Thanks Barry for your quick reply.
Le 16/05/2022 à 21:41, Barry Smith a écrit :
Loic,
From your code it looks like you are using a DM. Is it a DMDA or a
shell DM? If it is a DMDA then the process is intended to be pretty
straightforward. PCTELESCOPE should create a new DMDA that has the
same properties as the coarse grid DMDA but lives on a smaller number
of ranks. From this you can just provide multiple levels of coarsening
of the DMDA to produce the smaller multigrid problems.
It's a DMDA. So what you mean is to take the KSP of PCTELESCOPE, get the
DM which is the same of the coarse grid and build all the levels as
previously for the levels of PCTELESCOPE ?
Can you let us know more details of what exactly goes wrong with
attempts? It is likely straightforward to get things working, perhaps
due to our unclear documentation.
I essentially have wrong state or NULL matrices into the PCTELESCOPE.
Loic
Barry
On May 16, 2022, at 3:28 PM, Loic Gouarin
<[email protected]> wrote:
Hello,
I could have posted my message on the user list but it seems to me
that it's more in the petsc development part. Don't hesitate to tell
me if I'm wrong.
I am developing a code called cafes that studies fluid-particle
interactions in a Stokes fluid. For that, we implemented the whole
solver in matrix free on a cartesian grid. The preconditioner is
naturally a geometrical multigrid for the velocity. The set of matrix
free for each level is set by hand (I worked with Matthew on this a
long time ago now!).
I would have liked to implement telescope on this part. I started it
but I didn't get out of it. So I would like to have your help.
What I tried to do was to describe my classical multigrid up to a
level N1 and then take the coarse solver and apply the telescope
preconditioner to it up to a level N2. I don't know if this is the
best way but it seemed the most intuitive. The problem is that I
could not get the coarse matrix because the setup is not done yet (I
use a DMKSPSetComputeOperators for the matrices).
The construction of the matrix free for each level is here:
https://github.com/gouarin/cafes/blob/master/cafes/problem/stokes.hpp#L59-L88
The description of the Stokes preconditioner is described here :
https://github.com/gouarin/cafes/blob/master/cafes/problem/stokes.hpp#L109-L162
I have tried multiple implementations without success. For me, the
start is to add at line 105 something like
KSP coarse;
ierr = PCMGGetCoarseSolve(pc_i, &coarse);CHKERRQ(ierr);
PC pc_coarse;
ierr = KSPGetPC(coarse, &pc_coarse);CHKERRQ(ierr);
ierr = PCSetType(pc_coarse, PCTELESCOPE);CHKERRQ(ierr);
Then, to create the several matrices of the levels of TELESCOPE, I
thought using the same process as before with
DMKSPSetComputeOperators. But it doesn't work...
Could you tell me how I can make it work ?
Thanks,
Loic
