#715: Parents probably not reclaimed due to too much caching
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Reporter: robertwb | Owner: somebody
Type: defect | Status: needs_work
Priority: major | Milestone: sage-4.8
Component: coercion | Keywords: weak cache coercion
Work_issues: Get some timings | Upstream: N/A
Reviewer: | Author: Simon King
Merged: | Dependencies: #9138, #11900
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Comment(by SimonKing):
Perhaps as follows: We currently have one ensemble of buckets. Instead, we
could have two ensembles, say, `self.keys` and `self.refs`. Each bucket in
both ensembles is a list of length `3*n`. Let `X,Y,Z` be key, let h be
the hash of that triple and V the value associated with `X,Y,Z`.
Then, one could store `X,Y,Z` as `self.keys[h][i:i+3]`, with artificially
decrementing the reference count for X and Y (but not for Z, which usually
is True, False, None, operator.mul and so on), as suggested by Volker. And
`self.refs[h][i:i+3]` would be formed by a weak reference to X, a weak
reference to Y, and V. The two weak references have a callback function,
that tries to find a reference `self.refs[h][j]` when it became invalid,
and would delete the corresponding triple both in `self.refs[h]` and in
`self.keys[h]`.
Two weak references with callback function pointing to the same object are
distinct (they are only the same if they don't have a callback function).
Hence, each reference occurs in the `TripleDict` exactly once. Hence, it
makes sense to store the hash value of the ''triple'' `X,Y,Z` as
additional data both in the reference to X and to Y - which is possible
with `weakref.KeyedRef`. In that way, deleting an entry when a reference
became invalid would be much faster as with my current patch, since it is
not needed to search in ''all'' buckets.
I will try it tomorrow.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/715#comment:117>
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