Parallel Programming in Fortran 95 using OpenMP - People

70 5. The environment variables
5.1 OMP NUM THREADS
This environment variable specifies the number of threads to be used during execution of
the parallel regions inside an OpenMP-parallel program.
In principle, any integer number greater than 0 can be set as the number of threads to
be used, although if this number is larger than the number of available physical processors,
then the parallel running program will in general be very slow!
When dynamic adjustment of the number of threads is enabled, the value given in
OMP NUM THREADS represents the maximum number of threads allowed.
Examples showing the syntaxis corresponding to Linux/Unix systems using the csh
and bash shells are, respectively:
> setenv OMP_NUM_THREADS 16
> OMP_NUM_THREADS=16
> export OMP_NUM_THREADS
5.2 OMP SCHEDULE
This environment variable affects the way in which !$OMP DO and !$OMP PARALLEL DO
directives work, if they have their scheduling set to RUNTIME:
!$OMP DO SCHEDULE(RUNTIME)
The schedule type and chunk size for all such loops can be specified at run time
by setting this environment variable to any of the recognized schedule types and to an
optional chunk size, using the same notation as inside the SCHEDULE clause. The possible
schedule types are STATIC, DYNAMIC or GUIDED: descriptions of them can be found in the
explanation of the !$OMP DO directive.
The default value for the OMP SCHEDULE environment variable is OpenMP-implementation
dependent, so it is necessary to look at its documentation in order to know that.
If the optional chunk size is not set, a chunk size of 1 is assumed, except in the case of
a STATIC schedule, where the default chunk size is set equal to the total number of do-loop
iterations divided by the number of threads which act on the parallelized do-loop.
Examples showing the syntaxis corresponding to Linux/Unix systems using the csh
and bash shells are, respectively:
> setenv OMP_SCHEDULE "GUIDED,4"
> OMP_SCHEDULE=dynamic
> export OMP_SCHEDULE