[Mpi-forum] mpi info behavior

[Raffenetti, Ken]

We brought this to the forum because during our internal discussion is was shown that ROMIO will return any "ignored" key/value pairs set by the user in MPI_FILE_GET_INFO. Essentially, ROMIO duplicates the users info object, and only adjusts settings for keys it recognizes. Perhaps this is wrong behavior according to MPI-4.0?

This behavior was introduced into ROMIO somewhat recently in May 2020. There was a user request for it based on how OMPI-IO behaved. https://github.com/pmodels/mpich/pull/3954

Ken

On 10/13/21, 3:46 PM, "mpi-forum on behalf of Jim Dinan via mpi-forum" <mpi-forum-bounces at lists.mpi-forum.org on behalf of mpi-forum at lists.mpi-forum.org> wrote:

    Hi Rob,
    This seems reasonable to me. I think you can draw an analogy to an application setting "mpi_assert_no_any_tag = true" and checking the value returned by MPI_Comm_get_info to decide whether to use MPI_ANY_TAG on the communicator. I would suggest info values like "true" and "false" so that ROMIO can return back the same value passed by the user if the info key is accepted.

    As of MPI 4.0, MPI_File_get_info must return "all hints that are supported by the implementation and have default values specified; any user-supplied hints that were not ignored by the implementation; and any additional hints that were set by the implementation." So, true/false would also allow ROMIO to return a default value for the info key if the user hasn't set it.

    One thing you can't do with info today is fail the operation because of a key/value pair in the info argument. If there is an issue with a particular key/value pair, it is supposed to be ignored.

     ~Jim.


    On Mon, Oct 11, 2021 at 2:13 PM Latham, Robert J. via mpi-forum <mpi-forum at lists.mpi-forum.org> wrote:


    I'd like a way to tell users programmatically about features ROMIO supports.

    One way I imagined doing so was through the info object, though we're having some internal discussion about how kosher that is.  Any thoughts from the broader MPI community about an approach like this:


    ```
    MPI_Info_create(&info);
    MPI_Info_set(info, "romio_feature_xyz", "requested");
    MPI_File_open(...)
    MPI_File_get_info(fh, &info_used)
    MPI_File_get_info(info_used, "romio_feature_xyz", MPI_MAX_INFO_VAL-1,
        value, &flag);

    ```

    Possible points of contention:

    - we have examples of using hints for clients to pass information to implementations (striping_factor,  cb_buffer_size) and examples of implementations passing information to users (cb_nodes on Blue Gene), but we don't have too many examples of two-way hints.

    - will MPI_File_get_info return all info keys or just the ones the implementation understands?  The standard dictates what is required to be returned.  ROMIO (so de facto standard) will return the union of its internal keys and any keys (known or unknown) passed in by the user.

    - The use of "requested" as a hint value that the implementation can then either set to "enabled" or "disabled"  guards the situation where a user code is passing these infos to an older MPI IO implementation.  If the hint comes back exactly as given, then the implementation ignored this feature request and it is not supported. If the hint comes back "enabled" then the caller knows something about the implementation.

    Thanks
    ==rob


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