Chapter 2
OpenMP constructs
If only the previous parallel region constructor would exist, then the only possible thing to
do would be that all the threads perform exactly the same task, but this is not the aim of
parallelism. Therefore, OpenMP defines additional constructs which allow to distribute
a given task over the different threads and achieve in this way a real parallel working
program.
Four different groups of OpenMP directives or constructs exist. Each group has a
different aim and the selection of one directive or another inside the same group depends
on the nature of the problem to be solved. Therefore, it is good to understand the
principles of each of these directives in order to perform the correct choices.
2.1 Work-sharing constructs
The first group of OpenMP directives looks forward to divide a given work into pieces
and to give one or more of these pieces to each parallel running thread. In this way the
work, which would be done by a single thread in a serial program, is distributed over a
team of threads achieving a faster running program
1
.
All work-sharing constructs must be placed inside dynamic extends of parallel regions
in order to be effective. If this is not the case, the work-sharing construct will still
work, but a team with only one thread will be used. The reason is that a work-sharing
construct is not able of creating new threads; a task reserved to the
!$OMP PARALLEL/!$OMP
END PARALLEL
directive-pair.
The following restrictions need to be taken into account when using a work-sharing
construct:
• Work-sharing constructs must be encountered by all threads in a team or by none
at all.
1
Obviously, this is only true, if the team of threads is executed on more than one processor, which is
the case of SMP machines. Otherwise, when using a single processor, the computational overhead due to
the OpenMP directives as well as due to the need of executing several threads in a sequential way lead
to slower parallel programs than the corresponding serial versions!
9
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