CODEWITHSUNDEEP
c++openmp
user805547
user805547

Reputation: 1265

OpenMP Unexpected output

I have two threads with a parallel for but this one does not break up into threads:

#pragma omp parallel sections
  {
#pragma omp section
    {
      for(int i=0;i<4;++i)
        printf("Loop A %d %d\n", omp_get_thread_num(),2);
    }
#pragma omp section
    {
      for(int i=0;i<4;++i)
        printf("Loop B %d %d\n", omp_get_thread_num(),3);
    }           
  }

Output:

Running…

Loop A 0 2
Loop A 0 2
Loop A 0 2
Loop A 0 2
Loop B 0 3
Loop B 0 3
Loop B 0 3
Loop B 0 3

Upvotes: 1

Views: 129

Answers (2)

jfs
jfs

Reputation: 414825

It might just happen too fast. I've tried a larger number of iterations and it works as expected:

// gcc -std=c99 *.c -fopenmp && ./a.out
#include <stdio.h>
#include <omp.h>

enum { N = 10000,  M = N/2 };

static void loop(char* s, int d) {
  for(int i=0;i<N;++i)
    if (i % M == 0)
      printf("Loop %s %d %d %d\n", s, i, omp_get_thread_num(),d);
}

int main() {
#pragma omp parallel sections
  {
#pragma omp section
    loop("A", 2);
#pragma omp section
    loop("B", 3);
  }
}

Output

Loop A 0 7 2
Loop B 0 4 3
Loop A 5000 7 2
Loop B 5000 4 3

Upvotes: 1

Mysticial
Mysticial

Reputation: 471559

This can happen for a number of reasons. I'll list the possible ones I know:

1: Make sure you actually have a multi-core machine. If it's a single core machine (no HT), it'll only run with one thread.

2: If this is on Visual Studio, you need to enable OpenMP support. Just including the header is not enough:

Project -> Properties -> Configuration Properties -> C/C++ -> Language -> Open MP Support

change it to Yes (/openmp), and it should be enabled.

I ran your code with OpenMP setup properly and I get this:

Loop A 0 2
Loop B 2 3
Loop B 2 3
Loop B 2 3
Loop B 2 3
Loop A 0 2
Loop A 0 2
Loop A 0 2

So I think your code is correct.

Upvotes: 3

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