On Mon, Jul 27, 2009 at 4:35 AM, Tim Kroeger <span dir="ltr"><<a href="mailto:tim.kroeger@cevis.uni-bremen.de">tim.kroeger@cevis.uni-bremen.de</a>></span> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Dear all,<br>
<br>
In my application, there is a linear system to be solved in every time step. Steps 0 and 1 work well, but in step 2 PETSc fails to converge. I suspected that the system might be unsolvable in that step and checked that by writing matrix and the right hand side to files and loading them into "octave". Surprisingly, "octave" does find a solution to the system without any problems.<br>
<br>
The problem occurs even on a single core. I am using PETSc version 2.3.3-p11 with the GMRES solver and ILU preconditioner.<br>
<br>
Can anybody give me a hint which settings would PETSc reliably enable solving systems of the type that I face?</blockquote><div><br>If we could, we would already have retired. There are simply no iterative solvers that work for all systems. The<br>
best preconditioners are usually tailored to the particular equations being solved. I would suggest a search of<br>the literature for PCs for your equations.<br><br> Thanks,<br><br> Matt<br> </div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<br>
I have put matrix and right hand side on my homepage; they can be downloaded from <a href="http://www.mevis.de/%7Etim/m-and-v.tar.gz" target="_blank">www.mevis.de/~tim/m-and-v.tar.gz</a> (7MB). In octave, I used the following commands to find and check the solution:<br>
<br>
octave:1> matrix2<br>
octave:2> vector2<br>
octave:3> x=Mat_0\Vec_1;<br>
octave:4> res=Mat_0*x-Vec_1;<br>
octave:5> norm(res)<br>
ans = 1.0032e-12<br>
octave:6> norm(Vec_1)<br>
ans = 27.976<br>
octave:7> norm(Mat_0,"fro")<br>
ans = 2.5917e+22<br>
octave:8> norm(x)<br>
ans = 3855.3<br>
<br>
<br>
Best Regards,<br>
<br>
Tim<br><font color="#888888">
<br>
-- <br>
Dr. Tim Kroeger<br>
<a href="mailto:tim.kroeger@mevis.fraunhofer.de" target="_blank">tim.kroeger@mevis.fraunhofer.de</a> Phone +49-421-218-7710<br>
<a href="mailto:tim.kroeger@cevis.uni-bremen.de" target="_blank">tim.kroeger@cevis.uni-bremen.de</a> Fax +49-421-218-4236<br>
<br>
Fraunhofer MEVIS, Institute for Medical Image Computing<br>
Universitaetsallee 29, 28359 Bremen, Germany<br>
<br>
</font></blockquote></div><br><br clear="all"><br>-- <br>What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br>-- Norbert Wiener<br>