Archive for September, 2010

Panda3D and Cython

Sunday, September 12th, 2010 by Craig

This is about how to speed up your Python Code, and has no direct impact on Panda3D’s performance. For most projects, the vast majority of the execution time is inside Panda3D’s C++ or in the GPU, so no matter what you do, fixing your Python will never help. For the other cases where you do need to speed up your Python code, Cython can help. This is mainly addressed to people who prefer programming in Python, but know at least a little about C. I will not discuss how to do optimizations within Python, though if this article is relevant to you, you really should look into it.

Cython is an interesting programming language. It uses an extended version of python’s syntax to allow things like statically typed variables, and direct calls into C++ libraries. Cython compiles this code to C++. The C++ then compiles as a python extension module that you can import and use just like a regular python module. There are several benefits to this, but in our context the main one is speed. Properly written Cython code can be as fast as C code, which in some particular cases can be even 1000 times faster than nearly identical python code. Generally you won’t see 1000x speed increases, but it can be quite a bit. This does cause the modules to only work on the platform they were compiled for, so you will need to compile alternate versions for different platforms.

By default, Cython compiles to C, but the new 0.13 version supports C++. This is more useful as you probably use at least one C++ library, Panda3D. I decided to try this out, and after stumbling on a few simple issues, I got it to work, and I don’t even know C++.

Before I get to the details, I’ll outline why you might want to use Cython, rather than porting performance bottlenecks to C++ by hand. The main benefit is in the process, as well as the required skill set. If you have a large base of Python code for a project, and you decide some of it needs to be much faster, you have a few options. The common approach seems to be to learn C++, port the code, and learn how to make it so you can interface to it from python. With Cython, you can just add a few type definitions on variables where you need the performance increase, and compile it which gives you a Python modules that works just like the one you had. If you need to speed up the code that interfaces with Panda3D, you can swap the Python API calls for C++ ones. Using Cython allows you to just put effort into speeding up the parts of code you need to work on, and to do so without having to change very much. This is vastly different from ditching all the code and reimplementing it another language. It also requires you to learn a pretty minimal amount of stuff. You also get to keep the niceness of the Python syntax which may Python coders have come to appreciate.

There are still major reasons to actually code in C++ when working with Panda, but as someone who does not do any coding in C++, I won’t talk about it much. If you want to directly extend or contribute to Panda3D, want to avoid redundantly specifying your imports from header files (Cython will require you to re-specify the parts of API you are using rather than just using the header files shipped with Panda), or you simply prefer C++, C++ may be a better option. I mainly see Cython as a convenient option when you end up needing to speed up parts of a Python code-base; however, it is practical to undertake large projects from the beginning in Cython.

Cython does have some downsides as well. It is still in rather early development. This means you will encounter bugs in its translators as well as the produced code. It also lacks support for a few Python features, such as most uses of generators. Generally I haven’t had much trouble with these issues, but your experience may differ.

Cython does offer an interesting side benefit as well. It allows you to optionally   statically type variables and thus can detect more errors at compile time than Python.

To get started, first you need an install of Cython 0.13 (or probably any newer version). If you have a Cython install you can check the version with the -V command. You can pick up the source from the Cython Site, and install it by running “python setup.py install” from the Cython source directory. You will also need to have a compiler. The Cython site should help you get everything setup if you need additional guidance.

Then you should try out a sample to make sure you have everything you need, and that it’s all working. There is a nice C++ sample for Cython on the Cython Wiki. (This worked for me on Mac, and on Windows using MinGW or MSVC as a compiler).

As for working with Panda3D, there are a few things I discovered:

  • There are significant performance gains to be had by just compiling your existing Python modules as Cython. With a little additional work adding static typed variables, you can have larger performance gains without even moving over to Panda’s C++ API (Which means you don’t need to worry about linking against Panda3D which can be an issue).
  • Panda3D already has python bindings with nice memory management, so I recommend instancing all the objects using the python API, and only switching to the C++ one as needed.
  • You can use the ‘this’ property on Panda3D’s Python objects to get a pointer to the underlying C++ object.
  • On mac, you need to make sure libpanda (and is some cases, possibly others as well) is loaded before importing your extension module if you use any of Panda3D’s libraries.
  • On Windows, you need to specify the libraries you need when compiling (in my case, just libpanda)
  • The C++ classes and Python classes tend to have the same name. To resolve this, you can use “from x import y as z” when importing the python ones, or you can just import panda3d.core, and use the full name of the classes (like panda3d.core.Geom). There may be a way to rename the C++ classes on import too.
  • If using the Panda3D C++ API on Windows, you will need to use the MSVC compiler. You can get Microsoft Visual Studio 2008 Express Edition for free which includes the needed compiler.

Using this technique I got a 10x performance increase on my code for updating the vertex positions in my Geom. It avoided having to create python objects for all of the vertexes and passing them through the Python API which translates them back to C++ objects. It was just a matter of moving over one call in the inner loop to the other API. This, however, was done in already optimized Cython code that was simply loading vertex positions stored in a block of memory into the Geom. Most use cases would likely see less of a benefit. Overall though, I gained a lot of performance both from the change over to Cython, and from the change over to the C++ API. These changes only required relatively small changes to the speed critical portions of my existing python code.

I made a rather minimal example of using a Panda3D C++ API call from Cython. Place the setup.py and the testc.pyx files in the same directory, and from the said directory, run setup.py with your Python install you use with Panda3D. If everything is properly configured, this should compile the example Cython module, testc.pyx, to a python extension module and run it. If it works, it will print out a few lines ending with “done”. It is likely you may need to tweak the paths in setup.py. If not on Mac or Windows, you will get an error indicating where you need to enter your compiler settings (mostly just the paths to Panda3D’s libraries).

I would like to thank Lisandro Dalcin from the Cython-Users mailing list who helped me get this working on Windows.