Ubuntu Manpage: MPI_Win_allocate_shared - One-sided MPI call that allocates shared memory and returns a window object for

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

       INCLUDE 'mpif.h'
       MPI_WIN_ALLOCATE_SHARED(IZE, DISP_UNIT, INFO, COMM, BASEPTR, WIN, IERROR)
            INTEGER(KIND=MPI_ADDRESS_KIND) SIZE, BASEPTR
            INTEGER DISP_UNIT, INFO, COMM, WIN, 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.