ksp follow-up Qs

[Matthew Knepley]

2009/9/11 tsjb00 <tsjb00 at hotmail.com>

>  Many thanks for your replies! I have three follow-up questions. I use the
> AIJ format for the matrix, BCGS for KSP and ILU for PC. The matrix changes
> with time.
>
> Now I put PC and KSP definition in initialization. In each timestep, I
> redefine the matrix and rhs vector, use the KSPSetOperators and KSPSolve.
> When I run the program, I get the following error info, which seems to occur
> right before KSPSetOperators
>  [0]PETSC ERROR: --------------------- Error Message
> ------------------------------------
> [0]PETSC ERROR: Arguments are incompatible!
> [0]PETSC ERROR: Incompatible vector global lengths!
> [0]PETSC ERROR:
> ------------------------------------------------------------------------
> [0]PETSC ERROR: Petsc Release Version 3.0.0, Patch 5, Mon Apr 13 09:15:37
> CDT 2009
> [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: ../src_o_petsc/min3pp on a linux-gnu named pardiff by
> jinbei Fri Sep 11 08:49:22 2009
> [0]PETSC ERROR: Libraries linked from
> /home/jinbei/Soft/petsc-3.0.0-p5/linux-gnu-c-debug/lib
> [0]PETSC ERROR: Configure run at Thu Aug 20 11:36:14 2009
> [0]PETSC ERROR: Configure options --with-cc=gcc --with-fc=ifort
> --download-f-blas-lapack=1 --download-mpich=1 --with-shared=0
> [0]PETSC ERROR:
> ------------------------------------------------------------------------
> [0]PETSC ERROR: VecCopy() line 1685 in src/vec/vec/interface/vector.c
> [0]PETSC ERROR: KSPInitialResidual() line 60 in
> src/ksp/ksp/interface/itres.c
> [0]PETSC ERROR: KSPSolve_BCGS() line 44 in src/ksp/ksp/impls/bcgs/bcgs.c
> [0]PETSC ERROR: KSPSolve() line 385 in src/ksp/ksp/interface/itfunc.c
>
>
> What might cause the problem?
>

You cannot change the problem size dynamically with KSP, since it allocates
work vectors, etc. to support the computation. If the
size changes, everything will have to be recreated.


> Another question is, the BCGS+ILU combination is kind of slo w. Would you
> please give me some suggestions on how to optimize the solver performance?
> I've tried setting PC level to 1 and reducing maximum iterations, but
> doesn't really work well.
>

Preconditioning is highly problem dependent. I do not believe in block-box
preconditioners.


> The last question is, if I want to use the true residual norm (based on
> original A*x=y) and the norm of solution updates as the criteria of
> convergence, what functions would do? Any example in the package I can refer
> to?
>

http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/KSP/KSPSetNormType.html

If you want something more sophisticated, you can always use

http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/KSP/KSPSetConvergenceTest.html

  Matt


> Many thanks again and have a nice day!
>
>
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-- 
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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