I'd like to ask for some clarification on the expected outcome of a sequence of calls to non-blocking MPI-IO functions on a single file handle. To be concrete, assume a sequence of calls to MPI_File_iread_all(). My question concerns the result of making a call before the previous request has completed. Sec 13.6.5 of the standard says "The call initiates the operation which may progress independently of any communication, computation, or I/O.", which could be interpreted to mean there's no guarantee that the second call will start reading at the position of the file pointer after the completion of the previous request. In some testing that I've done, I get inconsistent results (seeming to depend on the number of ranks, but there could be more to that effect); I'm uncertain whether the inconsistency that I'm seeing is to be expected, or might point to a bug in the MPI library implementation.

Martin Pokorny
Software Engineer
Jansky Very Large Array correlator back-end and CASA software
National Radio Astronomy Observatory - New Mexico Operations
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