Setting number of threads

Overview

Teaching: 10 min
Exercises: 5 min

Questions

• How to control the number of threads created by OpenMP?

Objectives

• Learn to control the number of OpenMP number threads by environment variable and programmatically

Setting the number of threads

In our previous “Hello World” example we noticed that OpenMP created 40 threads. This is because, if not specified, the parallel construct will create as many threads as there are processing units on your computer (i.e. 40 if running in compute on Hawk).

This behaviour can be modified globally by environment variable from a job script:

export OMP_NUM_THREADS=8

…or dynamically from within your code:

If using Fortran:

!$OMP PARALLEL NUM_THREADS(4)

or the default value using the function

omp_set_num_threads(4)

If using C,C++:

#pragma omp parallel private(tnum) num_threads(4)

or the default value using the function

omp_set_num_threads(4)

In this example we will create a simple OpenMP program that does the following:

• Creates a first parallel region with a defined number of threads
• Each thread in the first parallel region obtains and prints its unique thread number
• Creates a second parallel region without specifying the number of threads
• Each thread in the second parallel region obtains and prints its unique thread number
• Only the master will print the total number of threads

// Load the OpenMP functions library
#include<omp.h>

int main()
{
  // Set variables
  int tnum=0;

  // Create a parallel block of four threads (including master thread)
/* #pragma omp parallel private(tnum) num_threads(4) */
/*   {     */
    tnum = omp_get_thread_num();
    printf("I am thread number: %d\n", tnum);
  /* } */

  // Create a parallel block, without specifying the number of threads
/* #pragma omp parallel private(tnum) */
/*   {     */
    tnum = omp_get_thread_num();
    printf("Second block, I am thread number %d\n", tnum);
  /* } */

  return 0;
}

Controlling the number of threads.

Let us begin by confirming that our code behaves in serial mode in the way we expect. For this you don’t need to make any modifications since the OpenMP directives are commented out at the moment. Go ahead and compile the code:

~/openmp/Intro_to_OpenMP.2019/c$ icc -qopenmp -o setthreads setthreads.c

Once we are satisfied with the serial version of the code, uncomment the OpenMP directives. Compile and run again. How does it behave? Try modifying the number of threads given to num_threads.

Using environment variables

This time try using the environment variable OMP_NUM_THREADS to control the number of threads that OpenMP creates by default. To do this, type in your command line:

~$ export OMP_NUM_THREADS=8

And run the program again (notice that no compilation is needed this time). What happens?

Key Points

• We can use the environment variable OMP_NUM_THREADS to control how many threads are created by OpenMP by default

• OpenMP function num_threads have a similar effect and can be used within your code