[Trilinos-Users] preconditioned GMRES: no convergence in explicit residual

[Pavel Jiránek]

Hi Nico,

Would it be possible that your preconditioner is not fixed? That might
explain why left/right preconditioned GMRES does not work while CG and
MINRES (which are sort of "flexible") do. You could probably try to use the
right preconditioning combined with flexible GMRES.

Pavel


On Fri, Jan 31, 2014 at 6:58 PM, Nico Schlömer <nico.schloemer at gmail.com>wrote:

> Hi all,
>
> I've got this matrix corresponding a finite-element discretization of
> the Poisson problem on the unit square (with Dirichlet BC, no issues
> here). Running the problem through Belos with CG and ML, Ifpack gives
> no problems at all. The PseudoBlockGMRES solver works fine when
> unpreconditioned, but yields something weird when using a
> preconditioner:
>
> ============== *snip* ==============
> *******************************************************
> ***** Belos Iterative Solver:  Pseudo Block Gmres
> ***** Maximum Iterations: 100
> ***** Block Size: 1
> ***** Residual Tests (SEQ):
> *****   Test 1 : Belos::StatusTestGenResNorm<>: (2-Norm Imp Res Vec) /
> (2-Norm Prec Res0), tol = 1e-13
> *****   Test 2 : Belos::StatusTestGenResNorm<>: (2-Norm Exp Res Vec) /
> (2-Norm Res0), tol = 1e-13
> *******************************************************
> Iter   0, [ 1] :    1.000000e+00            ---
> Iter   1, [ 1] :    6.120593e-01            ---
> Iter   2, [ 1] :    2.695238e-01            ---
> Iter   3, [ 1] :    5.704995e-02            ---
> Iter   4, [ 1] :    1.272673e-02            ---
> Iter   5, [ 1] :    2.783810e-03            ---
> Iter   6, [ 1] :    5.753817e-04            ---
> Iter   7, [ 1] :    2.045848e-04            ---
> Iter   8, [ 1] :    5.724907e-05            ---
> Iter   9, [ 1] :    1.633313e-05            ---
> Iter  10, [ 1] :    3.055064e-06            ---
> Iter  11, [ 1] :    4.947173e-07            ---
> Iter  12, [ 1] :    6.945227e-08            ---
> Iter  13, [ 1] :    1.859584e-08            ---
> Iter  14, [ 1] :    5.185228e-09            ---
> Iter  15, [ 1] :    5.922331e-10            ---
> Iter  16, [ 1] :    8.149474e-11            ---
> Iter  17, [ 1] :    1.010586e-11            ---
> Iter  18, [ 1] :    1.600644e-12            ---
> Iter  19, [ 1] :    3.034731e-13            ---
> Iter  20, [ 1] :    3.452280e-14   7.213315e-01
> [...]
> ============== *snap* ==============
>
> The ExpResVec never converges at all.
>
> What may cause this? Any known fix/workaround?
>
> Cheers,
> Nico
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