[mpich2-dev] MPI_Dims_create specification question

[William Gropp]

No, the point is that if the hardware topology is 2 x 4 x 1, you'd  
like Dims_create to give that back to you, rather than pretend it is  
2 x 2 x 2 .  Then Cart_create needs to map the ranks appropriately.   
In other words, there are two optimizations - the shape and the rank  
assignment.

Bill

On Jun 20, 2008, at 10:56 AM, Robert Latham wrote:

> On Fri, Jun 20, 2008 at 10:36:20AM -0500, Joe Ratterman wrote:
>> The system that we are building has a Cartesian network layout, so  
>> we want
>> to optimize the results of the MPI_Dims_create() function to most  
>> closely
>> reflect the physical layout of the nodes on the network.   
>> Recently, we had a
>> bug report from a customer who noted that when running on 8 nodes (an
>> example), MPI_Dims_create(3, [0, 0, 0]) doesn't always return  
>> [2,2,2].  They
>> consider this an error and quote the spec.
>>
>> Looking at the spec (latest 2.1 version :
>> http://www.hlrs.de/organization/par/services/models/mpi/mpi21/doc/ 
>> mpi-report-2.1draft-2008-06-13.pdf
>> username and password "mpi21") on page 247, the customer appears  
>> to be
>> correct.  There is no indication that we are allowed to attempt to  
>> make the
>> results reflect the HW.  Beyond the "must" requirements, there is  
>> this one
>> more: "The dimensions are set to be as close to each other as  
>> possible,
>> using an appropriate divisibility algorithm."  Why is there no  
>> mention of
>> the physical HW?
>
> I think you're misplacing your optimization?  Once you've got your
> dimensions, you pass them to MPI_Cart_create to optimize for
> underlying hardware, right?
>
> cart_create: http://www.mpi-forum.org/docs/mpi-11-html/ 
> node133.html#Node133
> dims_create: http://www.mpi-forum.org/docs/mpi-11-html/ 
> node134.html#Node134
>
> ==rob
>
> --
> Rob Latham
> Mathematics and Computer Science Division    A215 0178 EA2D B059 8CDF
> Argonne National Lab, IL USA                 B29D F333 664A 4280 315B
>

William Gropp
Paul and Cynthia Saylor Professor of Computer Science
University of Illinois Urbana-Champaign



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