[Mpi-forum] non-blocking I/O sequence

Martin Pokorny mpokorny at nrao.edu
Fri Feb 2 11:17:11 CST 2018

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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