[petsc-users] Trying to set up a field-split preconditioner
Dmitry Karpeyev
karpeev at mcs.anl.gov
Sat Jul 20 16:41:03 CDT 2013
What version of petsc are you using?
DMlibMesh needs to be updated to serve the splits via
DMCreateFieldDecomposition() correctly to petsc-3.4. Note that
DMCreateFieldDecomposition() is called by PCFieldSplit from PETSc, so you
wouldn't see those calls in the libMesh source.
Dmitry.
On Sat, Jul 20, 2013 at 6:35 AM, subramanya sadasiva <potaman at outlook.com>wrote:
> Hi Matt,
> Libmesh does create the DM. This is the relevant code.
>
> ierr = DMCreateLibMesh(libMesh::COMM_WORLD, this->system(),
> &dm);CHKERRABORT(libMesh::COMM_WORLD, ierr);
> ierr = DMSetFromOptions(dm); CHKERRABORT(libMesh::COMM_WORLD, ierr);
> ierr = DMSetUp(dm); CHKERRABORT(libMesh::COMM_WORLD, ierr);
> ierr = SNESSetDM(this->_snes, dm); CHKERRABORT(libMesh::COMM_WORLD, ierr);
>
> I am unable to tell from the code whether the DMCreateFieldDecomposition
> is run at all. I will see if running in the debugger helps me figure it out.
> Thanks,
> Subramanya
>
>
>
> Date: Sat, 20 Jul 2013 06:22:30 -0500
> Subject: Re: [petsc-users] Trying to set up a field-split preconditioner
> From: knepley at gmail.com
> To: potaman at outlook.com
> CC: petsc-users at mcs.anl.gov
>
> On Sat, Jul 20, 2013 at 6:19 AM, subramanya sadasiva <potaman at outlook.com>
> wrote:
> Hi Matt,
> The DM is created by the LibMesh code. The only thing I do directly with
> Petsc is set the solver prefixes which libmesh doesn't have an interface
> for at present . I have been able to set most options directly through
> command line options. this is the one case where that is not helping, and
> it might just be that I don't know how. Let me see if I am able to get this
> working.
>
> So does Libmesh create the PETSc solver? If so, it should be calling
> KSPSetDM() or SNESSetDM() or TSSetDM(). This
> is all I want to know. It seems like this is not the case since the
> PCFIELDSPLIT says it has no fields. Either that, or they have
> a bug in the DMCreateFieldDecomposition() implementation.
>
> Either way, it seems like the thing to do is run with -start_in_debugger,
> and break in PCFieldSplitSetDefaults() where that is called.
>
> Matt
>
>
> Thanks,
> Subramanya
>
> Date: Sat, 20 Jul 2013 06:07:47 -0500
> Subject: Re: [petsc-users] Trying to set up a field-split preconditioner
> From: knepley at gmail.com
> To: potaman at outlook.com
> CC: libmesh-users at lists.sourceforge.net; petsc-users at mcs.anl.gov
>
> On Fri, Jul 19, 2013 at 11:50 PM, subramanya sadasiva <potaman at outlook.com>
> wrote:
>
> Hi Matt,
> I see that there is an implementation of the interface
> to DMCreateFieldDecomposition.html .
> So does this sound right?
>
> Yes, that should automatically provide the field decomposition. Since this
> is not happening, something is
> wrong in the code. Is the DM set in your solver?
>
> 1. I get index sets ,and variable names from the dm create field
> decomposition
> 2. Once I have these , I create fieldsplits and name them using this..
> 3. And I guess I should be ready to go..
> One question that remains is that the fieldsplit is created on a full
> matrix. However, an algorithm such as VIRS operates only on a subset of
> this full DM. Will the fieldsplit and preconditioner created on the full DM
> carry over to the subsidiary DMs?
>
> VI is still new, and I have not tested in this case, but it is supposed to
> work.
>
> Thanks,
>
> Matt
>
> Thanks for all the help!
> Subramanya
>
>
> Date: Fri, 19 Jul 2013 23:09:10 -0500
> Subject: Re: [petsc-users] Trying to set up a field-split preconditioner
> From: knepley at gmail.com
> To: potaman at outlook.com
> CC: petsc-users at mcs.anl.gov
>
> On Fri, Jul 19, 2013 at 9:59 PM, subramanya sadasiva <potaman at outlook.com>
> wrote:
> Hi Matt,
> The DM being created is here (this is from Libmesh code
> (petscdmlibmesh.C )
>
> 01047 {
> 01048 PetscErrorCode ierr;
> 01049 PetscFunctionBegin;
> 01050 ierr = DMCreate(comm, dm); CHKERRQ(ierr);
> 01051 ierr = DMSetType(*dm, DMLIBMESH); CHKERRQ(ierr);
> 01052 ierr = DMLibMeshSetSystem(*dm, sys); CHKERRQ(ierr);
> 01053 PetscFunctionReturn(0);
> 01054 }
>
>
> This file has methods to access the variables assigned to the DM (this
> seems to be stored in a struct.)
