Difference between revisions of "Programming/OpenMP"

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* Serializing sections of code
 
* Serializing sections of code
 
* Synchronization of work among threads
 
* Synchronization of work among threads
 +
 +
=== Programming example ===
 +
 +
<pre style="background-color: #C8C8C8; color: black; border: 2px solid black; font-family: monospace, sans-serif;">
 +
 +
#include <stdio.h>
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#include <stdlib.h>
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#include <malloc.h>
 +
 +
/* compile with gcc -o test2 -fopenmp test2.c */
 +
 +
int main(int argc, char** argv)
 +
{
 +
    int i = 0;
 +
    int size = 20;
 +
    int* a = (int*) calloc(size, sizeof(int));
 +
    int* b = (int*) calloc(size, sizeof(int));
 +
    int* c;
 +
 +
    for ( i = 0; i < size; i++ )
 +
    {
 +
        a[i] = i;
 +
        b[i] = size-i;
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        printf("[BEFORE] At %d: a=%d, b=%d\n", i, a[i], b[i]);
 +
    }
 +
 +
    #pragma omp parallel shared(a,b) private(c,i)
 +
    {
 +
        c = (int*) calloc(3, sizeof(int));
 +
 +
        #pragma omp for
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        for ( i = 0; i < size; i++ )
 +
        {
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            c[0] = 5*a[i];
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            c[1] = 2*b[i];
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            c[2] = -2*i;
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            a[i] = c[0]+c[1]+c[2];
 +
 +
            c[0] = 4*a[i];
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            c[1] = -1*b[i];
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            c[2] = i;
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            b[i] = c[0]+c[1]+c[2];
 +
        }
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 +
        free(c);
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    }
 +
 +
    for ( i = 0; i < size; i++ )
 +
    {
 +
        printf("[AFTER] At %d: a=%d, b=%d\n", i, a[i], b[i]);
 +
    }
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}
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</pre>
 +
 +
==== Compiled ====
 +
 +
The program would be compiled in the following way, optional Intel compiler available too:
 +
 +
<pre style="background-color: #C8C8C8; color: black; border: 2px solid black; font-family: monospace, sans-serif;">
 +
 +
module load gcc/4.9.3
 +
gcc -o test2 -fopenmp test2.c
 +
 +
</pre>
 +
  
 
== Usage Examples ==
 
== Usage Examples ==

Revision as of 15:03, 30 January 2017

Programming Details

OpenMP is designed for multi-processor/core, shared memory machines. The underlying architecture can be shared memory UMA or NUMA.

It is an Application Program Interface (API) that may be used to explicitly direct multi-threaded, shared memory parallelism. Comprised of three primary API components:

  • Compiler Directives
  • Runtime Library Routines
  • Environment Variables

OpenMP compiler directives are used for various purposes:

  • Spawning a parallel region
  • Dividing blocks of code among threads
  • Distributing loop iterations between threads
  • Serializing sections of code
  • Synchronization of work among threads

Programming example


#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>

/* compile with gcc -o test2 -fopenmp test2.c */

int main(int argc, char** argv)
{
    int i = 0;
    int size = 20;
    int* a = (int*) calloc(size, sizeof(int));
    int* b = (int*) calloc(size, sizeof(int));
    int* c;

    for ( i = 0; i < size; i++ )
    {
        a[i] = i;
        b[i] = size-i;
        printf("[BEFORE] At %d: a=%d, b=%d\n", i, a[i], b[i]);
    }

    #pragma omp parallel shared(a,b) private(c,i)
    {
        c = (int*) calloc(3, sizeof(int));

        #pragma omp for
        for ( i = 0; i < size; i++ )
        {
            c[0] = 5*a[i];
            c[1] = 2*b[i];
            c[2] = -2*i;
            a[i] = c[0]+c[1]+c[2];

            c[0] = 4*a[i];
            c[1] = -1*b[i];
            c[2] = i;
            b[i] = c[0]+c[1]+c[2];
        }

        free(c);
    }

    for ( i = 0; i < size; i++ )
    {
        printf("[AFTER] At %d: a=%d, b=%d\n", i, a[i], b[i]);
    }
}

Compiled

The program would be compiled in the following way, optional Intel compiler available too:


module load gcc/4.9.3
gcc -o test2 -fopenmp test2.c


Usage Examples

Batch example

#!/bin/bash

#SBATCH -J openmpi-single-node
#SBATCH -N 1
#SBATCH --ntasks-per-node 28
#SBATCH -D /home/user/CODE_SAMPLES/OPENMP
#SBATCH -o %N.%j.%a.out
#SBATCH -e %N.%j.%a.err
#SBATCH -p compute
#SBATCH --exclusive

echo $SLURM_JOB_NODELIST

module purge
module load gcc/4.9.3

export I_MPI_DEBUG=5
export I_MPI_FABRICS=shm:tmi
export I_MPI_FALLBACK=no

/home/user/CODE_SAMPLES/OPENMP/demo


[username@login01 ~]$ sbatch demo.job
Submitted batch job 289552