cudaOpenMP failed to pass correctResult

I tried to increase the number of element n = 8129 to n = 2 * 3200 * 2 * 8192 * 8 and
then ran the code but unfortunately it failed to pass correctResult. If I increase n
to 2 * 3200 * 2 * 8192 * 4 then it passed the check. the GPU was GTX TITAN X with 12G
memory.

/*

  • Copyright 1993-2015 NVIDIA Corporation. All rights reserved.
  • Please refer to the NVIDIA end user license agreement (EULA) associated
  • with this source code for terms and conditions that govern your use of
  • this software. Any use, reproduction, disclosure, or distribution of
  • this software and related documentation outside the terms of the EULA
  • is strictly prohibited.

*/

/*

  • Multi-GPU sample using OpenMP for threading on the CPU side
  • needs a compiler that supports OpenMP 2.0
    */

#include <omp.h>
#include <stdio.h> // stdio functions are used since C++ streams aren’t necessarily thread safe
#include <helper_cuda.h>

using namespace std;

// a simple kernel that simply increments each array element by b
global void kernelAddConstant(unsigned int *g_a, const unsigned int b)
{
unsigned int idx = blockIdx.x * blockDim.x + threadIdx.x;
g_a[idx] += b;
}

// a predicate that checks whether each array element is set to its index plus b
int correctResult(unsigned int *data, const unsigned int n, const unsigned int b)
{
int success = 1;
for (unsigned int i = 0; i < n; i++)
{
if (data[i] != i + b)
{
printf("%u %u %u\n", i, data[i], i + b);
success = 0;
return success;
}
}

return success;

}

int main(int argc, char *argv)
{
int num_gpus = 0; // number of CUDA GPUs

printf("%s Starting...\n\n", argv[0]);

/////////////////////////////////////////////////////////////////
// determine the number of CUDA capable GPUs
//
cudaGetDeviceCount(&num_gpus);

if (num_gpus < 1)
{
    printf("no CUDA capable devices were detected\n");
    return 1;
}

/////////////////////////////////////////////////////////////////
// display CPU and GPU configuration
//
printf("number of host CPUs:\t%d\n", omp_get_num_procs());
printf("number of CUDA devices:\t%d\n", num_gpus);

for (int i = 0; i < num_gpus; i++)
{
    cudaDeviceProp dprop;
    cudaGetDeviceProperties(&dprop, i);
    printf("   %d: %s\n", i, dprop.name);
}

printf("---------------------------\n");

printf("sizeof(unsigned int) = %u\n", sizeof(unsigned int));

/////////////////////////////////////////////////////////////////
// initialize data
//
unsigned int n = 2 * 3200 * num_gpus * 8192 * 8;
unsigned int nbytes = n * sizeof(unsigned int);
unsigned int *a = 0;     // pointer to data on the CPU
unsigned int b = 3;      // value by which the array is incremented
a = (unsigned int *)malloc(nbytes);

if (0 == a)
{
    printf("couldn't allocate CPU memory\n");
    return 1;
}

for (unsigned int i = 0; i < n; i++)
    a[i] = i;

////////////////////////////////////////////////////////////////
// run as many CPU threads as there are CUDA devices
//   each CPU thread controls a different device, processing its
//   portion of the data.  It's possible to use more CPU threads
//   than there are CUDA devices, in which case several CPU
//   threads will be allocating resources and launching kernels
//   on the same device.  For example, try omp_set_num_threads(2*num_gpus);
//   Recall that all variables declared inside an "omp parallel" scope are
//   local to each CPU thread
//
//omp_set_num_threads(num_gpus);  // create as many CPU threads as there are CUDA devices
omp_set_num_threads(8*num_gpus);// create twice as many CPU threads as there are CUDA devices
#pragma omp parallel shared(n, nbytes, a, b)
{
    unsigned int cpu_thread_id = omp_get_thread_num();
    unsigned int num_cpu_threads = omp_get_num_threads();

// // set and check the CUDA device for this CPU thread
// int gpu_id = -1;
// checkCudaErrors(cudaSetDevice(cpu_thread_id % num_gpus)); // “% num_gpus” allows more CPU threads than GPU devices
// checkCudaErrors(cudaGetDevice(&gpu_id));
// printf(“CPU thread %d (of %d) uses CUDA device %d\n”, cpu_thread_id, num_cpu_threads, gpu_id);

    // set and check the CUDA device for this CPU thread
	int gpu_id = 0;
	checkCudaErrors(cudaSetDevice(gpu_id)); // "% num_gpus" allows more CPU threads than GPU devices
	checkCudaErrors(cudaGetDevice(&gpu_id));
	printf("CPU thread %d (of %d) uses CUDA device %d\n", cpu_thread_id,
			num_cpu_threads, gpu_id);


	unsigned int *d_a = 0;   // pointer to memory on the device associated with this CPU thread
    unsigned int *sub_a = a + cpu_thread_id * n / num_cpu_threads;   // pointer to this CPU thread's portion of data
    unsigned int nbytes_per_kernel = nbytes / num_cpu_threads;
    dim3 gpu_threads(128);  // 128 threads per block
    dim3 gpu_blocks(n / (gpu_threads.x * num_cpu_threads));

    checkCudaErrors(cudaMalloc((void **)&d_a, nbytes_per_kernel));
    checkCudaErrors(cudaMemset(d_a, 0, nbytes_per_kernel));
    checkCudaErrors(cudaMemcpy(d_a, sub_a, nbytes_per_kernel, cudaMemcpyHostToDevice));
    kernelAddConstant<<<gpu_blocks, gpu_threads>>>(d_a, b);

    checkCudaErrors(cudaMemcpy(sub_a, d_a, nbytes_per_kernel, cudaMemcpyDeviceToHost));
    checkCudaErrors(cudaFree(d_a));

}
printf("---------------------------\n");

if (cudaSuccess != cudaGetLastError())
    printf("%s\n", cudaGetErrorString(cudaGetLastError()));


////////////////////////////////////////////////////////////////
// check the result
//
bool bResult = correctResult(a, n, b);

if (a)
    free(a); // free CPU memory

// cudaDeviceReset causes the driver to clean up all state. While
// not mandatory in normal operation, it is good practice.  It is also
// needed to ensure correct operation when the application is being
// profiled. Calling cudaDeviceReset causes all profile data to be
// flushed before the application exits
cudaDeviceReset();

if (bResult) {
	printf("bResult = true\n");
} else {
	printf("bResult = false\n");
}

exit(bResult ? EXIT_SUCCESS : EXIT_FAILURE);

}

Problem solved.