New submission from Marco Sulla <launchpad....@marco.sulla.e4ward.com>:
I asked on SO why subclassing dict makes the subclass much slower in some operations. This is the answer by Monica (https://stackoverflow.com/a/59914459/1763602): Indexing and in are slower in dict subclasses because of a bad interaction between a dict optimization and the logic subclasses use to inherit C slots. This should be fixable, though not from your end. The CPython implementation has two sets of hooks for operator overloads. There are Python-level methods like __contains__ and __getitem__, but there's also a separate set of slots for C function pointers in the memory layout of a type object. Usually, either the Python method will be a wrapper around the C implementation, or the C slot will contain a function that searches for and calls the Python method. It's more efficient for the C slot to implement the operation directly, as the C slot is what Python actually accesses. Mappings written in C implement the C slots sq_contains and mp_subscript to provide in and indexing. Ordinarily, the Python-level __contains__ and __getitem__ methods would be automatically generated as wrappers around the C functions, but the dict class has explicit implementations of __contains__ and __getitem__, because the explicit implementations (https://github.com/python/cpython/blob/v3.8.1/Objects/dictobject.c) are a bit faster than the generated wrappers: static PyMethodDef mapp_methods[] = { DICT___CONTAINS___METHODDEF {"__getitem__", (PyCFunction)(void(*)(void))dict_subscript, METH_O | METH_COEXIST, getitem__doc__}, ... (Actually, the explicit __getitem__ implementation is the same function as the mp_subscript implementation, just with a different kind of wrapper.) Ordinarily, a subclass would inherit its parent's implementations of C-level hooks like sq_contains and mp_subscript, and the subclass would be just as fast as the superclass. However, the logic in update_one_slot (https://github.com/python/cpython/blob/v3.8.1/Objects/typeobject.c#L7202) looks for the parent implementation by trying to find the generated wrapper methods through an MRO search. dict doesn't have generated wrappers for sq_contains and mp_subscript, because it provides explicit __contains__ and __getitem__ implementations. Instead of inheriting sq_contains and mp_subscript, update_one_slot ends up giving the subclass sq_contains and mp_subscript implementations that perform an MRO search for __contains__ and __getitem__ and call those. This is much less efficient than inheriting the C slots directly. Fixing this will require changes to the update_one_slot implementation. Aside from what I described above, dict_subscript also looks up __missing__ for dict subclasses, so fixing the slot inheritance issue won't make subclasses completely on par with dict itself for lookup speed, but it should get them a lot closer. As for pickling, on the dumps side, the pickle implementation has a dedicated fast path (https://github.com/python/cpython/blob/v3.8.1/Modules/_pickle.c#L4291) for dicts, while the dict subclass takes a more roundabout path through object.__reduce_ex__ and save_reduce. On the loads side, the time difference is mostly just from the extra opcodes and lookups to retrieve and instantiate the __main__.A class, while dicts have a dedicated pickle opcode for making a new dict. If we compare the disassembly for the pickles: In [26]: pickletools.dis(pickle.dumps({0: 0, 1: 1, 2: 2, 3: 3, 4: 4})) 0: \x80 PROTO 4 2: \x95 FRAME 25 11: } EMPTY_DICT 12: \x94 MEMOIZE (as 0) 13: ( MARK 14: K BININT1 0 16: K BININT1 0 18: K BININT1 1 20: K BININT1 1 22: K BININT1 2 24: K BININT1 2 26: K BININT1 3 28: K BININT1 3 30: K BININT1 4 32: K BININT1 4 34: u SETITEMS (MARK at 13) 35: . STOP highest protocol among opcodes = 4 In [27]: pickletools.dis(pickle.dumps(A({0: 0, 1: 1, 2: 2, 3: 3, 4: 4}))) 0: \x80 PROTO 4 2: \x95 FRAME 43 11: \x8c SHORT_BINUNICODE '__main__' 21: \x94 MEMOIZE (as 0) 22: \x8c SHORT_BINUNICODE 'A' 25: \x94 MEMOIZE (as 1) 26: \x93 STACK_GLOBAL 27: \x94 MEMOIZE (as 2) 28: ) EMPTY_TUPLE 29: \x81 NEWOBJ 30: \x94 MEMOIZE (as 3) 31: ( MARK 32: K BININT1 0 34: K BININT1 0 36: K BININT1 1 38: K BININT1 1 40: K BININT1 2 42: K BININT1 2 44: K BININT1 3 46: K BININT1 3 48: K BININT1 4 50: K BININT1 4 52: u SETITEMS (MARK at 31) 53: . STOP highest protocol among opcodes = 4 we see that the difference between the two is that the second pickle needs a whole bunch of opcodes to look up __main__.A and instantiate it, while the first pickle just does EMPTY_DICT to get an empty dict. After that, both pickles push the same keys and values onto the pickle operand stack and run SETITEMS ---------- components: C API messages: 399625 nosy: Marco Sulla priority: normal severity: normal status: open title: dict subclassing is slow type: performance versions: Python 3.9 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue44921> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
