[petsc-users] V-cycle multigrid with matrix shells

[Sylvain Barbot]

Hi Jed,

You're right, I should experiment more with assembled matrices before
choosing a solver/preconditioner. Could you be more specific about the
command line options that I could test. I have a version of the code
with assembled sparse matrices (default format). I tried running with
the option -pc_type mg and got the following error:

[0]PETSC ERROR:
------------------------------------------------------------------------
[0]PETSC ERROR: Caught signal number 11 SEGV: Segmentation Violation,
probably memory access out of range
[0]PETSC ERROR: Try option -start_in_debugger or -on_error_attach_debugger
[0]PETSC ERROR: or see
http://www.mcs.anl.gov/petsc/petsc-as/documentation/troubleshooting.html#Signal[0]PETSC
ERROR: or try http://valgrind.org on GNU/linux and Apple Mac OS X to
find memory corruption errors
[0]PETSC ERROR: likely location of problem given in stack below
[0]PETSC ERROR: ---------------------  Stack Frames
------------------------------------
[0]PETSC ERROR: Note: The EXACT line numbers in the stack are not available,
[0]PETSC ERROR:       INSTEAD the line number of the start of the function
[0]PETSC ERROR:       is given.
[0]PETSC ERROR: [0] PCSetFromOptions_MG line 318 src/ksp/pc/impls/mg/mg.c
[0]PETSC ERROR: [0] PCSetFromOptions line 166 src/ksp/pc/interface/pcset.c
[0]PETSC ERROR: [0] KSPSetFromOptions line 320 src/ksp/ksp/interface/itcl.c
[0]PETSC ERROR: --------------------- Error Message
------------------------------------
[0]PETSC ERROR: Signal received!
[0]PETSC ERROR:
------------------------------------------------------------------------
[0]PETSC ERROR: Petsc Release Version 3.1.0, Patch 4, Fri Jul 30
14:42:02 CDT 2010
[0]PETSC ERROR: See docs/changes/index.html for recent updates.
[0]PETSC ERROR: See docs/faq.html for hints about trouble shooting.
[0]PETSC ERROR: See docs/index.html for manual pages.
[0]PETSC ERROR:
------------------------------------------------------------------------
[0]PETSC ERROR: ./planestrain on a darwin10. named
catalina.gps.caltech.edu by sbarbot Tue May 17 11:24:14 2011
[0]PETSC ERROR: Libraries linked from
/Users/sbarbot/Documents/work/petsc/darwin10.4.0-c-debug/lib
[0]PETSC ERROR: Configure run at Mon Aug  2 17:15:50 2010
[0]PETSC ERROR: Configure options
--with-blas-lapack-dir=/opt/intel/Compiler/11.1/084/Frameworks/mkl/lib/em64t
--with-mpi-dir=/shared/bin/
[0]PETSC ERROR:
------------------------------------------------------------------------
[0]PETSC ERROR: User provided function() line 0 in unknown directory
unknown file
application called MPI_Abort(MPI_COMM_WORLD, 59) - process 0

in the code, the matrix assembly for c%A is followed by:

    CALL KSPCreate(PETSC_COMM_WORLD,c%ksp,ierr)
    CALL KSPSetOperators(c%ksp,c%A,c%A,DIFFERENT_NONZERO_PATTERN,ierr)
    CALL KSPSetFromOptions(c%ksp,ierr)
    CALL KSPSetUp(c%ksp,ierr)


Thanks for your suggestions,
Sylvain

2011/5/12 Jed Brown <jed at 59a2.org>:
> On Thu, May 12, 2011 at 08:12, Sylvain Barbot <sylbar.vainbot at gmail.com>
> wrote:
>>
>> My primary concern now is computation speed, especially for
>> 3-D problems, where i have of the order 512^3 degrees of freedom. I
>> was planning to test Jacobi and SOR smoothers. Is there another
>> smoother you recommend for this kind of problem?
>
> It's worth trying ILU as a smoother, especially if you are using a stretched
> mesh. What discretization are you using? I still recommend just writing the
> matrix assembly and experimenting (you can do everything on the command
> line). Then, if the problem of interest responds well to SOR, you can easily
> put in an option to use matrix-free. That is, you would have one code with a
> runtime option of which levels to assemble versus handle matrix-free. The
> fastest choice will almost certainly depend on the problem and on the
> hardware that you are running it on, so it's well worth having both options
> available.