CUDA: cudaMemcpy only works in emulation mode

Question

I am just starting to learn how to use CUDA. I am trying to run some simple example code:


float *ah, *bh, *ad, *bd;
ah = (float *)malloc(sizeof(float)*4);
bh = (float *)malloc(sizeof(float)*4);
cudaMalloc((void **) &ad, sizeof(float)*4);
cudaMalloc((void **) &bd, sizeof(float)*4);
... initialize ah ...

/* copy array on device */
cudaMemcpy(ad,ah,sizeof(float)*N,cudaMemcpyHostToDevice);
cudaMemcpy(bd,ad,sizeof(float)*N,cudaMemcpyDeviceToDevice);
cudaMemcpy(bh,bd,sizeof(float)*N,cudaMemcpyDeviceToHost);

When I run in emulation mode (nvcc -deviceemu) it runs fine (and actually copies the array). But when I run it in regular mode, it runs w/o error, but never copies the data. It's as if the cudaMemcpy lines are just ignored.

What am I doing wrong?

Thank you very much, Jason

Oops. This seems to be a problem with cudaMalloc(). It is not allocating the memory on the device. Why is that? — Jason, Mar 19 '10 at 19:01
did you initialize device? use cuda get last error to print status — Anycorn, Mar 19 '10 at 20:34
@aaa: Using the runtime API (functions prefixed with cuda instead of cu) means you do not need to explicitly initialise a device, it will attach to the first compatible device on the first cuda call. — Tom, Mar 20 '10 at 17:24

Tom · Answer 1 · 2010-03-20T21:07:38.457

3

You should check for errors, ideally on each malloc and memcpy but just doing it once at the end will be sufficient (cudaGetErrorString(cudaGetLastError()).

Just to check the obvious:

You do have a CUDA capable GPU, right? Run the deviceQuery SDK sample to check the device is working correctly and all the drivers are installed and working.
N (in the memcpy) is equal to 4 (in the malloc), right?

edited Mar 20 '10 at 21:07

answered Mar 20 '10 at 17:22

Tom

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score 1 · Answer 2 · answered Oct 28 '10 at 03:30

See if you have a CUDA enabled device. Probably you can try running the code below and see what info you get:

#include <cstdio>

int main( void ) {
    cudaDeviceProp  prop;

    int count;
    cudaGetDeviceCount( &count );
    for (int i=0; i< count; i++) {
        cudaGetDeviceProperties( &prop, i );
        printf( "   --- General Information for device %d ---\n", i );
        printf( "Name:  %s\n", prop.name );
        printf( "Compute capability:  %d.%d\n", prop.major, prop.minor );
        printf( "Clock rate:  %d\n", prop.clockRate );
        printf( "Device copy overlap:  " );
        if (prop.deviceOverlap)
            printf( "Enabled\n" );
        else
            printf( "Disabled\n");
        printf( "Kernel execution timeout :  " );
        if (prop.kernelExecTimeoutEnabled)
            printf( "Enabled\n" );
        else
            printf( "Disabled\n" );

        printf( "   --- Memory Information for device %d ---\n", i );
        printf( "Total global mem:  %ld\n", prop.totalGlobalMem );
        printf( "Total constant Mem:  %ld\n", prop.totalConstMem );
        printf( "Max mem pitch:  %ld\n", prop.memPitch );
        printf( "Texture Alignment:  %ld\n", prop.textureAlignment );

        printf( "   --- MP Information for device %d ---\n", i );
        printf( "Multiprocessor count:  %d\n",
                    prop.multiProcessorCount );
        printf( "Shared mem per mp:  %ld\n", prop.sharedMemPerBlock );
        printf( "Registers per mp:  %d\n", prop.regsPerBlock );
        printf( "Threads in warp:  %d\n", prop.warpSize );
        printf( "Max threads per block:  %d\n",
                    prop.maxThreadsPerBlock );
        printf( "Max thread dimensions:  (%d, %d, %d)\n",
                    prop.maxThreadsDim[0], prop.maxThreadsDim[1],
                    prop.maxThreadsDim[2] );
        printf( "Max grid dimensions:  (%d, %d, %d)\n",
                    prop.maxGridSize[0], prop.maxGridSize[1],
                    prop.maxGridSize[2] );
        printf( "\n" );
    }
}

CUDA: cudaMemcpy only works in emulation mode

2 Answers2