[petsc-users] positive definite MPISBAIJ-Matrix gets singular in the case of more than one MPI-process

[Matthew Knepley]

On Tue, Jun 8, 2010 at 12:33 AM, Andreas Hauffe <
andreas.hauffe at tu-dresden.de> wrote:

> Dear all,
>
> I want to solve a linear System with a symmetric positive definite matrix.
> I
> use the external package MUMPS as solver with the following commands
>
>  call KSPCreate(PETSC_COMM_WORLD,ksp,ierr)
>  call KSPSetOperators(ksp,mat,mat,DIFFERENT_NONZERO_PATTERN,ierr)
>  call KSPSetType(ksp,KSPPREONLY,ierr)
>  call KSPGetPC(ksp,prec,ierr)
>  call PCSetType(prec,PCCHOLESKY,ierr)
>  call PCFactorSetMatSolverPackage(prec,MAT_SOLVER_MUMPS,ierr)
>  call KSPSetFromOptions(ksp,ierr)
>  call KSPSolve(ksp,vec1,vec2,ierr)
>
> and give the option "-mat_mumps_sym 1" (symmetric positive definite).
>
> If I run the program with one MPI-process everything works fine. For a low
> number of unknows it also works for more MPI-processes. If I increase the
> number of unknows (>50000) and use more then one MPI-process on one or more
> host, I get a MUMPS error that the matrix is singular. With one MPI-process
> everything is still fine. The critical value for the number of unknows
> depends also on the use of the debug-version or non-debug-version of PETSc.
> If I do not use the option "-mat_mumps_sym 1" everything also works.
>

It sounds like your matrix is not actually symmetric on multiple processes:

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

    Matt


> Right now the finite element assembling process (MatSetValue) is only done
> with rank 0. So the hole matrix gets all its values only from rank 0.
>
> Could someone explain this behaviour? Where is my error?
>
> Btw, I am currently using PETSc 3.0 since I work with SLEPc as well.
>
> Thank you very much,
> --
> Andreas Hauffe
>
>
> ----------------------------------------------------------------------------------------------------
> Technische Universität Dresden
> Institut für Luft- und Raumfahrttechnik / Institute of Aerospace
> Engineering
> Lehrstuhl für Luftfahrzeugtechnik / Chair of Aircraft Engineering
>
> D-01062 Dresden
> Germany
>
> phone   : (++49)351 463 38496
> fax     : (++49)351 463 37263
> mail    : andreas.hauffe at tu-dresden.de
> Website : http://tu-dresden.de/mw/ilr/lft
>
> ----------------------------------------------------------------------------------------------------
>



-- 
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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