[petsc-users] PCMG with matrix-free operators accessing DMDA

[Valeria Mele]

Thank you very much Barry.
Apparently I missed many things about "DMShell..." that I didn't find in
the current users manual, and I was trying to create the operators through
matCreateShell() and MatShellSetOperation().
If I use "DMShellSetCreate..." to define the matrices I shouldn't have any
doubt about the da to refer to.

Now I can go on.
Thank you again.

Best,
Valeria


---------------------------------------------------------------------------------------------
PhD Valeria Mele

University of Naples Federico II
Department of Mathematics and Applications "R. Caccioppoli"
Complesso Universitario M.S. Angelo, Via Cinthia
80126 Naples
---------------------------------------------------------------------------------------------

2016-07-29 12:19 GMT-05:00 Barry Smith <bsmith at mcs.anl.gov>:

>
> > On Jul 28, 2016, at 12:48 PM, Valeria Mele <valeria.mele at unina.it>
> wrote:
> >
> > Hi everyone,
> > this time I am using PETSc to do something that is more complicated than
> my usual and I want to do it at the highest possible abstraction level.
> >
> > To put it in a nutshell, my intent is to build a parallel multigrid to
> solve a linear system via DM, KSP and PCMG (I would like to use DMMG but
> probably I should have the same problems or more).
>
>    DMMG doesn't exist anymore. It was refactored away many years ago, its
> functionality is handled by PCMG and DM.
>
>
> >
> > I created the distributed object, da, with DMDACreate3d, even if it is
> distributed (as yet) only in the x-dimension and has 3 dof.
> > Then I create the KSP (type KSPRICHARDSON) and set the nonzero initial
> guess and PCMG as preconditioner. Here I start to tune the MG.
> >
> > The point is that I need to define all the operators as matrix-free,
> since they will do several operations on x to obtain y, and I am not
> familiar with the way to access all the elements or informations in the two
> levels involved and/or among the processors with a so-high level interface.
> >
> > So please (please please please please) tell me if I correctly
> understand the mechanism or I am on the wrong way and clear my doubts.
> >
> > That is, let's say that my operation for the shell are:
> >       • A_mult(Mat mat,Vec x, Vec y) //coefficients matrix
> > in this case the level is only one but should I write it taking into
> account only the local data (I think so) and accessing them via the
> informations in da?
>
>    Yes. You can use VecGetDM(x,&da) to get the DMDA object
>
> >
> > For example, if I use DMDAVecGetArray, DMDAVecGetCorners (or
> DMDAGetGhostCorners) and DMDAVecRestoreArray, will they retrieve
> informations from the right level each time (I am pretty sure that in some
> official examples it is done in this way)?
> >
> > Or should I handle just Vecs as local structures with their usual
> indices (through VecGetArray and VecRestoreArray)?
>
>    No, no, no because then you would need to mange all the structured grid
> information yourself, since the DMDA manages it for you you should use it.
>
> >       • P_mult(Mat mat,Vec x, Vec y) //interpolation matrix that is NOT
> conceptually the traspose of Restriction
> > in this case x and y will be from two different levels (respectively L
> and L+1), so, if I retrieve informations from the da... how can I access
> the two at different levels?
>
>    Use VecGetDM(x,  and VecGetDM(y to get access to both DMDA.
>
> >
> > I am sorry if it seems that they are trivial questions, and I will be
> grateful to anyone will help me.
>
>   Additional information. Since the PCMG will be requesting the matrices
> and the interpolation/restriction operations (rather than you setting them
> into each level of multigrid) you will need to use DMShellSetCreateMatrix()
> and DMShellSetCreateInterpolation() and DMShellSetCreateRestriction() to
> provide the routines that will create the Shell matrices you need to
> represent the operators on the levels and the restriction and interpolation
> (Even though you are using a DMDA you can still call these routines).
>
>    Barry
>
> >
> > Thanks a lot,
> > Valeria
> >
> >
> >
> >
> >
> >
> >
> ---------------------------------------------------------------------------------------------
> > PhD Valeria Mele
> >
> > University of Naples Federico II
> > Department of Mathematics and Applications "R. Caccioppoli"
> > Complesso Universitario M.S. Angelo, Via Cinthia
> > 80126 Naples
> >
> ---------------------------------------------------------------------------------------------
>
>
>
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