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.. _mpix_comm_agree:
MPIX_Comm_agree
===============
.. include_body
:ref:`MPIX_Comm_agree`, :ref:`MPIX_Comm_iagree` - Agree on a flag value
from all live processes and distributes the result back to all live
processes, even after process failures.
This is part of the User Level Fault Mitigation :ref:`ULFM extension <ulfm-label>`.
SYNTAX
------
C Syntax
^^^^^^^^
.. code-block:: c
#include <mpi.h>
#include <mpi-ext.h>
int MPIX_Comm_agree(MPI_Comm comm, int *flag)
int MPIX_Comm_iagree(MPI_Comm comm, int *flag, MPI_Request *request)
Fortran Syntax
^^^^^^^^^^^^^^
.. code-block:: fortran
USE MPI
USE MPI_EXT
! or the older form: INCLUDE 'mpif.h'
MPIX_COMM_AGREE(COMM, FLAG, IERROR)
INTEGER COMM, FLAG, IERROR
MPIX_COMM_IAGREE(COMM, FLAG, REQUEST, IERROR)
INTEGER COMM, FLAG, REQUEST, IERROR
Fortran 2008 Syntax
^^^^^^^^^^^^^^^^^^^
.. code-block:: fortran
USE mpi_f08
USE mpi_ext_f08
MPIX_Comm_agree(comm, flag, ierror)
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, INTENT(INOUT) :: flag
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPIX_COMM_IAGREE(COMM, FLAG, REQUEST, IERROR)
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, INTENT(INOUT), ASYNCHRONOUS :: flag
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
INPUT PARAMETERS
----------------
* ``comm``: Communicator (handle).
* ``flag``: Binary flags (integer).
OUTPUT PARAMETERS
-----------------
* ``flag``: Reduced binary flags (integer).
* ``request``: Request (handle, non-blocking only).
* ``ierror``: Fortran only: Error status (integer).
DESCRIPTION
-----------
This collective communication agrees on the integer value *flag* and
(implicitly) on the group of failed processes in *comm*.
On completion, all non-failed MPI processes have agreed to set the
output integer value of *flag* to the result of a *bitwise AND*
operation over the contributed input values of *flag*.
:ref:`MPIX_Comm_iagree` is the non-blocking variant of :ref:`MPIX_Comm_agree`.
PROCESS FAILURES
----------------
When an MPI process fails before contributing to the agree operation,
the *flag* is computed ignoring its contribution, and the operation
raises an error of class MPIX_ERR_PROC_FAILED.
When an error of class MPIX_ERR_PROC_FAILED is raised, it is consistently
raised at all MPI processes in the group(s) of *comm*.
After :ref:`MPIX_Comm_agree` raised an error of class MPIX_ERR_PROC_FAILED,
the group produced by a subsequent call to :ref:`MPIX_Comm_get_failed` on
*comm* contains every MPI process that didn't contribute to the
computation of *flag*.
WHEN THE COMMUNICATOR CONTAINS ACKNOWLEDGED FAILURES
----------------------------------------------------
If **all** MPI processes in the group of *comm* have acknowledged the failure
of an MPI process (using :ref:`MPIX_Comm_ack_failed`) prior to the call to
:ref:`MPIX_Comm_agree` (or :ref:`MPIX_Comm_iagree`), the MPIX_ERR_PROC_FAILED
error is not raised when the output value of *flag* ignores the
contribution of that failed process. Note that this is an uniform property:
if a non-contributing process is found to be not-acknowledged at any live
process in *comm*, all processes raise an error of class MPIX_ERR_PROC_FAILED.
**Example 1:** Using a combination of :ref:`MPIX_Comm_ack_failed` and
:ref:`MPIX_Comm_agree` users can propagate and synchronize the knowledge
of failures across all MPI processes in *comm*.
.. code-block:: c
Comm_get_failed_consistent(MPI_Comm c, MPI_Group * g) {
int rc; int T=1;
int size; int num_acked;
MPI_Group gf;
int ranges[3] = {0, 0, 1};
MPI_Comm_size(c, &size);
do {
/* this routine is not pure: calling MPI_Comm_ack_failed
* affects the state of the communicator c */
MPIX_Comm_ack_failed(c, size, &num_acked);
/* we simply ignore the T value in this example */
rc = MPIX_Comm_agree(c, &T);
} while( rc != MPI_SUCCESS );
/* after this loop, MPIX_Comm_agree has returned MPI_SUCCESS at
* all processes, so all processes have Acknowledged the same set of
* failures. Let's get that set of failures in the g group. */
if( 0 == num_acked ) {
*g = MPI_GROUP_EMPTY;
}
else {
MPIX_Comm_get_failed(c, &gf);
ranges[1] = num_acked - 1;
MPI_Group_range_incl(gf, 1, ranges, g);
MPI_Group_free(&gf);
}
}
WHEN THE COMMUNICATOR IS REVOKED
--------------------------------
This function never raises an error of class MPIX_ERR_REVOKED.
The defined semantics of :ref:`MPIX_Comm_agree` are maintained when *comm*
is revoked, or when the group of *comm* contains failed MPI processes.
In particular, :ref:`MPIX_Comm_agree` is a collective operation, even
when *comm* is revoked.
WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
------------------------------------------
When the communicator is an inter-communicator, the value of *flag* is
a *bitwise AND* operation over the values contributed by the remote
group.
When an error of class MPIX_ERR_PROC_FAILED is raised, it is consistently
raised at all MPI processes in the group(s) of *comm*, that is, both
the local and remote groups of the inter-communicator.
ERRORS
------
.. include:: ./ERRORS.rst
.. seealso::
* :ref:`MPIX_Comm_is_revoked`
* :ref:`MPIX_Comm_ack_failed`
|
