Parallel Programming in Fortran 95 using OpenMP - People

3.2. Other clauses 47
do i = 1, N
...
enddo
!$OMP END DO
!$OMP END PARALLEL
the do-loop is executed in parallel only, if the number of iterations is greater than
1000, otherwise the do-loop is executed in a serial fashion.
3.2.2 NUM THREADS(scalar integer expression)
For certain parallel running source code lines it could be desirable to use a fixed number
of threads, for example, if a certain set of tasks has been defined using an !$OMP SECTIONS
directive. The use of the clause would look as follows:
!$OMP PARALLEL NUM_THREADS(4)
where four threads are requested for the specific parallel region.
When a NUM THREADS clause is used at the beginning of a parallel region, it overwrites
any settings imposed by the environmental variable OMP NUM THREADS or by a call to the
run-time library subroutine OMP set num threads 3 . The specified number of threads only
affects the present parallel region and not the future ones.
3.2.3 NOWAIT
Under certain circumstances it is not necessary that all threads finish a given parallel
executed task at the same time, because the following operations are not depending on
what has been done before. In such cases it is possible to avoid the implied synchronization
present in many OpenMP directives by adding the NOWAIT clause to their closing-directive.
In the following example
!$OMP PARALLEL
!$OMP DO
do i = 1, 1000
a = i
enddo
!$OMP END DO NOWAIT
!$OMP DO
do i = 1, 1000
b = i
enddo
!$OMP END DO
!$OMP END PARALLEL
3 The run-time routine OMP set num threads, explained later on, allows to specify the number of
threads to be used in the following parallel regions.