[Cython] Speedup module-level lookup

Robert Bradshaw robertwb at math.washington.edu
Sat Jan 21 07:21:37 CET 2012

On Fri, Jan 20, 2012 at 10:00 PM, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Vitja Makarov, 19.01.2012 08:49:
>> 2012/1/19 Robert Bradshaw:
>>> On Wed, Jan 18, 2012 at 12:30 PM, Vitja Makarov wrote:
>>>> I tried to optimize module lookups (__pyx_m) by caching internal PyDict state.
>>>> In this example bar() is 1.6 time faster (500us against 842us):
>>>> C = 123
>>>> def foo(a):
>>>>     return C * adef bar():
>>>>     for i in range(10000):        foo(i)
>>>> Here is proof of
>>>> concept:https://github.com/vitek/cython/commit/1d134fe54a74e6fc6d39d09973db499680b2a8d9
>>>> So the question is: does it worth it?
>>> I think the right thing to do here is make all module-level globals
>>> into "cdef public" attributes, i.e. C globals with getters and setters
>>> for Python space. I'm not sure whether this would best be done by
>>> creating a custom dict or module subclass, but it would probably be
>>> cleaner and afford much more than a 1.6x speedup.
>> Yes, nice idea.
>> It's possible to subclass PyModuleObject and I didn't find any use of
>> PyModule_CheckExact() in CPython's sources:
>> import types
>> import sys
>> global_foo = 1234
>> class CustomModule(types.ModuleType):
>>     def __init__(self, name):
>>         types.ModuleType.__init__(self, name)
>>         sys.modules[name] = self
>>     @property
>>     def foo(self):
>>         return global_foo
>>     @foo.setter
>>     def foo(self, value):
>>         global global_foo
>>         global_foo = value
>> CustomModule('foo')
>> import foo
>> print foo.foo
> The one thing I don't currently see is how to get the module subtype
> instantiated in a safe and portable way.
> The normal way to create the module in Python 2.x is a call to
> Py_InitModule*(), which internally does a PyImport_AddModule(). We may get
> away with creating and registering the module object before calling into
> Py_InitModule*(), so that PyImport_AddModule() finds it there. At least,
> the internal checks on modules seem to use PyModule_Check() and not
> PyModule_CheckExact(), so someone seems to have already thought about this.
> In Python 3.x, the situation is different. There is no lookup involved and
> the module is always newly instantiated. That may mean that we have to copy
> the module creation code into Cython. But that doesn't look like a huge
> drawback (except for compatibility to potential future changes), because we
> already do most of the module initialisation ourselves anyway, especially
> now that we have CyFunction.

Or swap out its ob_type pointer after it's created... It's going to be
messy unless we can directly add hooks into its __dict__ though.

> I start feeling a bit like Linus Torvalds when he broke his minix
> installation and went: "ok, what else do I need to add to this terminal
> emulator in order to make it an operating system?"
> Stefan
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