switch between indep and collective modes

[Robert Latham]

On Fri, Jun 22, 2007 at 12:39:58AM -0500, Wei-keng Liao wrote:
> According to the API document, "the separation of the two modes is 
> necessary to provide consistent views of file data when moving between 
> MPI-IO collective and independent operations". In what situation the 
> inconsistency will occur if we don't enforce this mode switching?

Hi Wei-keng

I think we wanted to avoid a situation where an MPI-IO implementation
optimizes collective communication in such a way that indepedent
accesses might get unexpected data.  

ROMIO will use two-phase I/O for a collective noncontiguous write.  If
that is followed by an independent read, the results could be
undefined.   Explicitly entering independent mode means we can flush
any buffers the MPI-IO implementation might be using for collective
I/O. 

> Is pnetcdf's file consistency policy the same as MPI-IO?

I don't know what the consistency semantics for netcdf would be in a
parallel environment.  If two processes write different data to the
same indexes of a variable, it's not clear who would 'win'.  I don't
know if applications regularly read and write variables, or if they
usually just all-write or all-read.  

==rob

-- 
Rob Latham
Mathematics and Computer Science Division    A215 0178 EA2D B059 8CDF
Argonne National Lab, IL USA                 B29D F333 664A 4280 315B