Compiling OpenMP code on a cluster

Overview

Teaching: 15 min
Exercises: 10 min

Questions

• How do I compile and run OpenMP programs?

Objectives

• Be able to compile an OpenMP program

From Wikipedia:

OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared memory multiprocessing programming in C, C++, and Fortran, … . It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.

OpenMP programs are typically compiled with a standard compiler (e.g. gcc or gfortran), with an option applied to modify the compiler’s behaviour to understand OpenMP directives.

Compiling an OpenMP C program

There is an OpenMP program written in C in the workshop files. In this version of the program, each separate parallel process says ‘Hello world’, and reports it’s thread number (a thread is a “light-weight” process, and it’s number is like the rank of an MPI process).

We compile the program with good ‘ol gcc:

gcc -o hello-omp -fopenmp hello-omp.c

Notice the -fopenmp flag – this tells the compiler we are compiling an OpenMP program.

Running an OpenMP program

We run the compiled program, using the environment variable OMP_NUM_THREADS to control the number of threads:

OMP_NUM_THREADS=4 ./hello-omp 

Hello World from thread = 0
Number of threads = 4
Hello World from thread = 2
Hello World from thread = 1
Hello World from thread = 3

An alternative is to export the environment variable OMP_NUM_THREADS into our shell. Each time we run the program after this, we don’t need to set the number of threads:

export OMP_NUM_THREADS=4
./hello-omp 

The thread numbers are reported in a somewhat random order, much like the MPI program’s ranks were reported.

Compiling and running an OpenMP FORTRAN program

In the workshop files, there is a ‘hello world’ OpenMP program written in Fortran. We compile it with gfortran, and again supply the -fopenmp flag:

gfortran -o hello-omp-fortran -fopenmp hello-omp.f90

It is run in the exact same way that we ran the C program:

OMP_NUM_THREADS=4 ./hello-omp-fortran

 Hello World from thread =            3
 Hello World from thread =            0
 Number of threads =            4
 Hello World from thread =            1
 Hello World from thread =            2

Exercise: Submitting our program to the scheduler

Write a slurm submission script that runs our OpenMP program using 2 threads. Submit your script to slurm and checkout the job output.

Key Points

• gcc with the -fopenmp flag compiles OpenMP code written in C

• gfortran with the -fopenmp flag compiles OpenMP code written in Fortran

• The program generated is run as usual, but the number of processors is controlled with the OMP_NUM_THREADS environment variable