NAME

MPI_Win_allocate_shared - 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

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 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 MPI_Alloc_mem and MPI_Free_mem in MPI-3.1 § 8.2 also apply to MPI_Win_allocate_shared; in particular, see the rationale in MPI-3.1 § 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 MPI_Barrier for 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 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.

ERRORS

Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.