PARALLEL (TU*X only)

OpenMP Parallel Compiler Directive: Defines a parallel region.

Syntax

c$OMP PARALLEL [clause[[,] clause] ... ]
     block
c$OMP END PARALLEL

c
Is one of the following: C (or c), !, or * (see Syntax Rules for Parallel Directives).

clause
Is one of the following:

• COPYIN (list)
  
  
• DEFAULT ( PRIVATE | SHARED | NONE )
  
  
• FIRSTPRIVATE (list)
  
  
• IF (scalar_logical_expression)

  Specifies that the enclosed code section is to be executed in parallel only if the scalar_logical_expression evaluates to .TRUE.. Otherwise, the parallel region is serialized. If this clause is not used, the region is executed as if an IF(.TRUE.) clause were specified.

  This clause is evaluated by the master thread before any data scope attributes take effect.

  Only a single IF clause can appear in the directive.

• PRIVATE (list)
  
  
• REDUCTION( operator | intrinsic : list)
  
  
• SHARED (list)

block
Is a structured block (section) of statements or constructs. You cannot branch into or out of the block (the parallel region).

Rules and Behavior

The PARALLEL and END PARALLEL directive pair must appear in the same routine in the executable section of the code.

The END PARALLEL directive denotes the end of the parallel region. There is an implied barrier at this point. Only the master thread of the team continues execution at the end of a parallel region.

When a thread encounters a parallel region, it creates a team of threads and it becomes the master of the team. The master thread is a member of the team and it has a thread number of zero within the team. The number of threads in the team can be controlled by environment variables and library calls.

Once created, the number of threads in the team remains constant for the duration of that parallel region. However, you can explicitly change the number of threads used in the next parallel region by calling the run-time library routine omp_set_num_threads from a serial portion of the program. This routine overrides any value you may have set using the environment variable OMP_NUM_THREADS. For details on environment variables, see your user manual.

The code contained within the dynamic extent of the parallel region is executed on each thread, and the code path can be different for different threads.

If a thread executing a parallel region encounters another parallel region, it creates a new team and becomes the master of that new team. By default, nested parallel regions are always serialized and executed by a team of one thread.

See Also: Parallel Directives for Tru64 UNIX Systems, OpenMP Fortran API Compiler Directives (TU*X only), Compaq Fortran Parallel Compiler Directives (TU*X only), Parallel Library Routines on Tru64 UNIX Systems

Examples

You can use the PARALLEL directive in coarse-grain parallel programs. In the following example, each thread in the parallel region decides what part of the global array X upon which to work based on the thread number:

  c$OMP PARALLEL DEFAULT(PRIVATE) SHARED(X,NPOINTS)
        IAM = OMP_GET_THREAD_NUM( )
        NP = OMP_GET_NUM_THREADS( )
        IPOINTS = NPOINTS/NP
        CALL SUBDOMAIN(X,IAM,IPOINTS)
  c$OMP END PARALLEL

Assuming you previously used the environment variable OMP_NUM_THREADS to set the number of threads to six, you can change the number of threads between parallel regions as follows:

        CALL OMP_SET_NUM_THREADS(3)
  !$OMP PARALLEL
  ...
  !$OMP END PARALLEL
        CALL OMP_SET_NUM_THREADS(4)
  !$OMP PARALLEL DO
  ...
  !$OMP END PARALLEL DO