Skip to content

Latest commit

 

History

History
140 lines (104 loc) · 3.94 KB

MPI_Request_free.3.rst

File metadata and controls

140 lines (104 loc) · 3.94 KB
Raw

MPI_Request_free

:ref:`MPI_Request_free` - Frees a communication request object.

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Request_free(MPI_Request *request)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_REQUEST_FREE(REQUEST, IERROR)
     INTEGER REQUEST, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Request_free(request, ierror)
     TYPE(MPI_Request), INTENT(INOUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT/OUTPUT PARAMETER

  • request: Communication request (handle).

DESCRIPTION

This operation allows a request object to be deallocated without waiting for the associated communication to complete.

:ref:`MPI_Request_free` marks the request object for deallocation and sets request to MPI_REQUEST_NULL. Any ongoing communication that is associated with the request will be allowed to complete. The request will be deallocated only after its completion.

NOTES

Once a request is freed by a call to :ref:`MPI_Request_free`, it is not possible to check for the successful completion of the associated communication with calls to :ref:`MPI_Wait` or :ref:`MPI_Test`. Also, if an error occurs subsequently during the communication, an error code cannot be returned to the user -- such an error must be treated as fatal. Questions arise as to how one knows when the operations have completed when using :ref:`MPI_Request_free`. Depending on the program logic, there may be other ways in which the program knows that certain operations have completed and this makes usage of :ref:`MPI_Request_free` practical. For example, an active send request could be freed when the logic of the program is such that the receiver sends a reply to the message sent -- the arrival of the reply informs the sender that the send has completed and the send buffer can be reused. An active receive request should never be freed, as the receiver will have no way to verify that the receive has completed and the receive buffer can be reused.

Example:

CALL MPI_COMM_RANK(MPI_COMM_WORLD, rank)
IF(rank == 0) THEN
    DO i=1, n
      CALL MPI_ISEND(outval, 1, MPI_REAL, 1, 0, req, ierr)
      CALL MPI_REQUEST_FREE(req, ierr)
      CALL MPI_IRECV(inval, 1, MPI_REAL, 1, 0, req, ierr)
      CALL MPI_WAIT(req, status, ierr)
    END DO
ELSE IF (rank == 1) THEN
    CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
    CALL MPI_WAIT(req, status)
    DO I=1, n-1
       CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
       CALL MPI_REQUEST_FREE(req, ierr)
       CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
       CALL MPI_WAIT(req, status, ierr)
    END DO
    CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
    CALL MPI_WAIT(req, status)
END IF

This routine is normally used to free persistent requests created with either :ref:`MPI_Recv_init` or :ref:`MPI_Send_init` and friends. However, it can be used to free a request created with :ref:`MPI_Irecv` or :ref:`MPI_Isend` and friends; in that case the use can not use the test/wait routines on the request.

It is permitted to free an active request. However, once freed, you can not use the request in a wait or test routine (e.g., :ref:`MPI_Wait` ).

ERRORS

.. seealso::
   * :ref:`MPI_Isend`
   * :ref:`MPI_Irecv`
   * :ref:`MPI_Issend`
   * :ref:`MPI_Ibsend`
   * :ref:`MPI_Irsend`
   * :ref:`MPI_Recv_init`
   * :ref:`MPI_Send_init`
   * :ref:`MPI_Ssend_init`
   * :ref:`MPI_Rsend_init`
   * :ref:`MPI_Test`
   * :ref:`MPI_Wait`
   * :ref:`MPI_Waitall`
   * :ref:`MPI_Waitany`
   * :ref:`MPI_Waitsome`
   * :ref:`MPI_Testall`
   * :ref:`MPI_Testany`
   * :ref:`MPI_Testsome`