> So , I guess one should be able to add a bit of code to create sections as
> you mentioned somewhere around here.
>
> Okay, they have their own DM. It must implement one of the interfaces for
> field specification. They could provide
>
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMCreateFieldDecomposition.html
>
> or at a lower level
>
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMCreateSubDM.html
>
> which in turn can be constructed by specifying a default PetscSection
>
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMSetDefaultSection.html
>
> Matt
>
> Thanks,
> Subramanya
>
>
>
>
>
> Date: Fri, 19 Jul 2013 21:33:11 -0500
> Subject: Re: [petsc-users] Trying to set up a field-split preconditioner
> From: knepley at gmail.com
> To: potaman at outlook.com
> CC: petsc-users at mcs.anl.gov
>
> On Fri, Jul 19, 2013 at 9:17 PM, subramanya sadasiva <potaman at outlook.com>
> wrote:
> Hi Matt,
> I am using Libmesh so the DM stuff is actually in the background, and
> unfortunately the matrix doesn't have a saddle point,
> I thought that
>
> -ch_solve_fieldsplit_block_size 2
> -ch_solve_fieldsplit_0_fields 0
> -ch_solve_fieldsplit_1_fields 1
>
> The block_size argument presumes you are using a DA. Are you?
>
> The other two options just say select the first DM field as field 0 in
> this PC, and the same with the second field. The
> DM must inform the PC about the initial field decomposition.
>
> would inform the solver of the structure. If this doesn't work owing to
> the fact that the problem is only being solved on a section of the mesh
> (because of the reduced space method), I guess I will have to use the
> PetscSection. Does that sound right?
>
> First, I think the right people to do this are the Libmesh people (we will
> of course help them). Second, you have not said
> whether you are using a structured or unstructured mesh. What DM class
> does the solver actually see?
>
> Thanks,
>
> Matt
>
> Thanks,
> Subramanya
>
>
> Subject: Re: [petsc-users] Trying to set up a field-split preconditioner
> From: knepley at gmail.com
> To: potaman at outlook.com
> CC: petsc-users at mcs.anl.gov; libmesh-users at lists.sourceforge.net
>
> On Fri, Jul 19, 2013 at 7:56 PM, subramanya sadasiva <potaman at outlook.com>
> wrote:
> Hi,
> I am trying to set up a fieldsplit preconditioner for my Cahn Hilliard
> solver and I get the following error,
>
> You have to tell the PCFIELDSPLIT about the dofs in each field. So
>
> 1) You are probably not using a DA, since it would tell it automatically
>
> 2) If you have a saddle point, you can use
> -pc_fieldsplit_detect_saddle_point
>
> 3) If none of those apply, you can set a PetscSection describing your
> layout to the DM for the solver.
> Since this is new, I suspect you will need help, so mail back.
>
> Thanks,
>
> Matt
>
>
> [0]PETSC ERROR: --------------------- Error Message
> ------------------------------------
> [0]PETSC ERROR: Petsc has generated inconsistent data!
> [0]PETSC ERROR: Unhandled case, must have at least two fields, not 0!
> [0]PETSC ERROR:
> ------------------------------------------------------------------------
>
> These are the options that I am using,
> -ch_solve is just a prefix.
>
>
>
> -ch_solve_pc_type fieldsplit
> -ch_solve_pc_fieldsplit_type schur
> -ch_solve_fieldsplit_block_size 2
> -ch_solve_fieldsplit_0_field 1
> -ch_solve_fieldsplit_1_field 0
> -ch_solve_fieldsplit_0_ksp_type cg
> -ch_solve_fieldsplit_0_pc_type hypre
> -ch_solve_fieldsplit_0_pc_type_hypre boomeramg
> -ch_solve_fieldsplit_1_ksp_type cg
> -ch_solve_fieldsplit_1_pc_type hypre
> -ch_solve_fieldsplit_1_pc_type_hypre boomeramg
>
> Any ideas?
>
> Thanks,
> Subramanya
>
>
>
>
> --
> 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
>
>
>
> --
> 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
>
>
>
> --
> 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
>
>
>
> --
> 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
>
>
>
> --
> 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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