[petsc-users] using DM constructs

[Matthew Knepley]

On Sat, Jul 23, 2016 at 9:29 PM, Manav Bhatia <bhatiamanav at gmail.com> wrote:

> Thanks, Barry.
>
> This gives me a good perspective.
>
> Are there specific functions that need to be implemented/provided by a DM
> derived object? What would be a good resource to learn about this?
>

We talk a lot about this in the online tutorials.

   Matt


> Regards,
> Manav
>
>
> > On Jul 23, 2016, at 12:44 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> >
> >   Manav,
> >
> >     Each DM classes has two distinct interfaces:
> >
> >  One interface that is common to all DM which "speaks linear algebra
> (algebraic solvers)", for example DMCreateGlobalVector()
> >
> >  One interface that is specific to a particular DM (for example DMDA, or
> DMPlex or DMNetwork) it speaks in the language of the mesh/discretization
> model of the DM. So for example DMDA routines which are for structured
> grids speak in the language of structured grids and so you have things like
> DMDAGetCorners() which tells you the corners of the "box" of the structured
> grid you own. DMPlex speaks in a particular language of unstructured grids,
> DMNetwork speaks in the language of computations on networks (graphs) such
> as power grids where you have vertices and edges connecting vertices).
> DMForest speaks the languages of quad-tree and oct-tree grids.
> >
> >   The DM is PETSc's approach for communicating between
> mesh/discretization data and algebraic solvers. It is suppose to handle all
> the busywork of coordinating the interactions of the mesh/discretization
> data and algebraic solvers for the application developer so they don't need
> to do it themselves. For example with geometric multigrid the DMXXX object
> can fill up all the vectors and matrices that are needed for each level
> without requiring the user to loop over the levels and put the vectors and
> matrices themselves into the PCMG data structures.
> >
> >   IS are lower level basic data structures, often used by DMs. So one
> does not replace the use of IS with DM but one collects all the
> mesh/discretization interactions into a DMXXX and implements the DM
> operations (for example DMCreateGlobalVector()) using the data from DMXXX
> object.
> >
> >   In some sense libMesh is a DM for unstructured meshes with finite
> elements but it was written before we came up with the concept of DMs and
> so naturally doesn't use the DM interfaces. So one would not write libMesh
> using DMDA or DMPlex or something rather you would write DMlibMesh or write
> a new DMlibMesh2 by refactoring the libMesh interfaces to match the DM
> paradigm.
> >
> >   So if you are using libMesh and it satisfies your needs you should
> definitely not just switch to some DMXXX unless you have a good reason.
> Each DMXXX is for a particular class of problems/algorithms and you pick
> the DMXXX to use based on what you are doing. So use DMForest if you wish
> to use oct-trees, etc.
> >
> >   Barry
> >
> >
> >
> >> On Jul 23, 2016, at 11:50 AM, Manav Bhatia <bhatiamanav at gmail.com>
> wrote:
> >>
> >> Hi,
> >>
> >>  I am new to the DM constructs. I am curious if there is a compelling
> reason to move from handling IS sets to DM data structures.
> >>
> >>  My applications are built on top of libMesh. They used IS sets for a
> long time, and in recent years I have seen DM constructs in the library.
> However, I do not know why this is beneficial or necessary. The Petsc
> manual discusses DMDA for structured mesh, and I see reference to DMForest
> in the code (for unstructured mesh?) which is not discussed in the manual.
> >>
> >>  Is there a document that might provide the necessary background for DM
> and how best to derive from it, like in the libMesh source?
> >>
> >>  Any guidance would be appreciated.
> >>
> >> Regards,
> >> Manav
> >
>
>


-- 
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
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