<div dir="ltr"><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small">@Barry, Upon your suggestion, I added -pc_fieldsplit_detect_saddle_point to my previously used command line options. But, I get the same error message as earlier. </div><div class="gmail_default" style="font-family:verdana,sans-serif;font-size:small"><br clear="all"></div><div><div dir="ltr" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div><font size="2" face="verdana, sans-serif">-------------------------------------------</font></div><div><font size="2" face="verdana, sans-serif">Regards</font></div><div dir="ltr"><font face="verdana, sans-serif"><br></font><div><font face="verdana, sans-serif">Sthavishtha<br></font></div><div><br></div><div><br></div><div><br></div><div><br></div></div></div></div></div></div></div></div></div></div><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Apr 12, 2021 at 4:36 AM Barry Smith <<a href="mailto:bsmith@petsc.dev" target="_blank">bsmith@petsc.dev</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div><br></div><div>0 SNES Function norm 6.145506780035e-04 </div><div> 0 KSP Residual norm 2.013603254316e+41 </div><div><br></div><div> My guess is that your matrix has zeros or essentially zeros on some diagonal entries, this could be breaking </div><div><br></div><div><div>Relax down symmetric-SOR/Jacobi</div><div> Relax up symmetric-SOR/Jacobi</div></div><div><br></div><div>since the smoother would be dividing by those numbers. Standard multigrid methods generally cannot handle such matrices without adjustments.</div><div><br></div><div>On approach for such problems is to use PCFIELDSPLIT and split off the zero diagonal portion while using AMG on the rest -pc_fieldsplit_detect_saddle_point see PCFieldSplitSetDetectSaddlePoint().</div><div><br></div><div> Barry</div><div><br></div><div><br></div><div><br></div><div><br><blockquote type="cite"><div>On Apr 11, 2021, at 1:06 PM, Matthew Knepley <<a href="mailto:knepley@gmail.com" target="_blank">knepley@gmail.com</a>> wrote:</div><br><div><div dir="ltr"><div dir="ltr">On Sun, Apr 11, 2021 at 1:37 PM sthavishtha bhopalam <<a href="mailto:sthavishthabr@gmail.com" target="_blank">sthavishthabr@gmail.com</a>> wrote:<br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div style="font-family:verdana,sans-serif;font-size:small">Hello PETSc users</div><div style="font-family:verdana,sans-serif;font-size:small"><br></div><div style="font-family:verdana,sans-serif;font-size:small">I am trying to experiment with Hypre's BoomerAMG preconditioner which continually yields the error message "Linear solve did not converge due to DIVERGED_DTOL iterations 1". I would appreciate if someone can suggest some ways I could get BoomerAMG to yield converged results - the attached output shows a snippet of the error message. <u>Command Line options I used for BoomerAMG</u> : <i><span style="font-family:"comic sans ms",sans-serif">-pc_type hypre -pc_hypre_type boomeramg -pchypreboomeramgtol 1.0e-3 -pchypreboomeramgstrongthreshold 0.25 -ksp_type richardson -pc_hypre_boomeramg_max_iter 6 -snes_rtol 1.0e-3 -ksp_rtol 1.0e-3 -ksp_view -snes_view -ksp_monitor -snes_monitor -ksp_max_it 100 -ksp_converged_reason -snes_converged_reason</span></i></div><div style="font-family:verdana,sans-serif;font-size:small"><br></div><div style="font-family:verdana,sans-serif;font-size:small">I also tried using <span style="font-family:"comic sans ms",sans-serif"><i>-ksp_type gmres</i></span>, different values of <i><span style="font-family:"comic sans ms",sans-serif">-pc_hypre_boomeramg_max_iter</span></i>, <span style="font-family:"comic sans ms",sans-serif"><i>-pchypreboomeramgstrongthreshold</i></span>, <i><span style="font-family:"comic sans ms",sans-serif">-ksp_initial_guess_nonzero</span></i> but all yielded the same error message.<br></div><div style="font-family:verdana,sans-serif;font-size:small"><br></div><div style="font-family:verdana,sans-serif;font-size:small">However, the direct solver converges as required - the attached output shows a snippet of the norms from the SNES and KSP.<br></div><div style="font-family:verdana,sans-serif;font-size:small"><u>Command Line options I used for the direct solver</u> : <i><span style="font-family:"comic sans ms",sans-serif">-ksp_type gmres -pc_type lu -pc_factor_shift_type nonzero -pc_factor_mat_solver_type mumps -snes_converged_reason -ksp_converged_reason -ksp_rtol 1e-3 -snes_rtol 1e-3 -ksp_monitor -snes_monitor</span></i></div><div style="font-family:verdana,sans-serif;font-size:small"><br></div><div style="font-family:verdana,sans-serif;font-size:small">There is no particular reason for using the AMG here, but I just wanted to familiarize with it's options to see which of them need to be particularly tuned to yield converged and correct results.</div></div></blockquote><div><br></div><div>Hypre is only going to work for a very specific set of systems. What are you solving?</div><div><br></div><div> Thanks,</div><div><br></div><div> Matt</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div style="font-family:verdana,sans-serif;font-size:small">Thanks<br clear="all"></div><div><div dir="ltr"><div dir="ltr"><div><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div><font size="2" face="verdana, sans-serif">-------------------------------------------</font></div><div><font size="2" face="verdana, sans-serif">Regards</font></div><div dir="ltr"><font face="verdana, sans-serif"><br></font><div><font face="verdana, sans-serif">Sthavishtha <br></font></div><div><br></div><div><br></div><div><br></div><div><br></div></div></div></div></div></div></div></div></div></div></div>
</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div>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</div><div><br></div><div><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br></div></div></div></div></div></div></div></div>
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