[petsc-users] Problems about GMRES restart and Scaling

[Matthew Knepley]

On Wed, Mar 20, 2019 at 6:53 AM Yingjie Wu via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Dear PETSc developers:
> Hi,
> Recently, I used PETSc to solve a non-linear PDEs for thermodynamic
> problems. In the process of solving, I found the following two phenomena,
> hoping to get some help and suggestions.
>
> 1. Because my problem involves a lot of physical parameters, it needs to
> call a series of functions, and can not analytically construct Jacobian
> matrix, so I use - snes_mf_operator to solve it, and give an approximate
> Jacobian matrix as a preconditioner. Because of the large dimension of the
> problem and the magnitude difference of the physical variables involved, it
> is found that the linear step residuals will increase at each restart
> (default 30th linear step) . This problem can be solved by setting a large
> number of restart steps. I would like to ask the reasons for this
> phenomenon? What knowledge or articles should I learn if I want to find out
> this problem?
>

Make sure you non-dimensionalize the problem first, so that any scale
differences are real and not the result of units.


> 2. In my problem model, there are many physical fields (variables are
> realized by finite difference method), and the magnitude of variables
> varies greatly. Is there any Scaling interface or function in Petsc?
>

That is what Jacobi does.

 Thanks,

    Matt


> Thanks,
> Yingjie
>


-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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