<p>As alway, listen to Barry. You might also look at how the eigenvalues of this matrix is distributed. If they encircle the origin, for example, then GMRES will be pathologically bad, but the condition number is still small. It's worth comparing with bcgs and cgne. </p>
<div class="gmail_quote">On Oct 25, 2011 3:44 AM, "Klaij, Christiaan" <<a href="mailto:C.Klaij@marin.nl">C.Klaij@marin.nl</a>> wrote:<br type="attribution"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
>> The setting is Navier-Stokes, colocated FVM, matrix-free Krylov-Picard<br>
>> with SIMPLE preconditioning. It works for most case but stagnates<br>
>> for this particular case although SIMPLE as solver does converge nicely.<br>
>><br>
><br>
>> How are you applying the action of the linear operator? If you use finite<br>
>> differencing, it could be inaccurate. Is this incompressible or a low-Mach<br>
>> compressible formulation? Try -ksp_monitor_true_residual, if the true<br>
>> residual drifts from the unpreconditioned residual computed by FGMRES, the<br>
>> Krylov space could be losing orthogonality. You can try<br>
>> -ksp_gmres_modifiedgramschmidt. Are you losing a lot of progress in<br>
>> restarts?<br>
><br>
><br>
>It's incompressible Navier-Stokes. No finite differencing, the<br>
>action is computed directly without approximations. It's right<br>
>preconditioning, so preconditioned and true residual should be<br>
>the same. I don't get any progress, the residual is stagnating<br>
>from the very first iteration way before any restart.<br>
<br>
Now that I have the singular value monitor working this is what I<br>
get for a converging problem and the stagnating problem. Both<br>
problems are quite similar (same type of flow, same type of grid<br>
type, same BCs, just slightly different geometry). Any ideas?<br>
<br>
Converging problem:<br>
<br>
0 KSP Residual norm 2.365447773598e+00 % max 1 min 1 max/min 1<br>
1 KSP Residual norm 1.688717750865e+00 % max 2.43352 min 2.43352 max/min 1<br>
2 KSP Residual norm 1.441677051369e+00 % max 3.39561 min 0.658191 max/min 5.15901<br>
3 KSP Residual norm 1.309618066659e+00 % max 3.45429 min 0.477676 max/min 7.23145<br>
4 KSP Residual norm 1.236059684429e+00 % max 3.68325 min 0.418244 max/min 8.80645<br>
5 KSP Residual norm 1.178488235028e+00 % max 4.26371 min 0.37149 max/min 11.4773<br>
<br>
Stagnating problem (classical Gram-Schmidt):<br>
<br>
0 KSP Residual norm 6.466095059482e-02 % max 1 min 1 max/min 1<br>
1 KSP Residual norm 6.466093783326e-02 % max 1.07631 min 1.07631 max/min 1<br>
2 KSP Residual norm 6.466093747580e-02 % max 3.0223 min 1.06721 max/min 2.83196<br>
3 KSP Residual norm 6.466093697245e-02 % max 6.79697 min 1.0663 max/min 6.37436<br>
4 KSP Residual norm 6.466092195305e-02 % max 8.16759 min 1.06345 max/min 7.68027<br>
5 KSP Residual norm 6.466090166280e-02 % max 8.80407 min 1.06326 max/min 8.28026<br>
<br>
Stagnating problem (modified Gram-Schmidt):<br>
<br>
0 KSP Residual norm 6.470362245549e-02 % max 1 min 1 max/min 1<br>
1 KSP Residual norm 6.470334363709e-02 % max 1.08156 min 1.08156 max/min 1<br>
2 KSP Residual norm 6.470326556761e-02 % max 3.05856 min 1.07224 max/min 2.8525<br>
3 KSP Residual norm 6.470326223293e-02 % max 6.85267 min 1.0711 max/min 6.39777<br>
4 KSP Residual norm 6.470312640256e-02 % max 8.17917 min 1.06837 max/min 7.65572<br>
5 KSP Residual norm 6.470312619015e-02 % max 8.7711 min 1.06807 max/min 8.21213<br>
<br>
<br>
dr. ir. Christiaan Klaij<br>
CFD Researcher<br>
Research & Development<br>
E mailto:<a href="mailto:C.Klaij@marin.nl">C.Klaij@marin.nl</a><br>
T <a href="tel:%2B31%20317%2049%2033%2044" value="+31317493344">+31 317 49 33 44</a><br>
<br>
MARIN<br>
2, Haagsteeg, P.O. Box 28, 6700 AA Wageningen, The Netherlands<br>
T <a href="tel:%2B31%20317%2049%2039%2011" value="+31317493911">+31 317 49 39 11</a>, F <a href="tel:%2B31%20317%2049%2032%2045" value="+31317493245">+31 317 49 32 45</a>, I <a href="http://www.marin.nl" target="_blank">www.marin.nl</a><br>
<br>
</blockquote></div>