[Cython] SEP 201 draft: Native callable objects

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Thu May 31 21:29:40 CEST 2012

On 05/31/2012 08:50 PM, Robert Bradshaw wrote:
> On Thu, May 31, 2012 at 7:04 AM, Dag Sverre Seljebotn
> <d.s.seljebotn at astro.uio.no>  wrote:
>> [Discussion on numfocus at googlegroups.com please]
>> I've uploaded a draft-state SEP 201 (previously CEP 1000):
>> https://github.com/numfocus/sep/blob/master/sep201.rst
>> """
>> Many callable objects are simply wrappers around native code. This holds for
>> any Cython function, f2py functions, manually written CPython extensions,
>> Numba, etc.
>> Obviously, when native code calls other native code, it would be nice to
>> skip the significant cost of boxing and unboxing all the arguments.
>> """
>> The thread about this on the Cython list is almost endless:
>> http://thread.gmane.org/gmane.comp.python.cython.devel/13416/focus=13443
>> There was a long discussion on the key-comparison vs. interned-string
>> approach. I've written both up in SEP 201 since it was the major point of
>> contention. There was some benchmarks starting here:
>> http://thread.gmane.org/gmane.comp.python.cython.devel/13416/focus=13443
>> And why provide a table and not a get_function_pointer starting here:
>> http://thread.gmane.org/gmane.comp.python.cython.devel/13416/focus=13443
>> For those who followed that and don't want to read the entire spec, the
>> aspect of flags is new. How do we avoid to duplicate entries/check against
>> two signatures for cases like a GIL-holding caller wanting to call a nogil
>> function? My take: For key-comparison you can compare under a mask, for
>> interned-string we should have additional flags field.
>> The situation is a bit awkward: The Cython list consensus (well, me and
>> Robert Bradshaw) decided on what is "Approach 1" (key-comparison) in SEP
>> 201. I pushed for that.
>> Still, now that a month has passed, I just think key-comparison is too ugly,
>> and that the interning mechanism shouldn't be *that* hard to code up,
>> probably 500 lines of C code if one just requires the GIL in a first
>> iteration, and that keeping the spec simpler is more important.
>> So I'm tentatively proposing Approach 2.
> I'm still not convinced that a hybrid approach, where signatures below
> some cutoff are compiled down to keys, is not a worthwhile approach.
> This gets around variable-length keys (both the complexity and
> possible runtime costs for long keys) and allows simple libraries to
> produce and consume fast callables without participating in the
> interning mechanism.

I still think this gives us the "worst of both worlds", all the 
disadvantages and none of the advantages.

How many simple libraries are there really? Cython on one end, the 
magnificently complicated NumPy ufuncs on the other? Thinking big, 
perhaps PyPy and Julia? Cython, PyPy, Julia would all have to deal with 
long signatures anyway. And NumPy ufuncs are already complicated so even 
more low-level stuff wouldn't hurt.

> It's unclear how to rendezvous on a common interning interface without
> the GIL/Python, so perhaps requiring the GIL to use it not to onerous.
> An alternative is to acquire the GIL in the first/reference
> implementation (which could allow the interning function pointers to
> be cached by an external GIL-oblivions JIT for example). Presumably
> some other locking mechanism would be required if the GIL is not used,
> so the overhead would likely not be that great.

Yes. I guess a goal could be to make sure there's no ABI breakage 
if/when the GIL requirement is lifted.

Since modules can already have a reference to the interner by the time 
the first module interfacing with a GIL-less world is imported, this is 
non-trivial, but "every problem can be solved with another level of 
indirection", and particularly this one.

Good idea on separating out interning as a separate spec; that's 
definitely useful for interfaces etc. as well down the line. I can get 
to work on a string interning spec and implementation as SEP 202 in 
spare minutes over the next month or so, but I won't bother unless SEP 
201 will uses interning. My role in that depends on Travis' timeline as 
well, as my ETA is so unpredictable.


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