[mpich-discuss] nonblocking I/O sequence

Martin Pokorny mpokorny at nrao.edu
Tue Feb 6 09:07:33 CST 2018

I originally posted the following to the mpi-forum mailing list, but
have not received a reply. Perhaps someone on this list can help me out.

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 new 
request before a 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 from the position of the file pointer after the completion 
of the previous request. In some testing that I've done, when one 
request does not wait for the previous request to complete, I get 
inconsistent results in the sense that the following read does or does 
not start from the file pointer position at the completion of the 
previous read (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 

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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