XL Fortran Enterprise Edition for AIX : User's Guide - IBM

num
the maximum number of threads that can be used if dynamic adjustment
of the number of threads is enabled. If dynamic adjustment of the number
of threads is not enabled, the value of OMP_NUM_THREADS is the exact
number of threads that can be used. It must be a positive, scalar integer.
The default number of threads that a program uses when it runs is the number of
online processors on the machine.
If you specify the number of threads with both the PARTHDS suboption of the
XLSMPOPTS environment variable and the OMP_NUM_THREADS environment
variable, the OMP_NUM_THREADS environment variable takes precedence. The
omp_set_num_threads subroutine takes precedence over the
OMP_NUM_THREADS environment variable.
The following example shows how you can set the OMP_NUM_THREADS
environment variable:
export OMP_NUM_THREADS=16
OMP_SCHEDULE Environment Variable
The OMP_SCHEDULE environment variable applies to PARALLEL DO and
work-sharing DO directives that have a schedule type of RUNTIME. The syntax is
as follows:
►► OMP_SCHEDULE= sched_type
, chunk_size
►◄
sched_type
is either DYNAMIC, GUIDED, or STATIC.
chunk_size
is a positive, scalar integer that represents the chunk size.
This environment variable is ignored for PARALLEL DO and work-sharing DO
directives that have a schedule type other than RUNTIME.
If you have not specified a schedule type either at compile time (through a
directive) or at run time (through the OMP_SCHEDULE environment variable or
the SCHEDULE option of the XLSMPOPTS environment variable), the default
schedule type is STATIC, and the default chunk size is set to the following for the
first N - 1 threads:
chunk_size = ceiling(Iters/N)
It is set to the following for the Nth thread, where N is the total number of threads
and Iters is the total number of iterations in the DO loop:
chunk_size = Iters - ((N - 1) * ceiling(Iters/N))
If you specify both the SCHEDULE option of the XLSMPOPTS environment
variable and the OMP_SCHEDULE environment variable, the OMP_SCHEDULE
environment variable takes precedence.
The following examples show how you can set the OMP_SCHEDULE
environment variable:
export OMP_SCHEDULE="GUIDED,4"
export OMP_SCHEDULE="DYNAMIC"
Editing, Compiling, Linking, and Running XL Fortran Programs 65