OpenMP - Initializing 1D Array

Question

I am trying to initialize a dynamic array using OpenMP in C but it seems to be slower than the serial method. The function I am using is

int* createArray(int size, int num) {
    int i;
    int* M = (int*)malloc(size*sizeof(int));

    srand(time(NULL));
    double start = omp_get_wtime();

    #pragma omp parallel for num_threads(num)
    for (i = 0; i < size; i++) {
        M[i] = rand() % (MAX_NUMBER - MIN_NUMBER + 1) + MIN_NUMBER;
    }

    double end = omp_get_wtime();
    printf("Create Array %f\n",end-start);
    return M;
}

I get an array of the given size containing random numbers but the fewer threads I use the faster the function is. Am I doing something wrong?

There are lots of Qs and As here on SO explaining why not to use `rand` in OpenMP programs. For example:https://stackoverflow.com/questions/10624755/openmp-program-is-slower-than-sequential-one/10625090#10625090 — High Performance Mark, Jun 04 '17 at 18:47
Possible duplicate of [OpenMP program is slower than sequential one](https://stackoverflow.com/questions/10624755/openmp-program-is-slower-than-sequential-one) — Zulan, Jun 04 '17 at 21:46

PiCTo · Accepted Answer · 2017-06-04T18:23:01.707

In general, a parallel application running slower than the corresponding sequential implementation usually comes from either the overhead of starting the threads or the bottleneck of having threads that are not perfectly independent (e.g. through shared data).

Here, the former is true because you are calling rand(). This function uses somehow global variables which have to be shared between threads. A way to overcome this would be to use a private seed for each thread. Furthermore, did you notice that your array is not really random when you have multiple threads? You could make the seed provided to srand() a function of omp_get_thread_num() to solve this.

score 0 · Answer 2 · answered Jun 07 '17 at 21:00

I'm quite sure your program is suffering a problem called "false sharing" cache.

The article below explains it quite well. https://software.intel.com/en-us/articles/avoiding-and-identifying-false-sharing-among-threads

this often affects performance a lot. you can quickly have a test. add below to your omp pragma schedule(static, 16)

this should improve a lot. then you can dig further about false sharing.

OpenMP - Initializing 1D Array

2 Answers2