explict flush directive with OpenMP: when is it necessary and when is it helpful

Question

One OpenMP directive I have never used and don\'t know when to use is flush(with and without a list).

I have two questions:

1.) When i         


        

Answer 1

Not exactly an answer, but Michael Klemm's question is closed for comments. I think an excellent example of why flushes are so hard to understand and use properly is the following one copied (and shortened a bit) from the OpenMP Examples:

//http://www.openmp.org/wp-content/uploads/openmp-examples-4.0.2.pdf
//Example mem_model.2c, from Chapter 2 (The OpenMP Memory Model)
int main() {
   int data, flag = 0;
   #pragma omp parallel num_threads(2)
   {
      if (omp_get_thread_num()==0) {
         /* Write to the data buffer that will be read by thread */
         data = 42;
         /* Flush data to thread 1 and strictly order the write to data
            relative to the write to the flag */
         #pragma omp flush(flag, data)
         /* Set flag to release thread 1 */
         flag = 1;
         /* Flush flag to ensure that thread 1 sees S-21 the change */
         #pragma omp flush(flag)
      }
      else if (omp_get_thread_num()==1) {
         /* Loop until we see the update to the flag */
         #pragma omp flush(flag, data)
         while (flag < 1) {
            #pragma omp flush(flag, data)
         }
         /* Values of flag and data are undefined */
         printf("flag=%d data=%d\n", flag, data);
         #pragma omp flush(flag, data)
         /* Values data will be 42, value of flag still undefined */
         printf("flag=%d data=%d\n", flag, data);
      }
   }
   return 0;
}

Read the comments and try to understand.