MPI-IO

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 read and to write data that is partitioned on multiple processes, to one 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 (MPI_File_seek followed by MPI_File_read or by MPI_File_write), or even in a single operation (MPI_File_read_at or MPI_File_write_at). Usually the offset is computed as a function of the process rank.

File : mpi_rw_at.c

#include <mpi.h>

#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<NBRBLOCKS; ++i) {
        MPI_File_write(f, buffer, BLOCKSIZE, MPI_CHAR, MPI_STATUS_IGNORE);
        /* Advance (size-1)*BLOCKSIZE bytes */
        MPI_File_seek(f, (size-1)*BLOCKSIZE, MPI_SEEK_CUR);
    }

    MPI_File_close(&f);

    MPI_File_open(MPI_COMM_WORLD, filename, MPI_MODE_RDONLY, MPI_INFO_NULL, &f);

    /* Read data in a serial fashion for each process. Each process reads: aabbccdd */
    for (i=0; i<NBRBLOCKS; ++i) {
        MPI_File_read_at(f, rank*i*NBRBLOCKS*BLOCKSIZE, buffer, BLOCKSIZE, MPI_CHAR, MPI_STATUS_IGNORE);
    }

    MPI_File_close(&f);
    MPI_Finalize();

    return 0;
}

Using views

Using views, each process can see a section of the file, as if it were the entire file. In this way it is no longer necessary to compute the file offsets as a function of the process rank. Once the view is defined, it is then a lot simpler to perform operations on this file, without risking conflicts with operations performed by other processes. A view is defined using the function MPI_File_set_view. Here is a program identical to the previous example, but using views instead.

File : mpi_view.c

#include <stdio.h>
#include <mpi.h>

#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<NBRBLOCKS; ++i) {
        MPI_File_set_view(f, rank*BLOCKSIZE+i*size*BLOCKSIZE, MPI_CHAR, type_intercomp, "native", MPI_INFO_NULL);
        MPI_File_write(f, buffer, BLOCKSIZE, MPI_CHAR, MPI_STATUS_IGNORE);
    }

    MPI_File_close(&f);

    MPI_File_open(MPI_COMM_WORLD,
        filename,
        MPI_MODE_RDONLY,
        MPI_INFO_NULL,
        &f);

    /* Read data in a serial fashion for each process. Each process reads: aabbccdd */
    MPI_Type_contiguous(NBRBLOCKS*BLOCKSIZE, MPI_CHAR, &type_contiguous);
    MPI_Type_commit(&type_contiguous);
    MPI_File_set_view(f, rank*NBRBLOCKS*BLOCKSIZE, MPI_CHAR, type_contiguous, "native", MPI_INFO_NULL);
    for (i=0; i<NBRBLOCKS; ++i) {
        MPI_File_read(f,  buffer, BLOCKSIZE, MPI_CHAR, MPI_STATUS_IGNORE);
    }

    MPI_File_close(&f);
    MPI_Finalize();

    return 0;
}

Warning! Some file systems do not support file locks. Consequently some operations are not possible, in particular using views on disjoint file sections.

References

• OpenMPI documentation
• Course on parallel I/O (PDF)

Message Passing Interface