Barry Smith bsmith at mcs.anl.gov
Sun Feb 19 16:22:30 CST 2006


  ---Norm of residual=4.58868e+17

   This is usually indicative of a matrix with poor scaling or very small matrices
along the diagonal. I a very small pivot is encountered causing a "crazy"
preconditioner with tiny entries producing a really absurd preconditioned residual
norm (most likely the true residual norm is much more reasonable, use -ksp_truemonitor)
It has not converged.

    You can run (the latest PETSc 2.3.1) with -pc_factor_shift_positive_definite
to "damp the matrix used to construct the preconditioner to prevent zero or very small
pivots". For this size problem you also may as well just use the direct solver -pc_type lu

    Good luck,


On Sun, 19 Feb 2006, buket at be.itu.edu.tr wrote:

> Hello,
> I am trying to solve a linear system of vibroacoustic Problem with 12328
> dimension.
> When I use bcgs and ilu(1) with n=1 proc, I get:
> ---Norm of residual=4.58868e+17
> ---Norm of error(u-x)=75.0051
> ---Number of iteration=7
> ---Solution Time=3.896039 second
> When I use bcgs and ilu(2) with n=1 proc, I get a different error message:
> ---[0]PETSC ERROR: MatLUFactorNumeric_SeqAIJ() line 537 in
> src/mat/impls/aij/seq/aijfact.c
>   [0]PETSC ERROR:   Detected zero pivot in LU factorization!
>   [0]PETSC ERROR:   Zero pivot row 10816 value 5.0874e+06 tolerance 1e-12
> * rs 7.29416e+20!
>   [0]PETSC ERROR: MatLUFactorNumeric() line 1831 in
> src/mat/interface/matrix.c
>   ....................
> That system seems to converge immediately, at first, with 7 iters. But the
> norm of residual is increadibly high. Did you face with that situation?
> Can that be the result of divergence?
> Best Regards,
> Buket Benek

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