Simple OpenMP For Loop in C wrong output

Question

Trying to get a simple OpenMP loop going, but I keep getting weird outputs. It doesn't list from 1 to 1000 straight, but goes from 501 to 750, then 1 to 1000. I'm guessing there's a threading issue? I'm compiling and running on VS2013.

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

int main(void)
{
    int counter = 0;
    double root = 0;

    // OPEN MP SECTION
    printf("Open MP section: \n\n");
    getchar(); //Pause

#pragma omp parallel for 

    for (counter = 0; counter <= 1000; counter++)
    {
        root = sqrt(counter);
        printf("The root of %d is %.2f\n", counter, root);

    }


    return(0);


}

paxdiablo · Accepted Answer · 2015-01-19 02:53:51Z

The whole point of OpenMP is to run things in parallel, distributing work to different execution engines.

Hence, it's likely that the individual iterations of your loop are done out of order because that is the very nature of multi-threading.

While it may make sense for the calculations to be done in parallel (and hence possibly out of order), that's not really what you want for the printing of results.

One way to ensure the results are printed in the correct order is to defer the printing until after the parallel execution is complete. In other words, parallelise the calculation but serialise the output.

That of course means being able to store the information in, for example, an array, while the parallel operations are running.

In other words, something like:

// Make array instead of single value.

double root[1001];

// Parallelise just the calculation bit.

#pragma omp parallel for 
for (counter = 0; counter <= 1000; counter++)
    root[counter] = sqrt(counter);

// Leave the output as a serial operation,
//   once all parallel operations are done.

for (counter = 0; counter <= 1000; counter++)
    printf("The root of %d is %.2f\n", counter, root[counter]);

Mark Tolonen · Accepted Answer · 2015-01-04 05:40:08Z

1

Store the results in an array and get the printf out of the loop. It has to serialize to the display.

answered Jan 4, 2015 at 5:40

Mark Tolonen

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Comments

Alexander Stocko · Accepted Answer · 2015-01-04 05:40:43Z

0

Your code will not be run sequentially.

OpenMP Parallel Pragma:

#pragma omp parallel
{
  // Code inside this region runs in parallel.
  printf("Hello!\n");
}

'This code creates a team of threads, and each thread executes the same code. It prints the text "Hello!" followed by a newline, as many times as there are threads in the team created. For a dual-core system, it will output the text twice. (Note: It may also output something like "HeHlellolo", depending on system, because the printing happens in parallel.) At the }, the threads are joined back into one, as if in non-threaded program."'

http://bisqwit.iki.fi/story/howto/openmp/#ParallelPragma

answered Jan 4, 2015 at 5:40

Alexander Stocko

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