[petsc-users] Problems about Preconditioner in SNES

[Matthew Knepley]

On Wed, Dec 19, 2018 at 10:08 AM Yingjie Wu via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Respected Petsc developers:
> Hi,
> Recently, I developed a two-dimensional non-linear equations solver (SNES)
> using Petsc to solve temperature, velocity and pressure fields.  Since the
> coupling characteristics of the problem are complex, it is difficult to
> construct Jacobian matrix explicitly, so Matrix-Free method with
> Preconditioner is adopted.
>

Which preconditioner?


> I have the following questions about the Petsc's Preconditioner.
>
> 1. Is the preconditioning matrix updated in linear or non-linear steps?
> Because the elements in my preconditioning matrix are functions of
> variables (such as density is a function of temperature and pressure, and
> preconditioning matrix elements often contain density).
>

You are in control of this.


> 2.For some problems the preconditioning matrix element are constant, if it
> possible that the preconditioning matrix is constructed and decomposed only
> once at the beginning of the program and used for subsequent calculation?
>

Which preconditioner?


> 3. What is the difference between using Matrix-Free method and using
> normal Newton method for preconditioning matrix?
>

In MF, the PC cannot use matrix elements to form the preconditioner, only
the action of the operator.


> As far as I know, when using the Matrix-Free method, the type of
> preconditioning matrix needs to be defined as MATMFFD. Are there any other
> differences?
>

No, that is not the PC, only the action of the operator.

  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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