Intel® Fortran Compiler XE 13.1 User and Reference Guides

PARALLEL Directive (OpenMP*)

OpenMP* Fortran Compiler Directive: Defines a parallel region.

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

   block

c$OMP END PARALLEL

c

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

clause

Is one or more of the following:

  • COPYIN (list)
  • DEFAULT (PRIVATE | FIRSTPRIVATE | 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.

  • NUM_THREADS ( scalar_integer_expression)

    Specifies the number of threads to be used in a parallel region. The scalar_integer_expression must evaluate to a positive scalar integer value. Only a single NUM_THREADS 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).

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.

The number of threads in the team can be controlled by the NUM_THREADS clause, the environment variable OMP_NUM_THREADS, or by calling the run-time library routine OMP_SET_NUM_THREADS from a serial portion of the program.

NUM_THREADS supersedes the OMP_SET_NUM_THREADS routine, which supersedes the OMP_NUM_THREADS environment variable. Subsequent parallel regions, however, are not affected unless they have their own NUM_THREADS clauses.

Once specified, the number of threads in the team remains constant for the duration of that parallel region.

If the dynamic threads mechanism is enabled by an environment variable or a library routine, then the number of threads requested by the NUM_THREADS clause is the maximum number to use in the parallel region.

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.

Example

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

See Also


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