Tricks and Tips: Calling CUDA Thrust primitives from within a kernel

Starting from Thrust 1.8, CUDA Thrust primitives can be combined with the thrust::seq execution policy to run sequentially within a single CUDA thread (or sequentially within a single CPU thread). Below, an example is reported.

If one wants parallel execution within a thread, then using CUB must be considered, which provides reduction routines that can be called from within a threadblock, provided that the relevant card enables dynamic parallelism.

Here is the example with Thrust:

#include <stdio.h>

#include <thrust/reduce.h>
#include <thrust/execution_policy.h>

/********************/
/* CUDA ERROR CHECK */
/********************/
#define gpuErrchk(ans) { gpuAssert((ans), __FILE__, __LINE__); }
inline void gpuAssert(cudaError_t code, char *file, int line, bool abort=true)
{
   if (code != cudaSuccess)
   {
      fprintf(stderr,"GPUassert: %s %s %dn", cudaGetErrorString(code), file, line);
      if (abort) exit(code);
   }
}

__global__ void test(float *d_A, int N) {

    float sum = thrust::reduce(thrust::seq, d_A, d_A + N);

    printf("Device side result = %fn", sum);

}

int main() {

    const int N = 16;

    float *h_A = (float*)malloc(N * sizeof(float));
    float sum = 0.f;
    for (int i=0; i<N; i++) {
        h_A[i] = i;
        sum = sum + h_A[i];
    }
    printf("Host side result = %fn", sum);

    float *d_A; gpuErrchk(cudaMalloc((void**)&d_A, N * sizeof(float)));
    gpuErrchk(cudaMemcpy(d_A, h_A, N * sizeof(float), cudaMemcpyHostToDevice));

    test<<<1,1>>>(d_A, N);

}

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