adding SNESSetLinearSolve()

[Barry Smith]

    Lisandro,

       Go ahead add and add the features you'd like.

    Barry

On Nov 7, 2007, at 10:01 AM, Lisandro Dalcin wrote:

> On 11/1/07, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>> On Nov 1, 2007 9:24 AM, Lisandro Dalcin <dalcinl at gmail.com> > >>  
>>> For example, I would love to have SNESSetPresolve/SNESSetPostSolve
>>>> and
>>>> SNESSetPreStep/SNESSetPostStep, and perhaps a
>>>> SNESSetPreLinearSolve/SNESSetPostLinearSolve. Of course, this make
>>>> the
>>>> API grow with features that are not frecuently needed.
>>
>>   I'd like to understand why they are ever needed and if they can be
>> done
>> with current constructs?
>>
>
> Barry, sorry for the delay, but I was really busy. I'll try to  
> clarify my needs.
>
> * I'm using PETSc exclusively from Python. In such a dynamic language,
> implementing complicated application setups is far easier than in
> C/C++.
>
> * Of course, Python is no efficient for low-level stuff. Do I actually
> make use of a PETSc-based C++ application (PETSc-FEM) to assemble
> vectors and matrices, manage fixations/boundary conditions and linear
> constraints between dofs.
>
> * Some application setups will need PETSc solvers (SNES/TS) to provide
> a way to set user callback routines (or hooks). That's the reason I
> talked about adding SNESSetPresolve/SNESSetPostSolve and
> SNESSetPreStep/SNESSetPostStep. This way, PETSc can be adapted to the
> application needs if required, and not the other way.
>
> * Taking again SNES as an example: Why it provides SNESSetUpdate()???
> It is not used in any other place in PETSc sources. I guess at some
> point someone needed a user callback routine to be called just before
> computing the Jacobian, so it was added. I'm currently using it to
> compute/update auxiliar linear operators to be used in defining a
> problem-based preconditioner for NS equations. Of course, I could do
> this in computeJacobian() rutine, but then I would have to special
> case the use of this preconditioner in order to compute the auxiliar
> operator; in short my application has to adapt to PETSc.
>
> * Taking TS as an exaple. Why it provides SetPreStep/SetPostStep???
> They are also not even used in a TS example. I guess again that at
> some point someone needed it. And then, I'm asking to have more of
> that, and no less.
>
> In the end, perhaps the better way for me to go is to implement SNES
> and TS directly in Python and do what I want, it is not much hard and
> there should not be performace issues at this level. But I would
> really prefer to make SNES/TS more configurable (via setting general
> pourpose user callback routines) in order to not duplicate code and
> efforts.
>
> So the real quention you have to answer is the following: Should I
> continue fighting for making PETSc solver more 'user-configurable', or
> should I direct my efforts in implemening all those features directly
> in Python, where implementing truly polimorfic code is far, far easier
> than in C?
>
> In the near future, application written in C/C++/Fortran could take
> advantage of petsc4py to implement custom 'shell' matrices, linear
> solvers, preconditioners, nonlinear solvers and time steppers.
> Something like
> SNESCreate(comm,&snes)
> SNESSetType(snes, "python")
> SNESPythonCreate(snes, "package.module.MyNewtonSolver")
>
> Woud'nt that be a really nice feature?
>
> I'm not sure if all you consider Python (plus wrapped compiled code)
> as a feasible way to drive large-scale simulations, but I really
> believe it is. Furthermore, from the last three years I worked hard to
> make it possible: now Python users can use mpi4py, petsc4py, slepc4py
> and tao4py and manage applications with negligible overhead for medium
> to large data transfers/ problems.
>
>
> -- 
> Lisandro Dalcín
> ---------------
> Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
> Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
> Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
> PTLC - Güemes 3450, (3000) Santa Fe, Argentina
> Tel/Fax: +54-(0)342-451.1594
>