cuda 11 kernel doesn't run

Question

here is a demo.cu aiming to printf from the GPU device:

#include "cuda_runtime.h"
#include "device_launch_parameters.h"

#include <stdio.h>

__global__ void hello_cuda() {
        printf("hello from GPU\n");
}

int main() {
        printf("hello from CPU\n");
        hello_cuda <<<1, 1>>> ();
        cudaDeviceSynchronize();

        cudaDeviceReset();
        printf("bye bye from CPU\n");
        return 0;
}

it compiles and runs:

$ nvcc demo.cu
$ ./a.out

that's the output that I get:

hello from CPU
bye bye from CPU

Q: why there is no printing result from the GPU?

it does seem as I misconfigured the cuda toolkit or something, however I'm able to compile and run various programs from the cuda-samples. For example, matrixMul, or deviceQuery

any time you are having trouble with a CUDA code, it's good practice to include [proper CUDA error checking](https://stackoverflow.com/questions/14038589/what-is-the-canonical-way-to-check-for-errors-using-the-cuda-runtime-api). My guess is you are using CUDA 11 and therefore compiling for an architecture that doesn't match your GPU. What CUDA version are you using? What GPU are you trying to run on? (There is nothing wrong with the code you have shown, and the CUDA samples are compiled differently than how you are compiling here.) — Robert Crovella, Aug 31 '20 at 17:03
cuda 11, GPU: Tesla K80. thanks, I'll go over the CUDA error checking. I understand that my question doesn't localize to a particular issue. I'll keep my question alive for now unless I figure out the root cause — govordovsky, Aug 31 '20 at 17:11
add the following switch to your compile command line: `-arch=sm_37` like this: `nvcc -arch=sm_37 demo.cu` — Robert Crovella, Aug 31 '20 at 17:12
magic! it worked with that flag included. @robert, mind giving an official answer? and maybe elaborate more in your answer — govordovsky, Aug 31 '20 at 17:14
though it works, there is a warning. nvcc warning: The 'compute_35', 'compute_37', 'compute_50', 'sm_35', 'sm_37' and 'sm_50' architectures are deprecated, and may be removed in a future release — govordovsky, Aug 31 '20 at 17:15
right, that is the way the compiler (CUDA 11) lets you know that in the future, CUDA may drop support for your GPU (Tesla K80). — Robert Crovella, Aug 31 '20 at 17:20
I've edited your question title in an attempt to make it easier for others to find your question. — Robert Crovella, Aug 31 '20 at 17:35

Robert Crovella · Accepted Answer · 2022-12-21T14:12:39.020

If your device is of compute capability 3.0 or below, CUDA 11 dropped support for these GPUs. You'll need to use a prior CUDA version.

The CUDA compiler must compile for a GPU target (i.e. a device architecture). If you don't specify a target architecture on the compile command line, historically, CUDA has chosen a very flexible default architecture specification that can run on all GPUs that the CUDA version supports.

That isn't always the case, however, and its not the case with CUDA 11. CUDA 11 (and CUDA 12.0) compiles for a default architecture of sm_52 (compute capability 5.2, i.e. as if you had specified -arch=sm_52 on the command line). But CUDA 11 supports architectures down to sm_35 (compute capability 3.5). (CUDA 12 has dropped support for sm_3x GPUs.)

Therefore if you don't specify the target architecture on the compile command line with CUDA 11, and attempt to run on a GPU with an architecture that predates sm_52, any CUDA code (kernels) that you have written definitely won't work.

It's good practice, any time you are having trouble with a CUDA code, to use proper CUDA error checking, and if you had done that here you would have gotten a runtime error indication that would have immediately identified the issue (at least for someone who is familiar with CUDA errors).

The solution in these cases is to specify a compilation command that includes the GPU you intend to run on (this is usually good practice anyway). If you do that, and the architecture you specify is "deprecated", then the nvcc compiler will issue a warning letting you know that a future CUDA version may not support the GPU you are trying to run on. The warning does not mean anything you are doing is wrong or illegal or needs to be changed, but it means that in the future, a future CUDA version may not support that GPU.

If you want to suppress that warning, you can pass the -Wno-deprecated-gpu-targets switch on the compile command line.

The same problem can occur on windows, of course. In that case, you'll need to modify the following VS project setting to match the architecture for your device:

cuda 11 kernel doesn't run

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