[petsc-users] Problems about GMRES restart and Scaling

[Yingjie Wu]

Thank you very much for your reply.
I think my statement may not be very clear. I want to know why the linear
residual increases at gmres restart.
I think I should have no problem with the residual evaluation function,
because after setting a large gmres restart, the results are also in line
with expectations.
Thanks,
Yingjie

Matthew Knepley <knepley at gmail.com> 于2019年3月20日周三 下午8:00写道：

> 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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