> On 12 Feb 2016, at 15:20, Paul T. Bauman <ptbau...@gmail.com> wrote: > > Hi Barry, > > On Thu, Feb 11, 2016 at 2:45 PM, Barry Smith <bsm...@mcs.anl.gov> wrote: > > > On Feb 11, 2016, at 1:36 PM, Boris Boutkov <boris...@buffalo.edu> wrote: > > > > Hello All, > > > > I'm currently working on hooking into the PETSc multi-grid infrastructure > > by creating my own DMShell and providing my own interpolation and injection > > operators. The issue I am currently facing is that while PCSetUp_MG is > > running, PETSc begins by attempting to create an interpolation (through > > DMCreateInterpolation) for the finest grid that I am passing it. > > > > What would be a good way to try and let the PCSetUp_MG procedure know the > > first mesh it is receiving is the finest one, so there should be no > > interpolation provided to it? > > Boris, > > I do not understand the question, perhaps it is a matter of language. > Say you have two levels total: call them coarse and fine. the PCSetUp_MG will > try to create a single interpolation "for/to the fine grid" FROM the coarse > grid. This is necessary to do the geometric multigrid. You would then through > your DMSHELL provide the function that creates the interpolation matrix. The > PCSetUp_MG will not attempt to create an interpolation "for the coarse mesh" > since there is no mesh coarser than the coarse mesh. So > PCSetUp_MG() should be trying to create only the needed interpolations, so > you should not need to tell them not to try to create certain ones. > > The information I think we're missing is, more or less, the order of > operations during the MG setup and how PETSc is expecting us to behave with > our DMShell implementation. I'll stick with your coarse and fine grid. On our > side, we can construct the coarse and the fine grids, supplying the DMs and > accompanying PetscSections, at simulation setup time since we know what our > grid hierarchy is (we're focusing on hierarchical meshes to start with). > Then, for DMRefine and DMCoarsen, we can just return the correct DM that > we've built. (Maybe this the wrong way to do it?) We also have > implementations of DMCreateInterpolation and DMCreateInjection for moving > between each of the levels. > > Our main confusion is, once we've set up of all that, which of the DMs we've > constructed do we go ahead and give to SNES? The fine or the coarse? Is it > expected that we should be checking the DM supplied to, e.g., > DMCreateInterpolation for which of the operators PETSc is wanting? We were > guessing that we should supply the fine DM to SNES (and then not expecting > DMCreateInterpolation to be called yet). > > Clearly, there's some setup steps we're missing or we're just going about > this all the wrong way. Is there an existing example of setting up a problem > using a DMShell? I poked a little but couldn't find one.
AIUI, PETSc needs to know how many refinements generated the fine DM that is attached to the operator inside the PC. That means, I think, you need to call DMRefine on the coarse DM (and so on all the way down until you get to the fine DM). Cheers, Lawrence
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