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

[Nico Schlömer]

> Are you using left or right preconditioning, what
> happens when you switch to the other side?

Good question! I was using left preconditioning, but it doesn't matter
to which side it is applied: the result will be the same.

I checked the results that come out when not paying attention to the
large explicit residual: The solutions are indeed off if a
preconditioner is used (see attachments). I'm not sure what to make of
this.

I tried CG and MINRES in comparison, and both work well in all cases.

Any further ideas?

--Nico



On Fri, Jan 31, 2014 at 8:59 PM, Thornquist, Heidi K <hkthorn at sandia.gov> wrote:
> Hi Nico,
>
> This is interesting.  Are you using left or right preconditioning, what
> happens when you switch to the other side?
> I know that left preconditioning, because you are starting with a
> preconditioned RHS, can give a residual that is
> off from the residual of the original system.  But this is pretty extreme.
>
> I've tried running the preconditioned examples of pseudo block GMRES
> varying tolerances and I'm not seeing any abnormalities.
>
> Hmmmm....
>
> Heidi
>
> --
>
> Heidi K. Thornquist
> Electrical Models & Simulation
> Sandia National Laboratories
> Albuquerque, NM  87185-1323
>
>
>
>
>
> On 1/31/14 10:58 AM, "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
>>_______________________________________________
>>Trilinos-Users mailing list
>>Trilinos-Users at software.sandia.gov
>>http://software.sandia.gov/mailman/listinfo/trilinos-users
>
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