hash`? It is quite
likely that even with #12313 in place,
{{{#!python
for p in prime_range(1,1000):
k=GF(p)
a=k(1)
b=ZZ(a)
}}}
would still leak, due to the conversion `GF(p)->ZZ` being cached on `ZZ`.
That would make me believe we should probably not cache conversions. There
are too many of them, so it's too easy to let sage discover enough of them
that all objects in memory are in one connected component.
For coercion things are a little brighter because objects with lots of
inbound arrows (near-universal codomains?) are rarer.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/11521#comment:155>
Sage <http://www.sagemath.org>
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