CUDA kernel doesn't launch

Question

My problem is very much like this one. I run the simplest CUDA program but the kernel doesn't launch. However, I am sure that my CUDA installation is ok, since I can run complicated CUDA projects consisting of several files (which I took from someone else) with no problems. In these projects, compilation and linking is done through makefiles with a lot of flags. I think the problem is in the correct flags to use while compiling. I simply use a command like this: nvcc -arch=sm_20 -lcudart test.cu with a such a program (to run on a linux machine):

 __global__ void myKernel() 
{ 

    cuPrintf("Hello, world from the device!\n"); 


} 
int main() 
{ 
    cudaPrintfInit(); 
    myKernel<<<1,10>>>(); 
    cudaPrintfDisplay(stdout, true);    
    cudaPrintfEnd(); 
}

The program compiles correctly. When I add cudaMemcpy() operations, it returns no error. Any suggestion on why the kernel doesn't launch ?

I believe that for devices of compute capability 2.0 or higher you can simply call printf. And you might want to do some error checking to see if you get any error messages from your calls. — Bart
Also take note of the first comment in the question you linked to: stackoverflow.com/questions/9519272/cuda-kernel-not-launching - your code above has absolutely no error checking - those functions return a status for a reason, you know. — Paul R
@Bart Ok, I now use printf, and removed all cudaPrinf stuff, and the kernel has not yet printed! — Tarek

harrism harrism · Accepted Answer · 2012-08-29T02:02:56

The reason it is not printing when using printf is that kernel launches are asynchronous and your program is exiting before the printf buffer gets flushed. Section B.16 of the CUDA (5.0) C Programming Guide explains this.

The output buffer for printf() is set to a fixed size before kernel launch (see Associated Host-Side API). It is circular and if more output is produced during kernel execution than can fit in the buffer, older output is overwritten. It is flushed only when one of these actions is performed:

Kernel launch via <<<>>> or cuLaunchKernel() (at the start of the launch, and if the CUDA_LAUNCH_BLOCKING environment variable is set to 1, at the end of the launch as well),

Synchronization via cudaDeviceSynchronize(), cuCtxSynchronize(), cudaStreamSynchronize(), cuStreamSynchronize(), cudaEventSynchronize(), or cuEventSynchronize(),

Memory copies via any blocking version of cudaMemcpy*() or cuMemcpy*(),

Module loading/unloading via cuModuleLoad() or cuModuleUnload(),

Context destruction via cudaDeviceReset() or cuCtxDestroy().

For this reason, this program prints nothing:

#include <stdio.h>

__global__ void myKernel() 
{ 
  printf("Hello, world from the device!\n"); 
} 

int main() 
{ 
  myKernel<<<1,10>>>(); 
}

But this program prints "Hello, world from the device!\n" ten times.

#include <stdio.h>

__global__ void myKernel() 
{ 
  printf("Hello, world from the device!\n"); 
} 

int main() 
{ 
  myKernel<<<1,10>>>(); 
  cudaDeviceSynchronize();
}

CUDA kernel doesn't launch

3 Answers