There is some very bad behaviour related to CachedRepresentation caching that I'm observing on #24742 (but this is otherwise unrelated to that ticket):

sage: timeit('MatrixSpace(ZZ,3,3)')
625 loops, best of 3: 117 µs per loop

Now, we try again but we first create a strong reference:

sage: S = MatrixSpace(ZZ,3,3)
sage: timeit('MatrixSpace(ZZ,3,3)')
625 loops, best of 3: 4.13 µs per loop

This is much faster the second time! In the first example, the caching of CachedRepresentation.__classcall__ is pointless since there is no strong reference to the entry in the cache, so it gets deleted immediately whenever the "MatrixSpace(ZZ,3,3)" is deleted.

This is just the usual Py_DECREF of Python objects, it has nothing to do with the cyclic garbage collector: the behaviour remains the same even with gc.disable().

This makes me think that we might need a version of CachedRepresentation which keeps semi-strong references: these would only be deleted by the cyclic garbage collector but not by a simple Py_DECREF().


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