10.1.5 Number of Threads

The PARALLEL (or OMP_NUM_THREADS) environment variable controls the maximum number of threads available to the program. Setting the environment variable tells the runtime system the maximum number of threads the program can use. The default is 1. In general, set the PARALLEL or OMP_NUM_THREADS variable to the number of available virtual processors on the target platform.

The following example shows how to set it:

demo% setenv OMP_NUM_THREADS 4       C shell

-or-

demo$ OMP_NUM_THREADS=4               Bourne/Korn shell
demo$ export OMP_NUM_THREADS

In this example, setting PARALLEL to four enables the execution of a program using at most four threads. If the target machine has four processors available, the threads will map to independent processors. If there are fewer than four processors available, some threads could run on the same processor as others, possibly degrading performance.

The SunOS^TM operating system command psrinfo(1M) displays a list of the processors available on a system:

demo% psrinfo
0      on-line   since 03/18/2007 15:51:03
1      on-line   since 03/18/2007 15:51:03
2      on-line   since 03/18/2007 15:51:03
3      on-line   since 03/18/2007 15:51:03