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

Nico Schlömer nico.schloemer at gmail.com
Fri Jan 31 10:58:38 MST 2014

```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
```