#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: fix doctests | Upstream: N/A
Reviewer: | Author: Simon King
Merged: | Dependencies: #9138, #11900
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Comment(by SimonKing):
It turns out that using `TripleDictById` for the _action_maps cache makes
the segfault disappear.
If one uses `TripleDict` for _coercion_maps then
{{{
sage -t devel/sage-main/sage/modular/modsym/space.py
}}}
takes 30 seconds, but if one also uses `TripleDictById` then it only takes
23 seconds.
My conclusion:
* The old version of `TripleDict` was buggy: It uses `id(...)` for the
hash table, but `==` for comparison. I think that had to be fixed.
* The new version of `TripleDict` uses `hash(...)` for the hash table and
`==` for comparison. That should be fine, but (1) it leads to a segfault
and (2) it leads to a slowdown. After all, calling `hash(...)` is a lot
slower than determining the address.
* The new `TripleDictById` uses `id(...)` for the hash table and `... is
...` for comparison. Problem: It would probably not fix the memory leak.
However, the fact that using `TripleDictById` fixes the segfault makes me
wonder: Perhaps the segfault occurs when calling `hash(...)` on a parent?
Namely, in some cases, and action will already be constructed during
initialisation of a parent. But if the hash is determined based on cdef
data that aren't initialised, a segfault can easily occur.
I'll investigate that further. In any case, we need to keep an eye on the
potential slow-down.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/715#comment:48>
Sage <http://www.sagemath.org>
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