Question

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:
");
  for(j=0;j<8;j++)
  printf("%d",W[j]);
  printf("
");
  #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 	 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 
 Q[%d]= %d",id,i,Q[i]);
   };
 }
  printf("Result:
");
  for(j=0; j<8; j++)
   printf("%d 	",W[j]);
   printf("
");

}

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

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