numba-cuda-stencil.ipynb

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this example is great and seems to be everywhere on the internet, but I think there is a bug in using cupy-arrays. For one thing, you should get identical (?) performance feeding Numpy-Arrays, since the calculations are both done on gpu anyway. More importantly, I think that using cupy-arrays causes timeit to show only the kernel invocation time - nothing has actually been calculated. Can you please check this again? This is a top Google search result for numpy gpu stencils. Try to print the output, and the calculation will actually run. I get around 160 ms!

sadly the cpu version using parallel computing is still faster even for big arrays! (60 ms). The original stencil function is just slow in numba. Better do it manually:

@njit(nopython=True,parallel=True)
def smooth_cpu(x, out_cpu):

    for i in prange(1,np.shape(x)[0]-1):
        for j in range(1,np.shape(x)[1]-1):
            out_cpu[i, j] =  (x[i - 1, j - 1] + x[i - 1, j] + x[i - 1, j + 1] + x[i    , j - 1] + x[i    , j] + x[i    , j + 1] +x[i + 1, j - 1] + x[i + 1, j] + x[i + 1, j + 1]) / 9

edit: it seems I was wrong and it's mostly because of data transfer times as the cupy arrays are already on the GPU. I still think it needs a "cuda.synchronize()" for a fair comparison which increase running time quite alot.

It beeing referenced from Dask documentation as well...