The Great and Terrible implementation of MPI-2

function index


Perform an MPI fence synchronization on a MPI window
int MPI_Win_fence(
  int assert,
  MPI_Win win


[in] program assertion (integer)
[in] window object (handle)


The MPI call MPI_WIN_FENCE(assert, win) synchronizes RMA calls on win. The call is collective on the group of win. All RMA operations on win originating at a given process and started before the fence call will complete at that process before the fence call returns. They will be completed at their target before the fence call returns at the target. RMA operations on win started by a process after the fence call returns will access their target window only after MPI_WIN_FENCE has been called by the target process.

The call completes an RMA access epoch if it was preceded by another fence call and the local process issued RMA communication calls on win between these two calls. The call completes an RMA exposure epoch if it was preceded by another fence call and the local window was the target of RMA accesses between these two calls. The call starts an RMA access epoch if it is followed by another fence call and by RMA communication calls issued between these two fence calls. The call starts an exposure epoch if it is followed by another fence call and the local window is the target of RMA accesses between these two fence calls. Thus, the fence call is equivalent to calls to a subset of post, start, complete, wait.

A fence call usually entails a barrier synchronization: a process completes a call to MPI_WIN_FENCE only after all other processes in the group entered their matching call. However, a call to MPI_WIN_FENCE that is known not to end any epoch (in particular, a call with assert = MPI_MODE_NOPRECEDE) does not necessarily act as a barrier.

The assert argument is used to indicate special conditions for the fence that an implementation may use to optimize the MPI_Win_fence operation. The value zero is always correct. Other assertion values may be or'ed together. Assertions that are valid for MPI_Win_fence are:

the local window was not updated by local stores (or local get or receive calls) since last synchronization.
the local window will not be updated by put or accumulate calls after the fence call, until the ensuing (fence) synchronization.
the fence does not complete any sequence of locally issued RMA calls. If this assertion is given by any process in the window group, then it must be given by all processes in the group.
the fence does not start any sequence of locally issued RMA calls. If the assertion is given by any process in the window group, then it must be given by all processes in the group.

Advice to users.

Calls to MPI_WIN_FENCE should both precede and follow calls to put, get or accumulate that are synchronized with fence calls.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.
Other error; use MPI_Error_string to get more information about this error code.
Invalid MPI window object

Example Code

The following sample code illustrates MPI_Win_fence.

#include "mpi.h"
#include "stdio.h"

/* This does a transpose-cum-accumulate operation. Uses vector and hvector datatypes (Example 3.32 from MPI 1.1 Standard). Run on 2 processes */
#define NROWS 100
#define NCOLS 100

int main(int argc, char *argv[])
    int rank, nprocs, A[NROWS][NCOLS], i, j;
    MPI_Win win;
    MPI_Datatype column, xpose;
int errs = 0;

if (nprocs != 2)
        printf("Run this program with 2 processes\n");fflush(stdout);
if (rank == 0)
for (i=0; i<NROWS; i++)
for (j=0; j<NCOLS; j++)
                A[i][j] = i*NCOLS + j;
/* create datatype for one column */
MPI_Type_vector(NROWS, 1, NCOLS, MPI_INT, &column);
/* create datatype for matrix in column-major order */
MPI_Type_hvector(NCOLS, 1, sizeof(int), column, &xpose);

        MPI_Win_create(NULL, 0, 1, MPI_INFO_NULL, MPI_COMM_WORLD, &win);
        MPI_Win_fence(0, win);
        MPI_Accumulate(A, NROWS*NCOLS, MPI_INT, 1, 0, 1, xpose, MPI_SUM, win);

        MPI_Win_fence(0, win);
{ /* rank = 1 */
for (i=0; i<NROWS; i++)
for (j=0; j<NCOLS; j++)
                A[i][j] = i*NCOLS + j;
        MPI_Win_create(A, NROWS*NCOLS*
sizeof(int), sizeof(int), MPI_INFO_NULL,
        MPI_COMM_WORLD, &win);
        MPI_Win_fence(0, win);
        MPI_Win_fence(0, win);
for (j=0; j<NCOLS; j++)
for (i=0; i<NROWS; i++)
if (A[j][i] != i*NCOLS + j + j*NCOLS + i)
if (errs < 50)
                        printf("Error: A[%d][%d]=%d should be %d\n", j, i,
                            A[j][i], i*NCOLS + j + j*NCOLS + i);fflush(stdout);
if (errs >= 50)
            printf("Total number of errors: %d\n", errs);fflush(stdout);
return 0;