[mpich2-dev] MPI_Dims_create specification question

[William Gropp]

We should bring this up in the MPI Forum - this interpretation is  
clearly at odds with the rationale for virtual topologies at the  
beginning of this chapter.   I believe that the intent was for this  
requirement to be "subject to the constraints of underlying physical  
topology".

Bill

On Jun 20, 2008, at 10:36 AM, 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?
>
> Thanks,
> Joe Ratterman
> jratt at us.ibm.com
>

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



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