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I test the time to transfer data between CPU and GPU. And from the result, it seems that the time cost of transferring data can be neglected compared to the time to do the QR decomposition. It is weird that GPU performs much worse on QR decomposition of small matrices compared to CPU. As in the picture, the time to do QR over a 100600 matrix is close to the time to do QR over a 1006000 matrix.