How to select a GPU with CUDA?

Question

I have a computer with 2 GPUs; I wrote a CUDA C program and I need to tell it somehow that I want to run it on just 1 out of the 2 graphic cards; what is the command I need to type and how should I use it? I believe somehow that is related to the cudaSetDevice but I can't really find out how to use it.

Answer 1

It should be pretty much clear from documentation of cudaSetDevice, but let me provide following code snippet.

bool IsGpuAvailable()
{
    int devicesCount;
    cudaGetDeviceCount(&devicesCount);
    for(int deviceIndex = 0; deviceIndex < devicesCount; ++deviceIndex)
    {
        cudaDeviceProp deviceProperties;
        cudaGetDeviceProperties(&deviceProperties, deviceIndex);
        if (deviceProperties.major >= 2
            && deviceProperties.minor >= 0)
        {
            cudaSetDevice(deviceIndex);
            return true;
        }
    }

    return false;
}

This is how I iterated through all available GPUs (cudaGetDeviceCount) looking for the first one of Compute Capability of at least 2.0. If such device was found, then I used cudaSetDevice so all the CUDA computations were executed on that particular device. Without executing the cudaSetDevice your CUDA app would execute on the first GPU, i.e. the one with deviceIndex == 0 but which particular GPU is that depends on which GPU is in which PCIe slot.

EDIT:

After clarifying your question in comments, it seems to me that it should be suitable for you to choose the device based on its name. If you are unsure about your actual GPU names, then run this code which will print names of all your GPUs into console:

int devicesCount;
cudaGetDeviceCount(&devicesCount);
for(int deviceIndex = 0; deviceIndex < devicesCount; ++deviceIndex)
{
    cudaDeviceProp deviceProperties;
    cudaGetDeviceProperties(&deviceProperties, deviceIndex);
    cout << deviceProperties.name << endl;
}

After that, choose the name of the GPU that you want to use for computations, lets say it is "GTX XYZ". Call the following method from your main method, thanks to it, all the CUDA kernels will be executed on the device with name "GTX XYZ". You should also check the return value - true if device with such name is found, false otherwise:

bool SetGPU()
{
    int devicesCount;
    cudaGetDeviceCount(&devicesCount);
    string desiredDeviceName = "GTX XYZ";
    for(int deviceIndex = 0; deviceIndex < devicesCount; ++deviceIndex)
    {
        cudaDeviceProp deviceProperties;
        cudaGetDeviceProperties(&deviceProperties, deviceIndex);
        if (deviceProperties.name == desiredDeviceName)
        {
            cudaSetDevice(deviceIndex);
            return true;
        }
    }

    return false;
}

Of course you have to change the value of desiredDeviceName variable to desired value.

Answer 2

Searching more carefully in the internet I found this lines of code that select the GPU with more cores among all the devices installed in the Pc.

int num_devices, device;
cudaGetDeviceCount(&num_devices);
if (num_devices > 1) {
  int max_multiprocessors = 0, max_device = 0;
  for (device = 0; device < num_devices; device++) {
          cudaDeviceProp properties;
          cudaGetDeviceProperties(&properties, device);
          if (max_multiprocessors < properties.multiProcessorCount) {
                  max_multiprocessors = properties.multiProcessorCount;
                  max_device = device;
          }
  }
  cudaSetDevice(max_device);
}