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Subject: Re: [OMPI users] Parallel file write in fortran (+mpi)
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-02-02 18:31:49


+1 on Nick's responses.

AFAIK, if you don't mind getting garbage in the output file, it should be fine to do. Specifically: it should not cause OS issues (crash, reboot, corrupted filesystem, etc.) to do this -- but the file contents will likely be garbage.

That being said, this situation likely falls into the "Doc, it hurts when I do this..." category. Meaning: you know it's wrong, so you probably shouldn't be doing it anyway. :-)

On Feb 2, 2010, at 4:50 PM, Nicolas Bock wrote:

> Hi Laurence,
>
> I don't know whether it's as bad as a deadly sin, but for us parallel writes are a huge problem and we get complete garbage in the file. Take a look at:
>
> Implementing MPI-IO Atomic Mode and Shared File Pointers Using MPI One-Sided Communication, Robert Latham,Robert Ross, Rajeev Thakur, International Journal of High Performance Computing Applications, 21, 132 (2007).
>
> They describe an implemenation of a "mutex" like object in MPI. If you protect writes to the file with an exclusive lock you can serialize the writes and make use of NFS's close to open cache coherence.
>
> nick
>
>
> On Tue, Feb 2, 2010 at 08:27, Laurence Marks <L-marks_at_[hidden]> wrote:
> I have a question concerning having many processors in a mpi job all
> write to the same file -- not using mpi calls but with standard
> fortran I/O. I know that this can lead to consistency issues, but it
> can also lead to OS issues with some flavors of nfs.
>
> At least in fortran, there is nothing "wrong" with doing this. My
> question is whether this is "One of the Seven Deadly Sins" of mpi
> programming, or just frowned on. (That is, it should be OK even if it
> leads to nonsense files, and not lead to OS issues.) If it is a sin, I
> would appreciate a link to where this is spelt out in some "official"
> document or similar.
>
> --
> Laurence Marks
> Department of Materials Science and Engineering
> MSE Rm 2036 Cook Hall
> 2220 N Campus Drive
> Northwestern University
> Evanston, IL 60208, USA
> Tel: (847) 491-3996 Fax: (847) 491-7820
> email: L-marks at northwestern dot edu
> Web: www.numis.northwestern.edu
> Chair, Commission on Electron Crystallography of IUCR
> www.numis.northwestern.edu/
> Electron crystallography is the branch of science that uses electron
> scattering and imaging to study the structure of matter.
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-- 
Jeff Squyres
jsquyres_at_[hidden]