== Description == '''MPI-IO''' is a family of [[MPI]] routines that makes it possible to do file read and write operations in parallel. MPI-IO is a part of the MPI-2 standard. The main advantage of MPI-IO is that it allows, in a simple and efficient fashion, to write and to read data that is partitioned on multiple processes, to and from a single file that is common to all processes. This is particularly useful when the manipulated data are vectors or matrices that are cut up in a structured manner between the different processes involved. This page gives a few guidelines on the use of MPI-IO and some references to more complete documentation. == Using MPI-IO == === Operations through offsets === The simplest way to perform parallel read and write operations is to use offsets. Each process can read from or write to the file with a defined offset. This can be done in two operations ([http://www.open-mpi.org/doc/current/man3/MPI_File_seek.3.php MPI_File_seek] followed by [http://www.open-mpi.org/doc/current/man3/MPI_File_read.3.php MPI_File_read] or by [http://www.open-mpi.org/doc/current/man3/MPI_File_write.3.php MPI_File_write]), or even in a single operation ([http://www.open-mpi.org/doc/current/man3/MPI_File_read_at.3.php MPI_File_read_at] or [http://www.open-mpi.org/doc/current/man3/MPI_File_write_at.3.php MPI_File_write_at]). Usually the offset is computed as a function of the process rank. {{ File |name=mpi_rw_at.c |contents= #include #define BLOCKSIZE 80 #define NBRBLOCKS 32 int main(int argc, char** argv) { MPI_File f; char* filename = "testmpi.txt"; char buffer[TAILLEBLOC]; int rank, size; int i; /* MPI Initialization */ MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &rank); MPI_Comm_size(MPI_COMM_WORLD, &size); /* Buffer initialization */ memset(buffer, 'a'+rank, BLOCKSIZE); buffer[BLOCKSIZE - 1] = '\n'; MPI_File_open(MPI_COMM_WORLD, filename, (MPI_MODE_WRONLY {{!}} MPI_MODE_CREATE), MPI_INFO_NULL, &f); /* Write data alternating between the processes: aabbccddaabbccdd... */ MPI_File_seek(f, rank*BLOCKSIZE, MPI_SEEK_SET); /* Go to position rank * BLOCKSIZE */ for (i=0; i #include #define BLOCKSIZE 80 #define NBRBLOCKS 32 int main(int argc, char** argv) { MPI_File f; MPI_Datatype type_intercomp; MPI_Datatype type_contiguous; char* filename = "testmpi.txt"; char buffer[BLOCKSIZE]; int rank, size; int i; MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &rank); MPI_Comm_size(MPI_COMM_WORLD, &size); /* Buffer initialization */ memset(buffer, 'a'+rank, BLOCKSIZE); buffer[BLOCKSIZE - 1] = '\n'; MPI_File_open(MPI_COMM_WORLD, filename, (MPI_MODE_WRONLY {{!}} MPI_MODE_CREATE), MPI_INFO_NULL, &f); /* Write data alternating between the processes: aabbccddaabbccdd... */ MPI_Type_contiguous(BLOCKSIZE, MPI_CHAR, &type_intercomp); MPI_Type_commit(&type_intercomp); for (i=0; i