[petsc-users] preconditioning

[Sanjay Kharche]

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<mailto: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
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