[petsc-users] Trying to set up a field-split preconditioner

[subramanya sadasiva]

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. 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 		 	   		  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130719/366f65d5/attachment.html>