Make groupoids garbage collectable

[SimonKing]

Currently, the groupoid of an object P can not be garbage collected, even when deleting P.

Even worse: The persistence of Groupoid(P) also prevents P from being garbage collected.

The attached patch aims at solving it: An external reference to either P or Groupoid(P) is enough to keep both alive. But without an external reference, both P and Groupoid(P) become collectable.

Example from the docs:

            sage: P = GF(151)['x','y']
            sage: n = id(Groupoid(P))
            sage: import gc
            sage: _ = gc.collect()
            sage: n == id(Groupoid(P))   # indirect doctest
            True

        Thus, the groupoid is cached. But when deleting ``P``, its
        groupoid can be garbage collected as well::

            sage: del P
            sage: _ = gc.collect()
            sage: P = GF(151)['x','y']
            sage: n == id(Groupoid(P))
            False

        TESTS:

        We test against some corner cases::

            sage: Groupoid(None)
            Traceback (most recent call last):
            ...
            TypeError: Groupoid of None is not defined
            sage: Groupoid(1)
            Traceback (most recent call last):
            ...
            TypeError: 1 must either allow attribute assignment or be instances of <type 'sage.structure.parent.Parent'>
            sage: class A: pass
            sage: a = A()
            sage: Groupoid(a)
            Groupoid with underlying set <__main__.A instance at ...>
            sage: Groupoid(a) is Groupoid(a)
            True

Apply:

• trac12357_internal_strong_groupoid_cache.patch​
• trac12357_reviewer.patch​

[SimonKing]

I did not run the full test suite yet. It could be that some silly old doctests use the groupoids in a way they are not intended, such as Groupoid(1). If this is the case, then my patch has to change. Otherwise: Needs review!

[SimonKing]

To my surprise, there were no nasty old doctests. Hence, make ptest went well, with the patch from here and its dependencies!

[SimonKing]

Concerning timings:

Without the patch, the groupoid of P is stored in a dictionary, indexed by P. Hence, access to the cache is slow if hash(P) is slow.

Some data points:

sage: P = GF(151)['x','y']
sage: %timeit G = Groupoid(P)  # slow hash
625 loops, best of 3: 20.9 µs per loop
sage: %timeit G = Groupoid(ZZ) # fast hash
625 loops, best of 3: 1.75 µs per loop
sage: class Bar(Parent): pass
....: 
sage: P = Bar()
sage: %timeit G = Groupoid(P)  # slow hash
625 loops, best of 3: 15.3 µs per loop

But with the patch, it is stored as an attribute of P. Hence, it is slow if attribute access is slow. The point is that even slow attribute access is faster than slow hash, and slow attribute access is only little slower than fast hash:

sage: P = GF(151)['x','y']
sage: %timeit G = Groupoid(P)  # slow attribute access
625 loops, best of 3: 3.11 µs per loop
sage: %timeit G = Groupoid(ZZ) # slow attribute access
625 loops, best of 3: 3.1 µs per loop
sage: class Bar(Parent): pass
....: 
sage: P = Bar()
sage: %timeit G = Groupoid(P)  # fast attribute access
625 loops, best of 3: 1.59 µs per loop

Hence, I believe that the patch is not only fixing a memory leak, but has the potential to generate a speed-up.

[jpflori]

This is the simplest patch of the cache problems suite, and it does not depend on #715, so I begin with this one.

I'm a little bit confused by all those tests for None in your patch.

Isn't the first one sufficient?

More specifically, the second test "if S is not None" isn't superfluous ?

And if it is not None, can G be None ? (I guess there is some cython voodoo involved here)

To answer my own question, I guess it is none if S is not a Parent (as in your example with 1).

Once you answer me back, I'll post a reviewer patch with some additional minor corrections.

[SimonKing]

Yes, the test "if S is not None" in line 103 seems redundant, as S being None would result in an error being raised in line 92.

Concerning Cython voodoo: If S is not None and not of type Parent, then G == S in line 106 would result in a type error (since G is cdefined) - which is desired behaviour. But if S were None, then G == S in line 106 would not complain, and then line 109 would segfault. This is why there must be a test for the corner case - of course, one test would be enough.

[jpflori]

Ok, I've posted a reviewer patch removing the second test and adding some comments on the code.

Everything is fine for me (as long as further modification in #715 and #12313 do not affect the ticket here).

So I'll put is as positive review now.

[jpflori]

Reviewer patch

[SimonKing]

The reviewer patch looks fine to me. Thank you!

[jdemeyer]

Moving this to sage-pending for now since it depends on two non-positively reviewed tickets.

[SimonKing]

There is a conflict with #11943.

• The ticket here is pending anyway
• #11943 has only one dependency, that is already merged
• #11943 is more fundamental than the patch here.

Therefore, I suggest to rebase my patch with respect to #11943.

[SimonKing]

Done!

Apply trac12357_internal_strong_groupoid_cache.patch trac12357_reviewer.patch