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Subject: Re: [OMPI users] Problem with NFS + PVFS2 + OpenMPI
From: Davi Vercillo C. Garcia (davivercillo_at_[hidden])
Date: 2008-05-30 12:37:09


Hi,

Sorry but I made a mistake... I'm not trying to use PVFS over NFS but
PVFS over EXT3. I still don't know this error message...

On Thu, May 29, 2008 at 5:33 PM, Robert Latham <robl_at_[hidden]> wrote:
> On Thu, May 29, 2008 at 04:48:49PM -0300, Davi Vercillo C. Garcia wrote:
>> > Oh, I see you want to use ordered i/o in your application. PVFS
>> > doesn't support that mode. However, since you know how much data each
>> > process wants to write, a combination of MPI_Scan (to compute each
>> > processes offset) and MPI_File_write_at_all (to carry out the
>> > collective i/o) will give you the same result with likely better
>> > performance (and has the nice side effect of working with pvfs).
>>
>> I don't understand very well this... what do I need to change in my code ?
>
> MPI_File_write_ordered has an interesting property (which you probably
> know since you use it, but i'll spell it out anyway): writes end up
> in the file in rank-order, but are not necessarily carried out in
> rank-order.
>
> Once each process knows the offsets and lengths of the writes the
> other process will do, that process can writes its data. Observe that
> rank 0 can write immediately. Rank 1 only needs to know how much data
> rank 0 will write. and so on.
>
> Rank N can compute its offset by knowing how much data the proceeding
> N-1 processes want to write. The most efficent way to collect this is
> to use MPI_Scan and collect a sum of data:
>
> http://www.mpi-forum.org/docs/mpi-11-html/node84.html#Node84
>
> Once you've computed these offsets, MPI_File_write_at_all has enough
> information to cary out a collective write of the data.
>
> ==rob
>
> --
> Rob Latham
> Mathematics and Computer Science Division A215 0178 EA2D B059 8CDF
> Argonne National Lab, IL USA B29D F333 664A 4280 315B
> _______________________________________________
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> http://www.open-mpi.org/mailman/listinfo.cgi/users
>

-- 
Davi Vercillo Carneiro Garcia
Universidade Federal do Rio de Janeiro
Departamento de Ciência da Computação
DCC-IM/UFRJ - http://www.dcc.ufrj.br
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