[petsc-users] preconditioning

[Barry Smith]

   Hmm, zeros on the diagonal that usually indicates either a bug in your code or a "Stokes-like" problem; for Stokes-like problems we recommend using PCFIELDSPLIT, this requires your understanding of the "block structure" of your matrix. 

  Barry


> On Apr 21, 2015, at 1:35 PM, Sanjay Kharche <Sanjay.Kharche at manchester.ac.uk> wrote:
> 
> 
> Hi
> 
> I just tried Matt's suggestion and tried to guess other preconditioning options as well. I didnt have the time to think about their appropriateness.
> 
> All preconditioners worked with the actual ts/ex15. gmag took 13 times longer to finish than the case when I ran without any preconditioner. The fastest was the jacobi, followed by sor and bjacobi options to -pc_type I could not guess the names of the other listed preconditioners associated with ts and snes.
> 
> In my code however, I  found that sor and gamg did not work. I got error messages, and the first few lines are pasted below. My test was on 1 proc and 4 procs, both giving similar errors with differnent line numbers. The petsc part of my code is virtually identical to the example. Any comments on what is missing or can be changed?
> 
> cheers
> Sanjay
> 
> 
> 
> 
> [1]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
> [1]PETSC ERROR: Arguments are incompatible
> [1]PETSC ERROR: Zero diagonal on row 39
> [1]PETSC ERROR: Petsc Release Version 3.5.2, unknown 
> [1]PETSC ERROR: ./sk2d on a linux-gnu-c-debug named atriumpc.ex.ac.uk by sanjay Tue Apr 21 19:28:53 2015
> [1]PETSC ERROR: Configure options --with-cc=gcc --with-cxx=g++ --with-fc=gfortran --download-fblaslapack --download-mpich --download-sundials --download-scalapack --with-c2html=0
> [1]PETSC ERROR: #1 MatInvertDiagonal_SeqAIJ() line 1724 in /media/6c6201d2-4e2e-4b24-8d04-263b5bc06dfa/sanjay/petsc/src/mat/impls/aij/seq/aij.c
> [1]PETSC ERROR: #2 MatSOR_SeqAIJ() line 1756 in /media/6c6201d2-4e2e-4b24-8d04-263b5bc06dfa/sanjay/petsc/src/mat/impls/aij/seq/aij.c
> [1]PETSC ERROR: #3 MatSOR_MPIAIJ() line 1483 in /media/6c6201d2-4e2e-4b24-8d04-263b5bc06dfa/sanjay/petsc/src/mat/impls/aij/mpi/mpiaij.c
> [1]PETSC ERROR: #4 MatSOR() line 3643 in /media/6c6201d2-4e2e-4b24-8d04-263b5bc06dfa/sanjay/petsc/src/mat/interface/matrix.c
> [1]PETSC ERROR: #5 PCApply_SOR() line 35 in /media/6c6201d2-4e2e-4b24-8d04-263b5bc06dfa/sanjay/petsc/src/ksp/pc/impls/sor/sor.c
> [1]PETSC ERROR: #6 PCApply() line 440 in /media/6c6201d2-4e2e-4b24-8d04-263b5bc06dfa/sanjay/petsc/src/ksp/pc/interface/precon.c
> 
> 
> 
> From: Matthew Knepley [knepley at gmail.com]
> Sent: 21 April 2015 16:51
> To: Sanjay Kharche
> Cc: petsc-users at mcs.anl.gov
> Subject: Re: [petsc-users] preconditioning
> 
> On Tue, Apr 21, 2015 at 10:49 AM, Sanjay Kharche <Sanjay.Kharche at manchester.ac.uk> wrote:
> 
> Hi
> 
> I am working through the KSP examples, but I will ask for any comments in the mean time.
> 
> I have a TS code for 2D/3D heat equation, starting from ts/ex15. How can I easily modify this code to include a parallel preconditioner. As of now, I just need to add a preconditioner which is easy to program. The coefficients in my systems matrix are constant, but RHS changes at each iteration.
> 
> Use -pc_type gamg. You have the Laplacian + Identity, so this should work fine.
> 
>   Thanks,
> 
>     Matt
>  
> cheers
> Sanjay
> 
> 
> 
> -- 
> 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