DAcreate2d process layout order

[Sean Dettrick]

Barry Smith wrote:

> On Thu, 18 May 2006, Sean Dettrick wrote:
>
>> Hi Barry,
>> the order is determined by MPI_Cart_create.
>
>
>    Do you mean that MPI_Cart_create() orders across the 2nd (y-axis)
> fastest and then the first (x-axis)? Hmmm, maybe we should change the
> DA? Changing it once and for all (not supporting both) is probably
> not a big deal and shouldn't break much (I hope).

Hi Barry,

it depends, what do you call x and what do you call y?
MPI_Cart_coords returns a vector, coords - I tend to say x is coords[0], 
y is coords[1] and z is coords[2]. 

For what it's worth, there's a short code appended to this email, which 
produces:

rank = 0 has Cartesian coords = { 0, 0 }
rank = 1 has Cartesian coords = { 0, 1 }
rank = 2 has Cartesian coords = { 1, 0 }
rank = 3 has Cartesian coords = { 1, 1 }
rank = 0 has DA range x=[0,50) and y=[0,50)
rank = 1 has DA range x=[50,100) and y=[0,50)
rank = 2 has DA range x=[0,50) and y=[50,100)
rank = 3 has DA range x=[50,100) and y=[50,100)

>>>    I don't completely understand what goes wrong. Is it because YOUR
>>> application orders the processors related to geometry in the 
>>> following way?
>>>
>>>     ^ y direction
>>>     |
>>>        2   5  8
>>>        1   4  7
>>>        0   3  6
>>>
>>>                     -> x direction
>>>
>>> Or is this something inherent in MPI_Cart_create?
>>

For my interpretation of x and y, MPI_Cart_create produces the above 
layout.  But if I said x=coords[1] and y=coords[0], then it would match 
the one below.

>>>
>>> PETSc does it so
>>>
>>>     ^ y direction
>>>     |
>>>        6   7  8
>>>        3   4  5
>>>        0   1  2
>>>
>>>                     -> x direction
>>>
>>>   
>>

Code and makefile attached ... hopefully within the message size limit.
Just make cartcommtest.

Sean



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