[petsc-users] multigrid preconditioning and adaptivity

[Mark Adams]

You can just set the coarse grid matrix/operator instead of using
Galerkin.  If you have a shell (matrix free) P then you will need to create
and set this yourself.  Our Galerkin requires a matrix P.

On Mon, Mar 7, 2016 at 9:32 AM, Lukasz Kaczmarczyk <
Lukasz.Kaczmarczyk at glasgow.ac.uk> wrote:

>
> > On 7 Mar 2016, at 14:21, Lawrence Mitchell <
> lawrence.mitchell at imperial.ac.uk> wrote:
> >
> > On 07/03/16 14:16, Lukasz Kaczmarczyk wrote:
> >>
> >>> On 7 Mar 2016, at 13:50, Matthew Knepley <knepley at gmail.com
> >>> <mailto:knepley at gmail.com>> wrote:
> >>>
> >>> On Mon, Mar 7, 2016 at 6:58 AM, Lukasz Kaczmarczyk
> >>> <Lukasz.Kaczmarczyk at glasgow.ac.uk
> >>> <mailto:Lukasz.Kaczmarczyk at glasgow.ac.uk>> wrote:
> >>>
> >>>    Hello,
> >>>
> >>>    I run multi-grid solver, with adaptivity, works well, however It
> >>>    is some space for improving efficiency. I using hierarchical
> >>>    approximation basis, for which
> >>>    construction of interpolation operators is simple, it is simple
> >>>    injection.
> >>>
> >>>    After each refinement level (increase of order of approximation
> >>>    on some element) I rebuild multigrid pre-conditioner with
> >>>    additional level. It is a way to add dynamically new levels
> >>>    without need of rebuilding whole MG pre-conditioner.
> >>>
> >>>
> >>> That does not currently exist, however it would not be hard to add,
> >>> since the MG structure jsut consists of
> >>> arrays of pointers.
> >>>
> >>>
> >>>    Looking at execution profile I noticed that 50%-60% of time is
> >>>    spent on MatPtAP function during PCSetUP stage.
> >>>
> >>>
> >>> Which means you are using a Galerkin projection to define the coarse
> >>> operator. Do you have a direct way of defining
> >>> this operator (rediscretization)?
> >>
> >> Matt,
> >>
> >>
> >> Thanks for swift response. You are right, I using Galerkin projection.
> >>
> >> Yes, I have a way to get directly coarse operator, it is some sub
> >> matrix of whole matrix. I taking advantage here form hierarchical
> >> approximation.
> >>
> >> I could reimplement PCSetUp_MG to set the MG structure directly, but
> >> this probably not good approach, since my implementation which will
> >> work with current petsc version could be incompatible which  future
> >> changes in native MG data structures. The alternative option is to
> >> hack MatPtAP itself, and until petsc MG will use this, whatever
> >> changes you will make in MG in the future my code will work.
> >
> > Why not provide a shell DM to the KSP that knows how to compute the
> > operators (and how to refine/coarsen and therefore
> > restrict/interpolate).  Then there's no need to use Galerkin coarse
> > grid operators, and the KSP will just call back to your code to create
> > the appropriate matrices.
>
> Hello Lawrence,
>
> Thanks, it is good advice.
> I have already my DM shell, however I have not looked yet how make it in
> the context of MG. Now is probably time to do that.
>
> DM shell
> http://userweb.eng.gla.ac.uk/lukasz.kaczmarczyk/MoFem/html/group__dm.html
>
>
> Regards,
> Lukasz
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20160307/d87cfbcf/attachment.html>