SWIG overhead

[Bart Ogryczak]

Hi,
I´m looking for some benchmarks comparing SWIG generated modules with
modules made directly with C/Python API. Just how much overhead does
SWIG give? Doing profile of my code I see, that it spends quiet some
time in functions like _swig_setattr_nondinamic, _swig_setattr,
_swig_getattr.

 

[Roman Yakovenko]

Hi,
I´m looking for some benchmarks comparing SWIG generated modules with
modules made directly with C/Python API. Just how much overhead does
SWIG give? Doing profile of my code I see, that it spends quiet some
time in functions like _swig_setattr_nondinamic, _swig_setattr,
_swig_getattr.

Before you decide to go "low level" consider to use Boost.Python.
According to this( http://tinyurl.com/322d3p ) post it gives pretty
good performance.

P.S. The post does not contain numbers

 

[Phil Thompson]

Hi,
I´m looking for some benchmarks comparing SWIG generated modules with
modules made directly with C/Python API. Just how much overhead does
SWIG give? Doing profile of my code I see, that it spends quiet some
time in functions like _swig_setattr_nondinamic, _swig_setattr,
_swig_getattr.

There was a EuroPython paper describing some benchmarks. It's from 2004 so
things have probably changed a bit (SIP is now fully documented for example).

http://people.web.psi.ch/geus/talks/europython2004_geus.pdf

Phil

 

[Bart Ogryczak]

There was a EuroPython paper describing some benchmarks. It's from 2004 so
things have probably changed a bit (SIP is now fully documented for example).

http://people.web.psi.ch/geus/talks/europython2004_geus.pdf

Yeah, found that one googling around. But I haven´t fund anything more
up to date. I imagine, that the performance of all of these wrappers
has been improved since then. But the performance of Python/C API
would too?
Anyways, it´s not about exact number, it´s more about taking decision
if doing rewrite is worth it´s time.

 

[Diez B. Roggisch]

Yeah, found that one googling around. But I haven´t fund anything more

up to date. I imagine, that the performance of all of these wrappers
has been improved since then. But the performance of Python/C API
would too?
Anyways, it´s not about exact number, it´s more about taking decision
if doing rewrite is worth it´s time.

The wrappers essentially create the boilerplate-code that invokes the Python
C-API. So whatever improvements the latter has been developed, the wrappers
will benefit from it. I doubt that there are major performance penalties
associated with any of them.

More important for a wrapper-decision is the question how easy they are to
use.


Diez

 

[Bart Ogryczak]

The wrappers essentially create the boilerplate-code that invokes the Python
C-API. So whatever improvements the latter has been developed, the wrappers
will benefit from it.

Without doubt it´s true in case of SWIG, but if I understand
Python.Boost documentation correctly, it does *not* use Python/C API.

I doubt that there are major performance penalties associated with any of them.

Take a look at pages 23 and 24 of http://people.web.psi.ch/geus/talks/
europython2004_geus.pdf

More important for a wrapper-decision is the question how easy they are to use.

Well, SWIG is easy to use. But I´ve gotta make hundreds of millions of
calls, which do tasks as simple, as getting one int from an array and
returning it. With functions that simple SWIG´s overhead seems to be a
problem.

 

[Diez B. Roggisch]

Without doubt it´s true in case of SWIG, but if I understand
Python.Boost documentation correctly, it does *not* use Python/C API.

It has to. In the end, marshalling data between e.g. python datastructures
and C++ is done that way.

Take a look at pages 23 and 24 of http://people.web.psi.ch/geus/talks/
europython2004_geus.pdf

Ok, I see. I really wonder what SWIG does.

Well, SWIG is easy to use. But I´ve gotta make hundreds of millions of
calls, which do tasks as simple, as getting one int from an array and
returning it. With functions that simple SWIG´s overhead seems to be a
problem.

Still I think you should first use wrappers for ease of use. Then when you
hit a performance bottleneck, it might be worth wrapping that class
manually. However, it _might_ of course be that this isn't integrating too
seamless with the wrapped classes, but I can't say anything about that
really.

Diez

 

[Chris Mellon]

It has to. In the end, marshalling data between e.g. python datastructures
and C++ is done that way.

As I understand it, part of the Boost.Python internals is a C++
wrapper over the Python C api, and there's no separate code generation
phase because it uses template magic to generate the wrappers. So
while obviously the C API is used at some level, it's not visible to
the wrapper author.

Ok, I see. I really wonder what SWIG does.

SWIG generates a low-level C module, and then a Python one on top of
it. It allows for quite some versatility in wrapping (because you can
have (almost) arbitrary Python code generated). Most of the other
tools generate a .pyd which you import directly.

Still I think you should first use wrappers for ease of use. Then when you
hit a performance bottleneck, it might be worth wrapping that class
manually. However, it _might_ of course be that this isn't integrating too
seamless with the wrapped classes, but I can't say anything about that
really.

Personally, I'd favor *correctness* first, then ease of use, and then speed..

 

[John Roth]

Hi,
I´m looking for some benchmarks comparing SWIG generated modules with
modules made directly with C/Python API. Just how much overhead does
SWIG give? Doing profile of my code I see, that it spends quiet some
time in functions like _swig_setattr_nondinamic, _swig_setattr,
_swig_getattr.

This isn't exactly what you're looking for, but I tried SWIG a while
ago, back when I still had a C compiler, and then decided to use c-
types.

The reason: I've got an aversion to anything that generates huge
steaming piles of incomprehensible code. Especially when I found a
couple of bugs in a SWIG-generated module I'd gotten from someone
else. This wasn't SWIG's fault, it was the author's for not
understanding the API he was wrapping properly, but I had to dig into
the SWIG code before I could figure out what was going on.

Bottom line: the c-types module was a lot smaller, in Python, and
completely comprehensible. And while I didn't measure the performance,
I doubt if it was slower.

John Roth

 

[skip]

John> Bottom line: the c-types module was a lot smaller, in Python, and
John> completely comprehensible. And while I didn't measure the
John> performance, I doubt if it was slower.

One advantage SWIG (or Boost.Python) has over ctypes is that it will work
with C++.

Skip

 

[Roman Yakovenko]

As I understand it, part of the Boost.Python internals is a C++
wrapper over the Python C api,

That's true.

and there's no separate code generation
phase because it uses template magic to generate the wrappers.

Well, actually it depends on the size of the project. If project is
small you can use
Boost.Python without a code generator. For big projects you have to use the code
generator. The advantage is that generated code is pretty readable.

So
while obviously the C API is used at some level, it's not visible to
the wrapper author.

Boost.Python hides it pretty well