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Running OpenMP Code

To run your OpenMP code:

  1. Right click your OpenMP project and select “Clean and Build Project”.
  1. Right click your OpenMP project and select “Run project”.
  1. A terminal will open showing you the output of your program. The terminal will exit when your program finishes executing.

Manipulating the Number of Threads

This section explains how to manipulate the number of thread your process uses. There are a couple of ways to do this depending on whether you're running your code from SunStudio or straight from the command line.

Manipulating the Number of Threads from SunStudio

  1. There are a couple methods in the OpenMP API that will manipulate the number of threads your code uses. By default and for efficiency reasons, OpenMP will only start as many threads as there are processors on the system. To override this behavior you must set OMP_DYNAMIC to false with the method omp_set_dynamic.
  2. Once you've used omp_set_dynamic, you will be able to set the number of threads using the omp_set_num_threads method.
  3. The following are example usages in C/C++/Fortran:

C/C++

omp_set_dynamic(0);
omp_set_num_threads(10);

Fortran

call omp_set_dynamic(.FALSE.)
call omp_set_num_threads(10)

NOTE: On both the StarHPC remote cluster and standalone virtual machine image, the SunStudio projects included contain example usage of this code.

Manipulating the Number of Threads from the Command Line

This section assumes you will be executing your code manually from command line.

  • There are a couple environment variables that will manipulate the number of threads your code uses. By default and for efficiency reasons, OpenMP will only start as many threads as there are processors on the system. To override this behavior the environment variable OMP_DYNAMIC must be set to false by running in a terminal:
export OMP_DYNAMIC=FALSE

NOTE: On both the StarHPC remote cluster and standalone virtual machine image, OMP_DYNAMIC is set to FALSE by default for all terminal sessions. This means you do not have to export OMP_DYNAMIC for each terminal you open a new terminal.

  • After this variable is set to False, OpenMP will start any number of threads determined by the environment variable OMP_NUM_THREADS. If this variable is not set it defaults to the number of processors on the system. For example, to run your code with 4 threads, run in a terminal:
export OMP_NUM_THREADS=4

In the same terminal session, run your executable:

./yourexecutable

NOTE: The OMP_NUM_THREADS variable must be set each time you open a terminal.

  • To use more/less threads set OMP_NUM_THREADS accordingly.