A PyOpenCL tutorial on Windows with or without a GPU

I've been using CUDA and PyCUDA as GPGPU platforms for a few years now. They enable access to the incredible computational power of graphics cards through a simple C-like language. A recent Nvidia graphics card is nevertheless required in order to execute CUDA code. Some computers may not include a Nvidia GPU, but rather an AMD/ATI card or even an integrated graphics processor. Those computers thus cannot execute a CUDA program.

Whereas changing the GPU on a desktop computer is straightforward (one still needs to buy a new graphics card...), it is generally not possible to do so on a laptop. It prevents a number of developers to run, compile or debug CUDA code. It is also certainly a problem when distributing a software relying on CUDA, since not having a CUDA-enabled Nvidia GPU prevents one to even launch the program.

For these reasons, I've recently been interested in OpenCL, a computing framework equivalent to CUDA. The main difference is that OpenCL is an open standard that has been adopted not only by Nvidia, but also by Intel and AMD. It means that OpenCL programs may run on (recent) graphics card of either Nvidia or AMD. In addition, a very interesting feature is that OpenCL programs may run on a computer that does not even have a OpenCL-enabled GPU, but only a recent-enough CPU. OpenCL code can be compiled for CPUs through the Intel SDK for OpenCL (Intel-only CPUs) or the AMD APP SDK. OpenCL code is still accelerated through vectorization instructions on CPUs (e.g. SSE or AVX) and multicore CPUs.

This is really convenient since one can develop, debug and run an OpenCL program on virtually any computer, even if it does not include an OpenCL-enabled GPU. Moreover, distributing such a program is easier since the end-user does not necessarily need to have a specific graphics card. Of course, the performance of the program will be far less interesting than with such a GPU, but the program will still run correctly.

Here I detail my first experience with OpenCL on Python (using PyOpenCL). The installation part is actually the less trivial part (while still being relatively easy) and may be of interest to some people.

Installation of an OpenCL SDK

I'll explain how I could install OpenCL and PyOpenCL on an "old" laptop with an ATI Radeon Mobility HD 3650 graphics card that does not support OpenCL, an Intel Core 2 Duo CPU, and a 32-bits Windows 8 operating system.

First, one needs to install an OpenCL SDK. AMD and Intel have their own SDKs and I installed both. Later, we'll see that PyOpenCL allows us to choose, at compilation-time, which SDK to use. The installation of the SDKs is straightforward on Windows.

Installation of PyOpenCL

PyOpenCL is to OpenCL what PyCUDA is to CUDA: a Python wrapper to those GPGPU platforms. It is developed and maintained by Andreas Klöckner.

Installing PyOpenCL on Windows is easy when using the binary package provided by Christoph Gohlke. His webpage contains Windows binary installers for the most recent versions of hundreds of Python packages. It is of invaluable help for those Python users that use Windows. It is also possible yet much more complicated to build PyOpenCL from source on Windows.

PyOpenCL has a few uncommon dependencies (beyond Numpy), like decorator and PyTools (actually the latter includes decorator, so I'm not even sure one needs to install decorator separately). These libraries can be easily installed with the easy_install command-line tool provided by the Distribute library.

Some environment variables may need to be specified, like PYOPENCL_CTX or PYOPENCL_COMPILER_OUTPUT (see below). In order to do this, one needs to go to the advanced system parameters and add new variables in the User variables section (see here for details).

In order to quickly test the installation of PyOpenCL, try the following commands in a Python console:

import pyopencl
import pyopencl.array

We now turn to the execution of a very simple OpenCL program.

PyOpenCL Hello World script

Save the following code in a .py file and execute it. The OpenCL code should be compiled and executed correctly, and the squares of integers should be displayed. Some warnings may be displayed even if everything worked corectly. In addition, PyOpenCL may require the user to decide which SDK to use if several are installed. In order not to be asked everytime, one can set the PYOPENCL_CTX environment variable to the corresponding index (see above).

I could use both Intel and AMD SDKs. Ensure that you run Python in a command line as an administrator (otherwise it may cause issues with compilation, at least that's what I could note on my Windows 8 setup).

The actual code is self-explanatory, especially for someone who already knows CUDA. This program generates a Numpy array with all integers between 0 and 9, passes it to an OpenCL kernel that computes the square of an array of integers, dynamically compiles and executes this kernel, and copies back the output from the context memory to Python.

# import PyOpenCL and Numpy. An OpenCL-enabled GPU is not required,
# OpenCL kernels can be compiled on most CPUs thanks to the Intel SDK for OpenCL
# or the AMD APP SDK.
import pyopencl as cl
import numpy as np

# create an OpenCL context
ctx = cl.create_some_context()
queue = cl.CommandQueue(ctx)

# create the kernel input
a = np.array(np.arange(10), dtype=np.int32)

# kernel output placeholder
b = np.empty(a.shape, dtype=np.int32)

# create context buffers for a and b arrays
# for a (input), we need to specify that this buffer should be populated from a
a_dev = cl.Buffer(ctx, cl.mem_flags.READ_ONLY | cl.mem_flags.COPY_HOST_PTR,
# for b (output), we just allocate an empty buffer
b_dev = cl.Buffer(ctx, cl.mem_flags.WRITE_ONLY, b.nbytes)

# OpenCL kernel code
code = """
__kernel void test1(__global int* a, __global int* b) {
    int i = get_global_id(0);
    b[i] = a[i]*a[i];

# compile the kernel
prg = cl.Program(ctx, code).build()

# launch the kernel
event = prg.test1(queue, a.shape, a_dev, b_dev)

# copy the output from the context to the Python process
cl.enqueue_copy(queue, b, b_dev)

# if everything went fine, b should contain squares of integers

Here is the output I get on my installation.

C:\Python27\lib\site-packages\pyopencl\__init__.py:32: CompilerWarning: Built kernel retrieved from cache. Original from-source build had warnings:
Build on <pyopencl.Device 'Intel(R) Core(TM)2 Duo CPU     T6400  @ 2.00GHz' on 'Intel(R) OpenCL' at 0x2251b58> succeeded, but said:

Build started
Kernel <test1> was successfully vectorized
  warn(text, CompilerWarning)
C:\Python27\lib\site-packages\pyopencl\__init__.py:32: CompilerWarning: From-binary build succeeded, but resulted in non-empty logs:
Build on <pyopencl.Device 'Intel(R) Core(TM)2 Duo CPU     T6400  @ 2.00GHz' on 'Intel(R) OpenCL' at 0x2251b58> succeeded, but said:

Build started
Kernel <test1> was successfully vectorized
  warn(text, CompilerWarning)
[ 0  1  4  9 16 25 36 49 64 81]

Final notes

There is of course far more to tell about OpenCL that what I've shown here. The point was to show how one could use GPGPU on a standard computer that does not have a GPGPU-enabled graphics card, by using OpenCL and Python. Even if I don't have much experience in OpenCL yet, I think it has a promising future as an open and portable concurrent to CUDA.

Here are some interesting related links: