GPU Setup

SAS model evaluations can run on your graphics card (GPU) or they can run on the processor (CPU). In general, calculations performed on the GPU will run faster.

OpenCL Installation

Warning! GPU devices do not in general offer the same level of memory protection as CPU devices. If your code attempts to write outside allocated memory buffers unpredicatable behaviour may result (eg, your video display may freeze, or your system may crash, etc). Do not install OpenCL drivers without first checking for known issues (eg, some computer manufacturers install modified graphics drivers so replacing these may not be a good idea!). If in doubt, seek advice from an IT professional before proceeding further.

Check if you have OpenCL already installed


The following instructions are based on

  • Go to: Start -> Control Panel -> System & Security -> Administrative Tools

  • Double Click on Computer Managment

  • Click on Device Manager

  • Click open Display Adapters

  • Right-click on available adapter and select Properties

  • Click on Driver

  • Go to Driver Details

  • Scroll down and see if OpenCL is installed (look for OpenCL*.dll files)


For OS X operating systems higher than 10.6 OpenCL is shipped along with the system.

However, OpenCL has had a rocky history on Macs. Apple provide a useful compatibility table at

Note: some versions of OS/X have an OpenCL driver for the Intel CPU that does support precompiled OpenCL program. The first time that PyOpenCL compiles a program that only runs on the CPU (e.g., because the GPU does not support double precision programs), the program will run fine, but the next time it will give the unhelpful error:

pyopencl._cl.RuntimeError: clBuildProgram failed: BUILD_PROGRAM_FAILURE - clBuildProgram failed: BUILD_PROGRAM_FAILURE - clBuildProgram failed: BUILD_PROGRAM_FAILURE

To prevent this you will need to set the environment variable PYOPENCL_NO_CACHE=1 before reruning the program. Only do this if needed since it will increase the time it takes to restart the program when the cache is disabled.



Depending on the graphic card in your system, drivers can be obtained from different sources:



You cannot download OpenCL driver updates for your Mac. They are packaged with the normal quarterly OS X updates from Apple.


Intel provides OpenCL drivers for Intel processors at These can sometimes make use of special vector instructions across multiple processors, so it is worth installing if the GPU does not support double precision. You can install this driver alongside the GPU driver for NVIDIA or AMD.

GPU Selection

The logic for choosing the compute platform is a little bit complicated. If the model has the line single=False then it requires double precision. If the GPU is single precision only, then it will try running via OpenCL on the CPU. If the OpenCL driver is not available for the CPU then it will run as a normal program on the CPU.

For models with a large number of parameters or with a lot of code, the GPU may be too small to run the program effectively. In this case, you should try simplifying the model, maybe breaking it into several different models so that you don’t need IF statements in your code. If it is still too big, you can set opencl=False in the model file and the model will only run as a normal program on the CPU. This will not usually be necessary.

Device Selection

OpenCL drivers

If you have multiple GPU devices you can tell the program which device to use. By default, the program looks for one GPU and one CPU device from available OpenCL platforms. It prefers AMD or NVIDIA drivers for GPU, and prefers Intel or Apple drivers for CPU. Both GPU and CPU are included on the assumption that CPU is always available and supports double precision.

The device order is important: GPU is checked before CPU on the assumption that it will be faster. By examining ~/sasview.log you can see which device was used to run the model.

If you want to use a specific driver and devices, you can run the following from the python console:

import pyopencl as cl

This will provide a menu of different OpenCL drivers available. When one is selected, it will say “set PYOPENCL_CTX=…” Use that value as the value of SAS_OPENCL=driver:device.

To use the default OpenCL device (rather than CUDA or None), set SAS_OPENCL=opencl.

In batch queues, you may need to set XDG_CACHE_HOME=~/.cache (Linux only) to a different directory, depending on how the filesystem is configured. You should also set SAS_DLL_PATH for CPU-only modules.

CUDA drivers

If OpenCL drivers are not available on your system, but NVidia CUDA drivers are available, then set SAS_OPENCL=cuda or SAS_OPENCL=cuda:n for a particular device number n. If no device number is specified, then the CUDA drivers looks for look for CUDA_DEVICE=n or a file ~/.cuda-device containing n for the device number.

In batch queues, the SLURM command sbatch –gres=gpu:1 … will set CUDA_VISIBLE_DEVICES=n, which ought to set the correct device number for SAS_OPENCL=cuda. If not, then set CUDA_DEVICE=$CUDA_VISIBLE_DEVICES within the batch script. You may need to set the CUDA cache directory to a folder accessible across the cluster with PYCUDA_CACHE_DIR (or PYCUDA_DISABLE_CACHE to disable caching), and you may need to set environment specific compiler flags with PYCUDA_DEFAULT_NVCC_FLAGS. You should also set SAS_DLL_PATH for CPU-only modules.

No GPU support

If you don’t want to use OpenCL or CUDA, you can set SAS_OPENCL=None in your environment settings, and it will only use normal programs.

In batch queues, you may need to set SAS_DLL_PATH to a directory accessible on the compute node.

Device Testing

Unfortunately, not all vendors provide working OpenCL implementations for their GPU devices. For example, the HD 6000 Intel GPUs with double precision support fail for some of the double precision models.

The SasView user interface provides a Fitting OpenCL Options dialog for selecting amongst and testing the available devices. After a few minutes of seeming to freeze, the application will return a list of model tests which have passed. The same tests can be run directly from the python console using:

from sasmodels.model_tests import main as model_tests
model_tests("-v", "opencl", "all")

Compiler Selection

For models run as normal programs, you may need to specify a compiler. This is done using the SAS_COMPILER environment variable, and the SAS_OPENMP environment variable if OpenMP support is available for the compiler.

On Windows, set SAS_COMPILER=tinycc for the tinycc compiler, SAS_COMPILER=msvc for the Microsoft Visual C compiler, or SAS_COMPILER=mingw for the MinGW compiler. If TinyCC is available on the python path (it is provided with SasView), that will be the default. If you want one of the other compilers, be sure to have it available in your PATH so we can find it!

On Mac OS/X and Linux, set SAS_COMPILER=unix for the compiler. This will use the unix cc command to compile the model, with gcc style command line options. For OS/X you will need to install the Xcode package to make the compiler available.

Environment Variables

The following environment variables are used by the system:

SAS_MODELPATH=path - sets directory containing custom models
SAS_OPENCL=vendor:device|cuda:device|none - sets the target GPU device
XDG_CACHE_HOME=~/.cache - sets the pyopencl cache root (linux only)
SAS_COMPILER=tinycc|msvc|mingw|unix - sets the DLL compiler
SAS_OPENMP=1 - turns on OpenMP for the DLLs
SAS_DLL_PATH=path - sets the path to the compiled modules
SAS_NUMBA=1|2 - enables numba and numba.cuda calculations if available
PYOPENCL_NO_CACHE=1 - turns off caching for PyOpenCL

Document History

2019-11-06 Paul Kienzle