Question on using DMMG

[Barry Smith]

On Feb 19, 2009, at 3:48 PM, Randall Mackie wrote:

> Barry,
>
> I'm a little confused then, by how you set the size of the coarse grid
> in relation to the fine grid. What if my fine grid is 283x107x113? Can
> I just use a coarse grid of 3x3x3 and then the finer ones are  
> generated
> until it gets to my fine grid? I've been trying to follow ex22f.F,  
> where
> you set the coarse grid, and number of levels, but I can't figure out
> the relation between the grid size on each level and how you specify
                                                                                                ^ 
^^^^^^^^^^^^^^^
>
> the grid size at the finest level.

     We don't specify the grid size at the finest level; it just comes  
out from the coarse
grid size and the number of levels.
>
>
> In ex22f.F, when I run it, I see the first level is 27x27 (since the
> 3D grid is 3x3x3). Then at the next level it goes to 125x125 or 5x5x5.
> Then the final level is 729x729, or 9x9x9.

    By default, with nonperiodic domains the grid spacing (distance  
between grid points)
is cut in half with each refinement. So, for one dimension this looks  
like

       *                                  
*                              *
       +                +              +            +               +
       x       x       x      x      x     x      x      x       x

that's 3 points, then 5, then 9.

    To get the 113 with three levels then make it 29 on the coarsest,  
then it becomes 57 then it
becomes 113. For four levels start with 15.

     You can refine by other factors like 3 using  
DASetRefinementFactor() call this after
DACreate but before DMMGSetDA() You can also control this via the  
options database
see the manual page.

Barry


>
>
> Thanks, Randy
>
> Barry Smith wrote:
>> On Feb 19, 2009, at 3:22 PM, Randall Mackie wrote:
>>> I'm trying to modify my code to use the DMMG routines to solve my  
>>> system
>>> by multi-grid, instead of the usual krylov solver. In my previous  
>>> code,
>>> I would set up a DA for my finite difference grid, set the matrix
>>> preallocation using MATCreateMPIAIJ, then fill the matrix, etc.
>>>
>>> In the code I've modified to use DMMG, here is my sequence of  
>>> routines
>>> so far:
>>>
>>>    call DMMGCreate(...)
>>>    call DACreate3D(....) ! to set the DA on the finest level
>>>    call DMMGSetDM(...)
>>>
>>>
>>>    call DMMGSETKSP(...)
>>>
>>>
>>> Questions:
>>>
>>> 1) I want to set the DA on the finest level and use Galerkin to get
>>> the coarser matrices. So, is the DACreate above the way to do this?
>>   The current design always has you setting the coarsest DA; DMMG  
>> generates all the finer ones
>> automatically. This is independent of using Galerkin coarse grids  
>> or not.
>> To have DMMG use Galerkin to generate the coarser matrices call  
>> DMMGSetUseGalerkinCoarse()
>> after you se the number of levels or use the option -dmmg_galerkin
>>>
>>>
>>> 2) How do I set the matrix preallocation here for use in the DMMG  
>>> routines?
>>> Would I use the same MATCreateMPIAIJ as before, and if so, where  
>>> do I
>>> put that? In the routine that fills the Jacobian, or in the main  
>>> program
>>> someplace?
>>    DMMG manages all this for you; it creates the empty matrices  
>> (based on the DA stencil) and gives them
>> the right nonzero structure.
>>  See, for example, src/ksp/ksp/examples/tutorials/ex34.c
>>   Barry
>>>
>>>
>>>
>>> Thanks, Randy