MATLAB: Detect when no GPU is selected

gpugpudeviceParallel Computing Toolbox

In the documentation on gpuDevice() it says that,
"gpuDevice([]), with an empty argument (as opposed to no argument), deselects the GPU device and clears its memory of gpuArray and CUDAKernel variables. . This leaves no GPU device selected as the current device."
Is there a way to detect whether this 'no GPU' state is the current state? The intuitive way to check would be the following,
    g=gpuDevice;
    isempty(g.Index)
This does not work, however, because a call to gpuDevice with no input arguments always results in the selection of a device. Thus, g.Index can never be empty.

Best Answer

You can query device properties without selecting it using parallel.gpu.GPUDevice.getDevice. So, for instance, you can find the selected device without selecting it like this:
   selectedDevice = [];  % Nothing selected
   for i = 1:gpuDeviceCount
       if parallel.gpu.GPUDevice.getDevice(i).DeviceSelected
           selectedDevice = i;
           break;
       end
   end
For your purposes, you can use this handy internal:
   parallel.internal.gpu.isAnyDeviceSelected
Since you're writing your own CUDA code (using MEX, presumably), you can also just query device properties using the CUDA runtime API (e.g. cudaGetDeviceProperties), or the driver API (e.g. cuDeviceGetProperties).
In answer to your 'elaboration' question, I'd need to know what you're actually doing. If you are resetting the device (using, for instance, cudaDeviceReset) then there's nothing you can do to save your gpuArrays - the device memory has been cleared and every pointer into GPU memory invalidated. Since your GPU is crashing when you subsequently attempt to read those gpuArrays it seems like this is what you're doing. Your only hope would be to copy everything to the CPU and back after the reset. For instance, you could use the mex API to call out to a MATLAB function which walks through the workspace, finds all gpuArrays, copies them to CPU arrays, and then another to copy back afterwards. That's a bit crazy because it could be a huge task to check every workspace variable and its contents.
Perhaps all you really want to do is ensure your card doesn't crash. In that case MATLAB needs to know you reset the device, so call reset(gpuDevice) or gpuDevice([]) in MATLAB instead of doing it your own code.
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