Thank a lot I have missed this flag!
I added it and I get the CUDA processing time (updated gist) for my code.
Still I see a large difference in processing time for such a small increase of input tensor:


(256, 256, 256)

cudnn_convolution_backward 108.772us 5573916.893us 4 435.088us 22295667.572us
CudnnConvolutionBackward 121.100us 5573919.838us 4 484.401us 22295679.352us



(224, 224, 224)

cudnn_convolution_backward 125.165us 173178.101us 4 500.661us 692712.402us
CudnnConvolutionBackward 138.998us 173181.194us 4 555.993us 692724.777us



Is this an expected behavior going from 173ms to 5.6s for each of these operation ?