Concurrently initializing many arrays with random numbers using Curand and CUDA kernel

Question

I am trying to initialize 100 elements of each these parallel arrays with randomly generated numbers concurrently on the GPU. However, my routine is not producing a variety of random numbers. When I debug the code in Visual Studio I see one number for every element in the array. The object of this code is to optimize the CImg FilledTriangles routine to use the GPU where it can.

What am I doing wrong and how can I fix it? Here is my code:

__global__ void initCurand(curandState* state, unsigned long seed)
    int idx = threadIdx.x + blockIdx.x * blockDim.x;
    curand_init(seed, idx, 0, &state[idx]);
    __syncthreads();
}

/*
 * CUDA kernel that will execute 100 threads in parallel
*/

__global__ void initializeArrays(float* posx, float* posy,float* rayon, float* veloc, float* opacity
                                ,float * angle, unsigned char** color, int height, int width, curandState* state){

    int idx = threadIdx.x + blockIdx.x * blockDim.x;

    curandState localState = state[idx];
    __syncthreads();

    posx[idx] = (float)(curand_uniform(&localState)*width);
    posy[idx] = (float)(curand_uniform(&localState)*height);
    rayon[idx] = (float)(10 + curand_uniform(&localState)*50);
    angle[idx] = (float)(curand_uniform(&localState)*360);
    veloc[idx] = (float)(curand_uniform(&localState)*20 - 10);
    color[idx][0] = (unsigned char)(curand_uniform(&localState)*255);
    color[idx][1] = (unsigned char)(curand_uniform(&localState)*255);
    color[idx][2] = (unsigned char)(curand_uniform(&localState)*255);
    opacity[idx] = (float)(0.3 + 1.5*curand_uniform(&localState));
}

Here is the host code that prepares and calls these kernels: I am trying to create 100 threads (for each element) on one block in a grid.

 // launch grid of threads
      dim3 dimBlock(100);
      dim3 dimGrid(1);

      initCurand<<<dimBlock,dimGrid>>>(devState, unsigned(time(nullptr)));
      // synchronize the device and the host
    cudaDeviceSynchronize();
     initializeArrays<<<dimBlock, dimGrid>>>(d_posx, d_posy, d_rayon, d_veloc, d_opacity, d_angle,d_color, img0.height(), img0.width(), devState);

Preliminaries:

  // Define random properties (pos, size, colors, ..) for all triangles that will be displayed.
    float posx[100], posy[100], rayon[100], angle[100], veloc[100], opacity[100];
    // Define the same properties but for the device
    float* d_posx;
    float* d_posy;
    float* d_rayon;
    float* d_angle;
    float* d_veloc;
    float* d_opacity;
    //unsigned char d_color[100][3];
    unsigned char** d_color;
    curandState* devState;
    cudaError_t err;

    // allocate memory on the device for the device arrays
    err = cudaMalloc((void**)&d_posx, 100 * sizeof(float));
    err = cudaMalloc((void**)&d_posy, 100 * sizeof(float));
    err = cudaMalloc((void**)&d_rayon, 100 * sizeof(float));
    err = cudaMalloc((void**)&d_angle, 100 * sizeof(float));
    err = cudaMalloc((void**)&d_veloc, 100 * sizeof(float));
    err = cudaMalloc((void**)&d_opacity, 100 * sizeof(float));
    err = cudaMalloc((void**)&devState, 100*sizeof(curandState));
    errCheck(err);
    size_t pitch;
    //allocated the device memory for source array  
    err = cudaMallocPitch(&d_color, &pitch, 3 * sizeof(unsigned char),100);

Getting the results:

// get the populated arrays back to the host for use
     err = cudaMemcpy(posx,d_posx, 100 * sizeof(float), cudaMemcpyDeviceToHost);
     err = cudaMemcpy(posy,d_posy, 100 * sizeof(float), cudaMemcpyDeviceToHost);
     err = cudaMemcpy(rayon,d_rayon, 100 * sizeof(float), cudaMemcpyDeviceToHost);
     err = cudaMemcpy(veloc,d_veloc, 100 * sizeof(float), cudaMemcpyDeviceToHost);
     err = cudaMemcpy(opacity,d_opacity, 100 * sizeof(float), cudaMemcpyDeviceToHost);
     err = cudaMemcpy(angle,d_angle, 100 * sizeof(float), cudaMemcpyDeviceToHost);
     err = cudaMemcpy2D(color,pitch,d_color,100, 100 *sizeof(unsigned char),3, cudaMemcpyDeviceToHost);

Answer 1

definitely you will need to make a change from this:

err = cudaMalloc((void**)&devState, 100*sizeof(float));

to this:

err = cudaMalloc((void**)&devState, 100*sizeof(curandState));

If you ran your code through cuda-memcheck, you would have discovered this. Your initCurand kernel had plenty of out-of-bounds accesses due to this.

You should also be doing error checking on all cuda calls and all kernel launches. I believe your second kernel call is failing due to a messed up operation on your color[][] array.

Normally when we create an array with cudaMallocPitch, we need to access it using the pitch parameter. C doubly-subscripted arrays by themselves won't work, because C has no inherent knowledge of the actual array width.

I was able to fix it by making the following changes:

__global__ void initializeArrays(float* posx, float* posy,float* rayon, float* veloc, float* opacity,float * angle, unsigned char* color, int height, int width, curandState* state, size_t pitch){

    int idx = threadIdx.x + blockIdx.x * blockDim.x;

    curandState localState = state[idx];
    __syncthreads();

    posx[idx] = (float)(curand_uniform(&localState)*width);
    posy[idx] = (float)(curand_uniform(&localState)*height);
    rayon[idx] = (float)(10 + curand_uniform(&localState)*50);
    angle[idx] = (float)(curand_uniform(&localState)*360);
    veloc[idx] = (float)(curand_uniform(&localState)*20 - 10);
    color[idx*pitch] = (unsigned char)(curand_uniform(&localState)*255);
    color[(idx*pitch)+1] = (unsigned char)(curand_uniform(&localState)*255);
    color[(idx*pitch)+2] = (unsigned char)(curand_uniform(&localState)*255);
    opacity[idx] = (float)(0.3 + 1.5*curand_uniform(&localState));
}

and

 initializeArrays<<<dimBlock, dimGrid>>>(d_posx, d_posy, d_rayon, d_veloc, d_opacity, d_angle,d_color, img0.height(), img0.width(), devState, pitch);

and

unsigned char* d_color;

with those changes, I was able to eliminate the errors I found and the code spit out various random values. I haven't inspected all the values, but that should get you started.