Hi Jeroen,
Am Montag, 3. Dezember 2018 11:46:50 UTC+1 schrieb Jeroen Demeyer:
>
> I am studying the coercion model in detail, looking for optimization
> opportunities. One source of slow-down is the use of weak references.
>
> Over time, more and more places in Sage use weak references. But I'd
> like to look at the big picture and see where weak references should be
> used and where not.
>
> Take coercion maps for example. The coercion model stores a weak
> reference to the coercion maps and the maps also store a weak reference
> to the domain (but not the codomain).
>
> It's not clear to me why this double weak reference is needed, but maybe
> I'm missing something. It seems more logical to use strong references in
> the coercion map but then store a weak reference to the map.
>
I was involved in the development of the weakly cached coercion system, but
I am afraid I don't recall all the rationales behind the construction.
Let's try to explain anyway...
First, I summarise how currently coercion data is stored:
1. Each parent has a cache of coercion maps for which the parent is
codomain. The cache uses a hash table (MonoDict) with a weak reference to
the domain (the key) and a strong reference to the coercion map.
2. In some cases, coercion between two parents P,Q involves the creation
of a new parent R, such that both P and Q coerce into R. That's a
complicated construction, therefore the result is cached. This cache
currently is a global hash table (TripleDict), with keys being P and Q that
are weakly referenced. There is a weak reference to both coercion maps (P
to R and Q to R).
3. A similar scheme is used for actions. Here, in addition, the operator
(operator.mul, operator.add etc) is used as key.
How does that prevent memory leaks? Let there be a coercion map f from Q to
P, and maps gP from P to R and gQ from Q to R. Let C be the global cache.
By => resp. -> I mean a strong resp. weak reference.
- We have
- C->gP, C->gQ, C->Q, C->P
- P=>f, P->Q
- f=>Q, f->P, fP=>R, fQ=>R, fP->P, fQ->Q
- R=>fP, R=>fQ, R->P, R->Q
- There thus is a reference cycle involving a coercion map and its
codomain. All other references in the above graph are weak.
- If there is no external strong reference chain from a global object
to P, then the pair (fP,fQ) is removed from C, and P together with f will
be collected.
- If there is no external strong reference chain from a global object
to Q, then the pair (fP,fQ) is removed from C and f is removed from P's
cache of incoming coercion maps.
- Mild problem: If there is an external strong reference to, say, f,
then it is possible that Q becomes garbage collected anyway, and we would
end up with an invalid map.
I was just drawing the above graph on a napkin, and if I see that
correctly, changing ANY weak reference of the above graph into a *strong*
reference would create a situation where an external strong reference chain
from a global object to one parent would extend *internally* (i.e., inside
of the coercion system) to *another* parent (or it would introduce a strong
reference chain from the global object C to some parent) --- and that's a
memory leak.
So, Jeroen, I guess changing some weak references into strong references
won't work. But I'd be happy to stand corrected.
Best regards,
Simon
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