MPI_File_get_info fails to return hints as required by MPI standard

[wkliao]

When calling MPI_File_get_info() right after MPI_File_open() with
MPI_INFO_NULL, the returned info_used does not contain any the
hints used by OpenMPI, as the number of keys in info_used is 0.
(I am using 5.0.8 release).

According MPI 4.0 quoted below, info_used should contain all hints
implemented in OpenMPI and their default values.

MPI_File_get_info returns a new info object containing the hints of the file 
associated with fh. The current setting of all hints related to this file is
returned in info used. An MPI implementation is required to return all hints
that are supported by the implementation and have default values specified;
...

MPI_File_get_info() needs to be fixed.

[edgargabriel]

@wkliao do you have a testcase that is reproducing the issue correctly as well as a precise description of the environment where you ran the tests? Just from the high-level perspective, I think it is not impossible that on a standard posix file system we do not retain any info objects, so zero could be the correct value. For example striping_unit and striping_factor are correctly handled on a lustre file system (so info_used should return a value > 0 if you use these info objects on lustre), but we do not use that information on a standard posix file system (which includes xfs, ext4, beegfs).

[wkliao]

I ran this program below on a redhat server, one MPI process. The file system is a local disk, xfs.

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

int main(int argc, char **argv)
{
    int nkeys;
    MPI_File fh;
    MPI_Info info_used;

    MPI_Init(&argc,&argv);

    MPI_File_open(MPI_COMM_WORLD, "foo", MPI_MODE_CREATE | MPI_MODE_RDWR, MPI_INFO_NULL, &fh);

    MPI_File_get_info(fh, &info_used);

    MPI_Info_get_nkeys(info_used, &nkeys);
    printf("MPI File Info: nkeys = %d\n",nkeys);

    MPI_File_close(&fh);

    MPI_Info_free(&info_used);

    MPI_Finalize();
    return 0;
}

When using MPICH, it reports 23 keys. For OpenMPI, it is 0.

If I set hint cb_nodes in an info object and passed it to MPI_File_open(), such as,

    MPI_Info_set(info, "cb_nodes", "4");

Then MPI_File_get_info() will return an info object that has 1 key.
This means cb_nodes is a hint accepted by OpenMPI in my running environment.
In this case, OpenMPI should have a default value for this hint at least.

[wkliao]

FYI. I tested 5.0.7 on Perlmutter at NERSC (the default version installed
on the system) and used its Lustre there (by creating a file in a folder
configured with stripe count of 16 and stripe size of 1 MB).
I got the number of keys also equal to 0.