# ksp tutorial ex22

Wee-Beng Tay zonexo at gmail.com
Fri Dec 11 00:44:04 CST 2009

```Hi,

I'm also looking at the fortran version of ex22. I have some questions:

1. M,N,P are global dimension in each direction of the array. I'm abit
confused. Do you mean the no. of grid pts at the coarse level? Hence, if my
problem has 27x27 grid pts, do I use M=N=9 with 3 multigrid levels or M=N=3
with 9 lvls? What is the guideline in choosing the number of multigrid
levels?

2. What is stencil width (s)? Seems to be 1 in both ex22 and ex29.

3. What are arrays containing the number of nodes in each cell along the
x/y/z coordinates? Is it ok to just use the default PETSC_NULL_INTEGER?

4. There's a subroutine ComputeJacobian which computes the jacobian.
However, what is it actually calculating? Is it simply calculating the
coefficient arising from the discretization of the matrix?

5. My objective is to solve for the poisson equation arising from the Navier
Stokes equations. ie div grad P = F, which F can be calculated separately.
The boundary conditions are Neumann which is dp/dx, dp/dy=0. Is this a good
example to follow? Or is ex29 a better example, although I need time to
digest the C syntax.

thanks alot!

On Fri, Dec 11, 2009 at 4:14 AM, Ryan Yan <vyan2000 at gmail.com> wrote:

> Hi Barry,
> Thank you very much for the confirm.
>
> Yan
>
>
> On Thu, Dec 10, 2009 at 3:12 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>> On Dec 10, 2009, at 2:09 PM, Ryan Yan wrote:
>>
>>  Hi all,
>>> I was looking at the ex22 of ksp.
>>>
>>> If I call:
>>> ...
>>> ierr = DMMGCreate(PETSC_COMM_WORLD,3,PETSC_NULL,&dmmg);CHKERRQ(ierr);
>>> ierr =
>>> DACreate3d(PETSC_COMM_WORLD,DA_NONPERIODIC,DA_STENCIL_STAR,-3,-3,-3,PETSC_DECIDE,PETSC_DECIDE,PETSC_DECIDE,1,1,0,0,0,&da);CHKERRQ(ierr);
>>>  ...
>>>
>>> How many grid pts should I have on the finest level? Is it 27,
>>> considering I am set 3 as the global dimension in each direction of the
>>> array?
>>>
>>> When I run ex22 with -ksp_view, I saw that  on the finest level there are
>>> 9 pts on each direction.
>>>
>>> Is that because I have 3 level?
>>>
>>
>>   Yes. The DA you create initially is for the coarsest grid, finer ones
>> are automatically created by the DMMG.
>>
>>   Barry
>>
>>
>>> Yan
>>>
>>>
>>
>
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