Question on using DMMG

[Randall Mackie]

Thanks Barry, this was useful and I now understand how to set the levels and
the grid size at the coarsest level.

I'm trying to run a simple test problem, and it seems to be taking *forever*
in the routine setting the Jacobian. In my old code, where I use MATCreateMPIAIJ
to set up the matrix with the correct pre-allocation, it takes no time to fill
this matrix.

Here's a sketch of my ComputeJacobian routine:

	call DMMGGetDA
	Call DAGetGlobalIndices

	Loop through model:

	  v(ic)=fact
           col(ic)=ltog(idltog +.....)

	  etc

	  call MatSetValues(...)



I suspect this is a problem of the default stencil that is set up, with
many more entries than I'm using. Any way to just use MATMPIAIJSetPreallocation,
or do I need to use DASetBlockFills?


Thanks, Randy


Barry Smith wrote:
> 
> 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
>