how to use omp barrier on a while loop with no equal number of iterations for threads

Question

Hi! I'm trying to implement the listranking problem (known also by shortcutting) with omp to have the sums prefixes of the array W. I don't know if i use correctly the flush pragma.. And i have a warning when compiling "barrier region may not be closely nested inside of work-sharing, critical, ordered, master or explicit task region"

#include <stdio.h> 
#include <stdlib.h>
#include <math.h>
#include <omp.h>

main(int argc, char *argv[])
{ 
  int Q[9]={1,2,3,4,5,6,7,8,0};
  int W[8]={1,2,3,4,5,6,7,8};
  int i,j=6,id;

  printf("Before:\n");
  for(j=0;j<8;j++)
  printf("%d",W[j]);
  printf("\n");
  #pragma omp parallel for shared(Q,W) private(id) num_threads(7)
  for (i=6; i>=0; i--)
  {
    id= omp_get_thread_num();
    while((Q[i] !=0)&& (Q[Q[i]] !=0))
    { 
      #pragma omp flush(W)

       W[i]=W[i]+W[Q[i]];

      #pragma omp flush(W)

       printf("Am %d \t W[%d]= %d",id,i,W[i]);

     #pragma omp barrier    
     #pragma omp flush(Q)
     Q[i]=Q[Q[i]];
     #pragma omp flush(Q)
     printf("Am %d \n Q[%d]= %d",id,i,Q[i]);
   };
 }
  printf("Result:\n");
  for(j=0; j<8; j++)
   printf("%d \t",W[j]);
   printf("\n");

}

PLEAAAAAAAAAAAASE HELP!

Answer 1

You can't use a barrier inside an omp parallel for, you can pretty much only use a barrier inside an omp parallel region.

The reason for this is because if your loop is from 1 to N, a barrier inside will effectively create N threads which will have a negative perf impact if N is large.

I didn't lookup the algorithm here, but two reasonable choices are to refactor to use 2 parallel for loops one after the other where the barrier is, or to refactor your algorithm to use a #pragma parallel region.

I looked up the list ranking algorithm, you will be well served to find an implementation of prefix sum or scan if you must use openmp.

-Rick