The XLSMPOPTS environment variable sets options for programs using loop parallelization. For example, to have a program run-time create 4 threads and use dynamic scheduling with chunk size of 5, you would set the XLSMPOPTS environment variable as shown below:
XLSMPOPTS=PARTHDS=4:SCHEDULE=DYNAMIC=5
Additional environment variables set options for program parallelization using OpenMP-compliant directives.
Run-time Options for Parallel Processing
OpenMP Run-time Options for Parallel Processing