[petsc-users] Linear solver diverging

[Matthew Knepley]

On Wed, Apr 1, 2015 at 11:07 AM, bichinhoverde <
bichinhoverde at spwinternet.com.br> wrote:

> The system I am trying to solve is a 2D Poisson equation, but split into 5
> 1D equations. Something like this:
>
> p_xx + p_yy = f
> p_xx - d(p_x)/dx = 0
> p_yy - d(p_y)/dy = 0
> p_x - d(p)/dx = 0
> p_y - d(p)/dy = 0
>
> If I use standard finite differences, it works fine with gmres and jacobi.
> But when I switch to compact finite differences, only the direct solver
> works.
>
> It is on a staggered mesh, so p, p_xx and p_yy are on the cell center and
> p_x and p_y are on the edges.
>
> I recently submitted a paper and the reviewer complained about the direct
> linear solver. So I wanted to do a performance comparison.
>

I am not sure why the direct solver is a problem, other than unthinking
prejudice.

If you want to use an iterative solver here, you will likely need to use
PCFIELDSPLIT since you have saddle-point
structure from the constraints (I think).

  Thanks,

    Matt


> On Wed, Apr 1, 2015 at 12:58 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> On Wed, Apr 1, 2015 at 10:51 AM, bichinhoverde <
>> bichinhoverde at spwinternet.com.br> wrote:
>>
>>> I have a linear system which I can easily solve using the Pardiso
>>> library, which is a direct solver. But when I try Petsc, the solver does
>>> not converge. It reduces the norm a little but then gets stuck.
>>>
>>> I tried several combinations of pc_type and ksp_type. It seems that
>>> pc=jacobi is better, since my linear system does not have the diagonal
>>> element in some rows.
>>>
>>> Is there some advice you can give me? Some way to analyse the matrix and
>>> find out why it does not work?
>>>
>>
>> Without knowing anything about the system, there is nothing you can say.
>>
>> Why do you want to change from using a direct solver?
>>
>> Have you looked in the literature for iterative solvers that work on this
>> problem?
>>
>>   Thanks,
>>
>>     Matt
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to which their
>> experiments lead.
>> -- Norbert Wiener
>>
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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