OpenMP hello world in C

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Load modules

We will use the new software stack in this tutorial:

[jarunanp@eu-login-11 ~]$ env2lmod  
[jarunanp@eu-login-11 ~]$ module list

Currently Loaded Modules:
  1) StdEnv   2) gcc/4.8.5

openMP Hello World

• Go to $SCRATCH and create a work directory

 [jarunanp@eu-login-11 ~]$ cd $SCRATCH
 [jarunanp@eu-login-11 jarunanp]$ pwd
 /cluster/scratch/jarunanp
 [jarunanp@eu-login-11 jarunanp]$ mkdir test_openmp
 [jarunanp@eu-login-11 jarunanp]$ cd test_openmp
 [jarunanp@eu-login-11 test_openmp]$ 

• Create a file named hello_omp.c and edit it with a text editor (vi, nano or emacs)

// OpenMP header
#include <omp.h>
#include <stdio.h>

int main(int argc, char* argv[])
{
    //Parallel region code
    #pragma omp parallel                   
    {
      printf("Hello world from thread %d\n", 
             omp_get_thread_num());
    }  
}

• Compile the code

[jarunanp@eu-login-11 test_openmp]$ gcc -o hello_omp -fopenmp hello_omp.c
[jarunanp@eu-login-11 test_openmp]$ ll
total 20
-rwxr-x--- 1 user user-group 8680 Jun 14 10:55 hello_omp
-rw-r----- 1 user user-group  231 Jun 14 10:55 hello_omp.c

• Request an interactive session with a compute node

[jarunanp@eu-login-11 test_openmp]$ bsub -n 4 -Is bash
Generic job.
Job <175495032> is submitted to queue <normal.4h>.
<<Waiting for dispatch ...>>
<<Starting on eu-ms-001-08>>
FILE: /sys/fs/cgroup/cpuset/lsf/euler/job.175495032.32022.1623661994/tasks
[jarunanp@eu-ms-001-08 test_openmp]$ 

• Execute the code

[jarunanp@eu-ms-001-08 test_openmp]$ export OMP_NUM_THREADS=4
[jarunanp@eu-ms-001-08 test_openmp]$ ./hello_omp
Hello world from thread 1
Hello world from thread 3
Hello world from thread 0
Hello world from thread 2
[jarunanp@eu-ms-001-08 openmp]$ 

[jarunanp@eu-ms-001-08 test_openmp]$ export OMP_NUM_THREADS=8
[jarunanp@eu-ms-001-08 test_openmp]$ ./hello_omp
Hello world from thread 0
Hello world from thread 5
Hello world from thread 3
Hello world from thread 4
Hello world from thread 1
Hello world from thread 7
Hello world from thread 2
Hello world from thread 6
[jarunanp@eu-ms-001-08 test_openmp]$

• Unset the environment variable OMP_NUM_THREADS

[jarunanp@eu-ms-001-08 test_openmp]$ unset OMP_NUM_THREADS

• Exit the interactive session

[jarunanp@eu-ms-001-08 test_openmp]$ exit
exit
[jarunanp@eu-login-11 test_openmp]$

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