This sounds like it has to do with the condition of your system, not any parallel problem. Errors<br>in the solution can only be reduced to about (condition number) * (residual norm).<br><br> Matt<br><br><div class="gmail_quote">
On Wed, Oct 7, 2009 at 5:26 PM, Matt Funk <span dir="ltr"><<a href="mailto:mafunk@nmsu.edu">mafunk@nmsu.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div style="font-family: 'Arial'; font-size: 12pt; font-weight: 400; font-style: normal;">Hi,<br>
<p style="margin: 0px; text-indent: 0px;"><br></p>i have a problem for which i am not exaclty sure about what to do.<br>
I set up a simple 2D rectangular domain and decompose it into four equal boxes. I then build the petsc matrix based on this layout as well as the corresponsing RHS vector.<br>
<p style="margin: 0px; text-indent: 0px;"><br></p>I print out the matrix and RHS vector right before my KSPSolve call, and right after that call i print out the solution vector 'x'.<br>
<p style="margin: 0px; text-indent: 0px;"><br></p>I do this for 2 runs. <br>
1) 1 processor<br>
2) 4 processors.<br>
<p style="margin: 0px; text-indent: 0px;"><br></p>For both runs i do a difference (i.e. on the output files using diff) on all 3 quantities (the matrix, the RHS vector and the solution vector).<br>
<p style="margin: 0px; text-indent: 0px;"><br></p>The 'diff' command reports no difference between the files for the matrix and RHS vector.<br>
<p style="margin: 0px; text-indent: 0px;"><br></p>However, the soltution vector is different between the 2 runs. How different depends a little on what precond/solver combination i use and the tolerances.<br>
<p style="margin: 0px; text-indent: 0px;"><br></p>However, for example for BJacobi/GMRES with reltol=abstol=1e-12 the vector element with the maximum difference is on the order 1e-05. This is only after the first timestep. My problem has some nonlinearlity to it such that this will become a problem later on.<br>
<p style="margin: 0px; text-indent: 0px;"><br></p>The worst difference i have seen is if i use hypre's euclid. It was on the order of 1e-02.<br>
<p style="margin: 0px; text-indent: 0px;"><br></p><p style="margin: 0px; text-indent: 0px;"><br></p>So my question is whether someone has an idea why this is happening (i suppose it is related to the parallel communication) and if there is way to fix it. <br>
<p style="margin: 0px; text-indent: 0px;"><br></p><p style="margin: 0px; text-indent: 0px;"><br></p>thanks<br><font color="#888888">
matt</font></div></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>