[petsc-users] Linear solver diverging

[bichinhoverde]

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.



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
>
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