[petsc-users] Question concerning ilu and bcgs

[Matthew Knepley]

On Wed, Feb 18, 2015 at 12:33 AM, Sun, Hui <hus003 at ucsd.edu> wrote:

>  I have a matrix system Ax = b, A is of type MatSeqAIJ or MatMPIAIJ,
> depending on the number of cores.
>
>  I try to solve this problem by pc_type ilu and ksp_type bcgs, it does
> not converge. The options I specify are:
>
> -pc_type hypre -pc_hypre_type pilut -pc_hypre_pilut_maxiter 1000
> -pc_hypre_pilut_tol 1e-3 -ksp_type b\
>
> cgs -ksp_rtol 1e-10 -ksp_max_it 1000 -ksp_monitor_short
> -ksp_converged_reason
>

1) Run with -ksp_view, so we can see exactly what was used

2) ILUT is unfortunately not a well-defined algorithm, and I believe the
parallel version makes different decisions
    than the serial version.

  Thanks,

    Matt


> The first a few lines of the output are:
>
>   0 KSP Residual norm 1404.62
>
>   1 KSP Residual norm 88.9068
>
>   2 KSP Residual norm 64.73
>
>   3 KSP Residual norm 71.0224
>
>   4 KSP Residual norm 69.5044
>
>   5 KSP Residual norm 455.458
>
>   6 KSP Residual norm 174.876
>
>   7 KSP Residual norm 183.031
>
>   8 KSP Residual norm 650.675
>
>   9 KSP Residual norm 79.2441
>
>  10 KSP Residual norm 84.1985
>
>
>  This clearly indicates non-convergence. However, I output the sparse
> matrix A and vector b to MATLAB, and run the following command:
>
> [L,U] = ilu(A,struct('type','ilutp','droptol',1e-3));
>
> [ux1,fl1,rr1,it1,rv1] = bicgstab(A,b,1e-10,1000,L,U);
>
>
>  And it converges in MATLAB, with flag fl1=0, relative residue
> rr1=8.2725e-11, and iteration it1=89.5. I'm wondering how can I figure out
> what's wrong.
>
>
>  Best,
>
> Hui
>



-- 
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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