The examples in this section illustrate the application of the MPI-/ consistency and semantics guarantees. These address
The simplest way to achieve consistency for conflicting accesses is to obtain sequential consistency by setting atomic mode. For the code below, process 1 will read either 0 or 10 integers. If the latter, every element of b will be 5. If nonatomic mode is set, the results of the read are undefined.
/* Process 0 */ int i, a[10] ; int TRUE = 1; for ( i=0;i<10;i++) a[i] = 5 ; MPI_File_open( MPI_COMM_WORLD, "workfile", MPI_MODE_RDWR | MPI_MODE_CREATE, MPI_INFO_NULL, &fh0 ) ; MPI_File_set_view( fh0, 0, MPI_INT, MPI_INT, "native", MPI_INFO_NULL ) ; MPI_File_set_atomicity( fh0, TRUE ) ; MPI_File_write_at(fh0, 0, a, 10, MPI_INT, &status) ; /* MPI_Barrier( MPI_COMM_WORLD ) ; */
/* Process 1 */ int b[10] ; int TRUE = 1; MPI_File_open( MPI_COMM_WORLD, "workfile", MPI_MODE_RDWR | MPI_MODE_CREATE, MPI_INFO_NULL, &fh1 ) ; MPI_File_set_view( fh1, 0, MPI_INT, MPI_INT, "native", MPI_INFO_NULL ) ; MPI_File_set_atomicity( fh1, TRUE ) ; /* MPI_Barrier( MPI_COMM_WORLD ) ; */ MPI_File_read_at(fh1, 0, b, 10, MPI_INT, &status) ;A user may guarantee that the write on process 0 precedes the read on process 1 by imposing temporal order with, for example, calls to MPI_BARRIER.
Alternatively, a user can impose consistency with nonatomic mode set:
/* Process 0 */ int i, a[10] ; for ( i=0;i<10;i++) a[i] = 5 ; MPI_File_open( MPI_COMM_WORLD, "workfile", MPI_MODE_RDWR | MPI_MODE_CREATE, MPI_INFO_NULL, &fh0 ) ; MPI_File_set_view( fh0, 0, MPI_INT, MPI_INT, "native", MPI_INFO_NULL ) ; MPI_File_write_at(fh0, 0, a, 10, MPI_INT, &status ) ; MPI_File_sync( fh0 ) ; MPI_Barrier( MPI_COMM_WORLD ) ; MPI_File_sync( fh0 ) ;
/* Process 1 */ int b[10] ; MPI_File_open( MPI_COMM_WORLD, "workfile", MPI_MODE_RDWR | MPI_MODE_CREATE, MPI_INFO_NULL, &fh1 ) ; MPI_File_set_view( fh1, 0, MPI_INT, MPI_INT, "native", MPI_INFO_NULL ) ; MPI_File_sync( fh1 ) ; MPI_Barrier( MPI_COMM_WORLD ) ; MPI_File_sync( fh1 ) ; MPI_File_read_at(fh1, 0, b, 10, MPI_INT, &status ) ;The ``sync-barrier-sync'' construct is required because:
The following program represents an erroneous attempt to achieve consistency by eliminating the apparently superfluous second ``sync'' call for each process.
/* ---------------- THIS EXAMPLE IS ERRONEOUS --------------- */ /* Process 0 */ int i, a[10] ; for ( i=0;i<10;i++) a[i] = 5 ; MPI_File_open( MPI_COMM_WORLD, "workfile", MPI_MODE_RDWR | MPI_MODE_CREATE, MPI_INFO_NULL, &fh0 ) ; MPI_File_set_view( fh0, 0, MPI_INT, MPI_INT, "native", MPI_INFO_NULL ) ; MPI_File_write_at(fh0, 0, a, 10, MPI_INT, &status ) ; MPI_File_sync( fh0 ) ; MPI_Barrier( MPI_COMM_WORLD ) ;
/* Process 1 */ int b[10] ; MPI_File_open( MPI_COMM_WORLD, "workfile", MPI_MODE_RDWR | MPI_MODE_CREATE, MPI_INFO_NULL, &fh1 ) ; MPI_File_set_view( fh1, 0, MPI_INT, MPI_INT, "native", MPI_INFO_NULL ) ; MPI_Barrier( MPI_COMM_WORLD ) ; MPI_File_sync( fh1 ) ; MPI_File_read_at(fh1, 0, b, 10, MPI_INT, &status ) ; /* ---------------- THIS EXAMPLE IS ERRONEOUS --------------- */The above program also violates the MPI-/ rule against out-of-order collective operations and will deadlock for implementations in which MPI_FILE_SYNC blocks.