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MPI_Win_allocate_shared.3.rst

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MPI_Win_allocate_shared

:ref:`MPI_Win_allocate_shared` |mdash| One-sided MPI call that allocates shared memory and returns a window object for RMA operations.

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Win_allocate_shared (MPI_Aint size, int disp_unit, MPI_Info info,
                             MPI_Comm comm, void *baseptr, MPI_Win *win)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_WIN_ALLOCATE_SHARED(SIZE, DISP_UNIT, INFO, COMM, BASEPTR, WIN, IERROR)
     INTEGER(KIND=MPI_ADDRESS_KIND) SIZE, BASEPTR
     INTEGER DISP_UNIT, INFO, COMM, WIN, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Win_allocate_shared(size, disp_unit, info, comm, baseptr, win, ierror)
     USE, INTRINSIC :: ISO_C_BINDING, ONLY : C_PTR
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: size
     INTEGER, INTENT(IN) :: disp_unit
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(C_PTR), INTENT(OUT) :: baseptr
     TYPE(MPI_Win), INTENT(OUT) :: win
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

  • size: Size of window in bytes (nonnegative integer).
  • disp_unit: Local unit size for displacements, in bytes (positive integer).
  • info: Info argument (handle).
  • comm: Communicator (handle).

OUTPUT PARAMETERS

  • baseptr: Initial address of window.
  • win: Window object returned by the call (handle).
  • ierror: Fortran only: Error status (integer).

DESCRIPTION

:ref:`MPI_Win_allocate_shared` is a collective call executed by all processes in the group of comm. On each process, it allocates memory of at least size bytes that is shared among all processes in comm, and returns a pointer to the locally allocated segment in baseptr that can be used for load/store accesses on the calling process. The locally allocated memory can be the target of load/store accesses by remote processes; the base pointers for other processes can be queried using the function :ref:`MPI_Win_shared_query`. The call also returns a window object that can be used by all processes in comm to perform RMA operations. The size argument may be different at each process and size = 0 is valid. It is the user's responsibility to ensure that the communicator comm represents a group of processes that can create a shared memory segment that can be accessed by all processes in the group. The discussions of rationales for :ref:`MPI_Alloc_mem` and :ref:`MPI_Free_mem` in MPI-3.1 section 8.2 also apply to :ref:`MPI_Win_allocate_shared`; in particular, see the rationale in MPI-3.1 section 8.2 for an explanation of the type used for baseptr. The allocated memory is contiguous across process ranks unless the info key alloc_shared_noncontig is specified. Contiguous across process ranks means that the first address in the memory segment of process i is consecutive with the last address in the memory segment of process i - 1. This may enable the user to calculate remote address offsets with local information only.

The following info keys are supported:

alloc_shared_noncontig
If not set to true, the allocation strategy is to allocate contiguous memory across process ranks. This may limit the performance on some architectures because it does not allow the implementation to modify the data layout (e.g., padding to reduce access latency).
blocking_fence
If set to true, the osc/sm component will use :ref:`MPI_Barrier` for :ref:`MPI_Win_fence`. If set to false a condition variable and counter will be used instead. The default value is false. This info key is Open MPI specific.

For additional supported info keys see :ref:`MPI_Win_create`.

NOTES

Common choices for disp_unit are 1 (no scaling), and (in C syntax) sizeof(type), for a window that consists of an array of elements of type type. The later choice will allow one to use array indices in RMA calls, and have those scaled correctly to byte displacements, even in a heterogeneous environment.

Calling :ref:`MPI_Win_free` will deallocate the memory allocated by :ref:`MPI_Win_allocate_shared`. It is thus erroneous to manually free baseptr.

C NOTES

While baseptr is a void * type, this is to allow easy use of any pointer object for this parameter. This argument is really a void ** type.

ERRORS

.. seealso::
   * :ref:`MPI_Alloc_mem`
   * :ref:`MPI_Free_mem`
   * :ref:`MPI_Win_allocate`
   * :ref:`MPI_Win_create`
   * :ref:`MPI_Win_shared_query`
   * :ref:`MPI_Win_free`