Ben,
Thanks a bunch. I still haven=92t had a chance to look at the code or =
compile,
run it yet but I hope it will run without any glitches. I will modify my
existing code along the same lines and see where it leads me. Again,
appreciate your effort in adding this example !!
Derek,
I understand your point of view but even Petsc can work with just =
residuals
to obtain the Jacobian matrix (numerical Jacobian). But when the dof's =
are
in the order of millions in a 3D case and I wouldn=92t even bother =
finding the
numerical Jacobian and then eventually solving it. Doing it matrix-free =
with
good preconditioners to reduce linear iterations can provide great CPU
improvements.
I agree about not tying LibMesh too much to PETSc but since there is a
PetscNonlinearSolver class, I definitely want to use its full potential =
when
solving nonlinear problems. And hence exposure to the actual snes object
either through a property or function is necessary to have better =
control of
how the nonlinear system is being solved. And that was my original =
request !
There could always be a class for TrilinosNonlinearSolver or
...NonlinearSolver that could solve the system with Newton or FPI scheme
from the corresponding packages but nevertheless, user needs control of =
the
base objects. Transparency here could affect performance. Just my 2 =
cents.
Vijay
-----Original Message-----
From: Derek Gaston [mailto:friedmud@...
Sent: Wednesday, January 23, 2008 10:40 AM
To: Benjamin Kirk
Cc: Vijay M; libmesh-devel@...
Subject: Re: [Libmesh-devel] [Libmesh-users] Support for Matrix-free
algorithms
This is an interesting development...
One thing I would warn against is tying ourselves too close to Petsc.
I am currently waiting on a project to get green lighted where I will
be tying Trilinos into libMesh... where we will have access to other
matrix free solvers (like NOX, which can work with just residuals...
doing finite differenced jacobians). So coming up with a good
interface where you actually don't interact with Petsc primitives
would be ideal. In most other places in the code you guys have done
well at this exact thing... making it pretty easy to tie in a new set
of linear/non-linear/transient solvers... no reason to go backwards
now!
So far, what you have here looks great!
Just some food for thought...
Derek
On Jan 23, 2008 8:39 AM, Benjamin Kirk <benjamin.kirk@...> wrote:
> Here is a really, really raw example, the comments are not clear right
now,
> but I wanted to keep you informed. This requires the latest svn =
branch to
> work.
>
> Unpack it in the ./examples directory and run make.
>
> Run it as
>
> $ ./ex19-dbg -snes_view -r 4
> for a successive approximation which will converge linearly, and
>
> $ ./ex19-dbg -snes_view -r 4 -snes_mf_operator
> for a matrix-free approach in the iterative solver which will converge
> quadratically.
>
> > Anyway, I do have a question regarding using PETSc object with =
LibMesh.
I
> > have been trying to use Petsc objects Mats, Vecs and SNES solver =
with
> > Libmesh but the one thing I cant seem figure out is how to set the
> > nonlinear_solver public attribute of say a NonlinearImplicitSystem
object to
> > a PETSc SNES object which I have created and initialized separately.
Since,
> > the SNES object used in the wrapper PetscNonlinearSolver is private, =
I
don=B9t
> > understand how this can be done.
> >
> > Have I missed something and taken a completely wrong path on this ? =
I
would
> > very much appreciate any comments that you can provide to help me =
out
here.
>
> The user interface is totally up for discussion since I am the only =
one
who
> as exercised it to date. (I am sure Roy will have some comments!) It
seems
> to me the right approach will be to add a method which gives the user
access
> to the SNES object? From there the KSP, Mat, Vec, PC, etc... can be
> accessed. This would be similar to the approach used in the
> PetscLinearSolver.
>
> -Ben
>
>
> =
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