<div dir="ltr"><div><div><div>It seems the problem is solved if I call KSPSetOperators() before KSPSolve(). <br><br></div>I have one more question. How is a norm of the RHS calculated to check relative tolerance in iterative solvers? Is it norm2 or norm infinity?<br>
<br></div>Best,<br></div>Nori<br><br></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Jan 7, 2014 at 11:39 AM, Norihiro Watanabe <span dir="ltr"><<a href="mailto:norihiro.w@gmail.com" target="_blank">norihiro.w@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div>Matthew, Thank you for your quick response. I followed your suggestion and confirmed that the matrices are identical (I checked the norm1 of a matrix resulted from MatAXPY()). <br>
<br>The next step is to check whether PETSc objects are correctly reset before reassembling equations. A question I have is, do I have to reset or recreate a KSP object? The coefficient matrix, RHS and solution vectors are all set zero by calling MatZeroEntries() and VecSet().<br>
<br><br></div>Best,<br></div>Nori<br><br></div><div class="gmail_extra"><div><div class="h5"><br><br><div class="gmail_quote">On Mon, Jan 6, 2014 at 7:36 PM, Matthew Knepley <span dir="ltr"><<a href="mailto:knepley@gmail.com" target="_blank">knepley@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>On Mon, Jan 6, 2014 at 12:06 PM, Norihiro Watanabe <span dir="ltr"><<a href="mailto:norihiro.w@gmail.com" target="_blank">norihiro.w@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div><div>Hi,<br><br>I got a strange problem with PETSc 3.4 that linear equations assembled in our FEM codes cannot be solved within 5000 iterations, whereas linear equations loaded from binary files can be solved with only 24 iterations using ksp/examples/tutorials/ex10.c. The binary files were created by the FEM codes using MatView() and VecView() right before calling KSPSolve(). Linear solver types are -ksp_type bcgs -pc_type bjacobi. I set the same tolerance to both programs. <br>
</div><br>As you can see from attached log files, both programs calculate the same "true resid norm" at the beginning but different "preconditioned resid norm". Does it mean both programs are actually solving the same problem but somewhat with different preconditoner? It would be so helpful if you have any clue for this problem. </div>
</div></div></blockquote><div><br></div></div><div>Yes, that would be the likely conclusion. It appears that the parallel partition is the same, that the number of nonzeros in both</div><div>matrices is the same, and that the solvers are identical. Thus I would make sure that the matrices are indeed identical. For example,</div>
<div>you can MatLoad the matrix you used in ex10, and MatAXPY() to get the difference. It is a common error to forget to MatZeroEntries()</div><div>before reassembling the FEM operator which can produce the convergence degredation you see.</div>
<div><br></div><div> Thanks,</div><div><br></div><div> Matt</div><div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div>
<div><div><div><div>
just tell you background of this: Currently I'm facing a convergence problem in linear solvers for
solving transient heat transport problems using FEM. At early time
steps, PETSc converges quickly (<20 iterations). Later, iteration numbers increase
as time steps increase (>5000 after 19 time steps). I'm in the middle of
checking where is a problem for the slow convergence. Because I don't get such slow convergence with other linear solvers (BiCGSTAB+Jacobi), I suspect the FEM codes are missing some PETSc functions or options to be used. As I wrote above, if I use ex10.c with the binary files, the convergence problem is solved, which means something going wrong in the FEM codes.<br>
</div><br><br></div><div>Thank you in advance,<br></div><div>Nori<br><br>
</div></div></div></div></div>
</blockquote></div></div><span><font color="#888888"><br><br clear="all"><div><br></div>-- <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
</font></span></div></div>
</blockquote></div><br><br clear="all"><br></div></div><span class="HOEnZb"><font color="#888888">-- <br>Norihiro Watanabe
</font></span></div>
</blockquote></div><br><br clear="all"><br>-- <br>Norihiro Watanabe
</div>