[Python-Dev] Micro-benchmarks for PEP 580

[Stefan Behnel]

INADA Naoki schrieb am 11.07.2018 um 02:12:
> On Tue, Jul 10, 2018 at 10:20 PM Antoine Pitrou wrote:
>> On Tue, 10 Jul 2018 21:59:28 +0900
>> INADA Naoki wrote:
>>>
>>> Then, the function is called from another C extension like this:
>>>
>>> PyObject_CallFunction(func, "n", 42);
>>>
>>> Currently, we create temporary long object for passing argument.
>>> If there is protocol for exposeing format used by PyArg_Parse*, we can
>>> bypass temporal Python object and call myfunc_impl directly.

Note that this is not fast at all. It actually has to parse the format
description at runtime. For really fast calls, this should be avoided, and
it can be avoided by using a str object for the signature description and
interning it. That relies on signature normalisation, which requires a
proper formal specification of C/C++ signature strings, which ... is pretty
much the can of worms that Antoine mentioned.


>> This is another can of worms to open.  If you're worried about the
>> added complexity of PEP 580, what you're proposing is one or two orders
>> of magnitude more complicated.
> 
> This is just an example of possible optimization, to explain why I want
> example application first.
> I know Cython is important for data scientists.  But I don't know how
> it used typically.
> 
> If my idea has 50% gain and current PEP 580 has only 5% gain,
> why we should accept PEP 580?

Because PEP 580 is also meant as a preparation for a fast C-to-C call
interface in Python.

Unpacking C callables is quite an involved protocol, and we have been
pushing the idea around and away in the Cython project for some seven
years. It's about time to consider it more seriously now, and there are
plans to implement it on top of PEP 580 (which is also mentioned in the PEP).


> And PEP 576 seems much simpler and straightforward way to expose
> FASTCALL.

Except that it does not get us one step forward on the path towards what
you proposed. So, why would *you* prefer it over PEP 580?


>>> I think optimization like this idea can boost application performance
>>> using Cython heavily.
>>
>> You can already define the C signature of a function in Cython and
>> intra-Cython calls will benefit from it where possible.  Cooperation
>> from core Python is not necessary for that.
> 
> Why not allow it for extensions written in C, without Cython?

It should be. They just need a simpler protocol, which is PEP 580.

Stefan