Parallel Programming in Fortran 95 using OpenMP - People

30 2. OpenMP constructs
2.3.6 !$OMP ORDERED/!$OMP END ORDERED
In certain do-loops some of the statements executed at each iteration need to be evaluated
in the same order as if the do-loop would be executed sequentially. For example:
do i = 1, 100
A(i) = 2 * A(i-1)
enddo
In this do-loop it is necessary to have computed iteration 59 before being able of
computing correctly iteration 60, for instance. In such a situation a parallel treatment of
the entire do-loop is not possible! The following more general case
do i = 1, 100
block1
block2
block3
enddo
in which block2 needs to be evaluated in the correct order while the other two blocks
can be executed in any order, is going to be target. One possible solution to this problem
could be to split the do-loop into three do-loops and to parallelize only the first and the
last one. Another option is to use the present directive-pair to indicate to the compiler
the part of the code that needs to be executed sequentially. The result would be:
!$OMP DO ORDERED
do i = 1, 100
block1
!$OMP ORDERED
block2
!$OMP END ORDERED
block3
enddo
!$OMP END DO
where it is mandatory to add the ORDERED clause to the !$OMP DO opening-directive (see
page 52 for further information regarding the ORDERED clause). The sequence in which the
different blocks of code are executed, for the case of having a team with three threads, is
shown graphically in figure 2.7.
The !$OMP ORDERED/!$OMP END ORDERED directive-pair only makes sense inside the dynamic
extent of parallelized do-loops. On one hand the directive-pair allows only one
thread at a time inside its scope and on the other hand it allows the entrance of the
threads only following the order of the loop iterations: no thread can enter the ORDERED
section until it is guaranteed that all previous iterations have been completed.