Difference between revisions of "Programming/OpenMP"
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The following modules are available for OpenMP: | The following modules are available for OpenMP: | ||
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== Batch example == | == Batch example == | ||
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#!/bin/bash | #!/bin/bash | ||
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#SBATCH -N 1 | #SBATCH -N 1 | ||
#SBATCH --ntasks-per-node 28 | #SBATCH --ntasks-per-node 28 | ||
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#SBATCH -o %N.%j.%a.out | #SBATCH -o %N.%j.%a.out | ||
#SBATCH -e %N.%j.%a.err | #SBATCH -e %N.%j.%a.err | ||
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module purge | module purge | ||
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export I_MPI_DEBUG=5 | export I_MPI_DEBUG=5 | ||
Revision as of 15:00, 7 February 2017
Contents
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]);
}
}
Compilation
The program would be compiled in the following way, optional Intel compiler available too:
[username@login01 ~]$ module load gcc/4.9.3 [username@login01 ~]$ gcc -o test2 -fopenmp test2.c
Modules Available
The following modules are available for OpenMP:
- module add gcc/4.9.3 (GNU compiler)
- module add intel/compiler/64/2016.2.181 (Intel compiler)
Note: OpenMP is a library directive within the compiler and does not require any additional module to be loaded.
Usage Examples
Batch example
#!/bin/bash #SBATCH -J openmpi-single-node #SBATCH -N 1 #SBATCH --ntasks-per-node 28 #SBATCH -o %N.%j.%a.out #SBATCH -e %N.%j.%a.err #SBATCH -p compute #SBATCH --exclusive echo $SLURM_JOB_NODELIST module purge module add gcc/4.9.3 export I_MPI_DEBUG=5 export I_MPI_FABRICS=shm:tmi export I_MPI_FALLBACK=no /home/user/CODE_SAMPLES/OPENMP/demo
[username@login01 ~]$ sbatch demo.job Submitted batch job 289552