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 Hi Barry, I have not used any other solver parameters in the code and the full set of solver related command line options are those I mentioned in the previous email. Below is the output from -ksp_view: KSP Object: (back_) 1 MPI process type: 
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<div dir="ltr"><div>Hi Barry,</div><div><br></div><div>I have not used any other solver parameters in the code and the full set of solver related command line options are those I mentioned in the previous email. <br></div><div><br></div><div>Below is the output from -ksp_view:</div><div><br></div><div>KSP Object: (back_) 1 MPI process<br>  type: gmres<br>    restart=30, using Classical (unmodified) Gram-Schmidt Orthogonalization with no iterative refinement<br>    happy breakdown tolerance 1e-30<br>  maximum iterations=10000, initial guess is zero<br>  tolerances: relative=1e-08, absolute=1e-50, divergence=10000.<br>  left preconditioning<br>  using PRECONDITIONED norm type for convergence test<br>PC Object: (back_) 1 MPI process<br>  type: fieldsplit<br>    FieldSplit with Schur preconditioner, blocksize = 1, factorization FULL<br>    Preconditioner for the Schur complement formed from S itself<br>    Split info:<br>    Split number 0 Defined by IS<br>    Split number 1 Defined by IS<br>    KSP solver for A00 block<br>      KSP Object: (back_fieldsplit_0_) 1 MPI process<br>        type: gmres<br>          restart=30, using Classical (unmodified) Gram-Schmidt Orthogonalization with no iterative refinement<br>          happy breakdown tolerance 1e-30<br>        maximum iterations=10000, initial guess is zero<br>        tolerances: relative=1e-05, absolute=1e-50, divergence=10000.<br>        left preconditioning<br>        using PRECONDITIONED norm type for convergence test<br>      PC Object: (back_fieldsplit_0_) 1 MPI process<br>        type: lu<br>          out-of-place factorization<br>          tolerance for zero pivot 2.22045e-14<br>          matrix ordering: nd<br>          factor fill ratio given 5., needed 8.83482<br>            Factored matrix follows:<br>              Mat Object: (back_fieldsplit_0_) 1 MPI process<br>                type: seqaij<br>                rows=30150, cols=30150<br>                package used to perform factorization: petsc<br>                total: nonzeros=2649120, allocated nonzeros=2649120<br>                  using I-node routines: found 15019 nodes, limit used is 5<br>        linear system matrix = precond matrix:<br>        Mat Object: (back_fieldsplit_0_) 1 MPI process<br>          type: seqaij<br>          rows=30150, cols=30150<br>          total: nonzeros=299850, allocated nonzeros=299850<br>          total number of mallocs used during MatSetValues calls=0<br>            using I-node routines: found 15150 nodes, limit used is 5<br>    KSP solver for S = A11 - A10 inv(A00) A01<br>      KSP Object: (back_fieldsplit_1_) 1 MPI process<br>        type: gmres<br>          restart=30, using Classical (unmodified) Gram-Schmidt Orthogonalization with no iterative refinement<br>          happy breakdown tolerance 1e-30<br>        maximum iterations=10000, initial guess is zero<br>        tolerances: relative=1e-08, absolute=1e-50, divergence=10000.<br>        left preconditioning<br>        using PRECONDITIONED norm type for convergence test<br>      PC Object: (back_fieldsplit_1_) 1 MPI process<br>        type: none<br>        linear system matrix = precond matrix:<br>        Mat Object: (back_fieldsplit_1_) 1 MPI process<br>          type: schurcomplement<br>          rows=15000, cols=15000<br>            Schur complement A11 - A10 inv(A00) A01<br>            A11<br>              Mat Object: (back_fieldsplit_1_) 1 MPI process<br>                type: seqaij<br>                rows=15000, cols=15000<br>                total: nonzeros=74700, allocated nonzeros=74700<br>                total number of mallocs used during MatSetValues calls=0<br>                  not using I-node routines<br>            A10<br>              Mat Object: 1 MPI process<br>                type: seqaij<br>                rows=15000, cols=30150<br>                total: nonzeros=149550, allocated nonzeros=149550<br>                total number of mallocs used during MatSetValues calls=0<br>                  not using I-node routines<br>            KSP solver for A00 block viewable with the additional option -back_fieldsplit_0_ksp_view<br>            A01<br>              Mat Object: 1 MPI process<br>                type: seqaij<br>                rows=30150, cols=15000<br>                total: nonzeros=149550, allocated nonzeros=149550<br>                total number of mallocs used during MatSetValues calls=0<br>                  using I-node routines: found 15150 nodes, limit used is 5<br>  linear system matrix = precond matrix:<br>  Mat Object: (back_) 1 MPI process<br>    type: seqaij<br>    rows=45150, cols=45150<br>    total: nonzeros=673650, allocated nonzeros=673650<br>    total number of mallocs used during MatSetValues calls=0<br>      has attached null space<br>      using I-node routines: found 15150 nodes, limit used is 5</div><div><br></div><div>Thanks again!</div><div><br></div><div>-Colton<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, May 22, 2024 at 3:39 PM 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>  Are you using any other command line options or did you hardwire any solver parameters in the code with, like, KSPSetXXX() or PCSetXXX() Please send all of them.</div><div><br></div><div>  Something funky definitely happened when the true residual norms jumped up.</div><div><br></div>  Could you run the same thing with -ksp_view and don't use any thing like -ksp_error_if_not_converged so we can see exactly what is being run.<div><br></div><div>  Barry</div><div><br id="m_6479905935081366205m_-972133632825488841lineBreakAtBeginningOfMessage"><div><br><blockquote type="cite"><div>On May 22, 2024, at 3:21 PM, Colton Bryant <<a href="mailto:coltonbryant2021@u.northwestern.edu" target="_blank">coltonbryant2021@u.northwestern.edu</a>> wrote:</div><br><div><div dir="ltr" id="m_6479905935081366205m_-972133632825488841pfptBannerebho4i4" style="writing-mode:revert;color:revert;font-family:revert;font-feature-settings:revert;font-kerning:revert;font-size:revert;font-size-adjust:revert;font-stretch:revert;font-variant-alternates:revert;font-variant-caps:revert;font-variant-east-asian:revert;font-variant-ligatures:revert;font-variant-numeric:revert;font-weight:revert;text-orientation:revert;zoom:revert;letter-spacing:revert;background-blend-mode:revert;background-image:revert;background-position:revert;background-repeat:revert;background-size:revert;border-collapse:revert;box-sizing:revert;break-after:revert;break-before:revert;break-inside:revert;caption-side:revert;clear:revert;columns:revert;column-fill:revert;column-gap:revert;column-rule:revert;column-span:revert;empty-cells:revert;float:revert;image-orientation:revert;isolation:revert;line-break:revert;line-height:revert;list-style:revert;mix-blend-mode:revert;object-fit:revert;object-position:revert;outline:revert;overflow:revert;quotes:revert;table-layout:revert;text-align:left;text-align-last:revert;text-decoration:revert;text-emphasis:revert;text-indent:revert;text-justify:revert;text-overflow:revert;text-transform:revert;text-underline-position:revert;vertical-align:revert;white-space:revert;word-break:revert;border-spacing:revert;word-spacing:revert;background-clip:revert;background-origin:revert;text-combine-upright:revert;display:block;opacity:revert;background-color:rgb(208,216,220);border-bottom:revert;border-left:revert;border-right:revert;height:revert;max-height:revert;max-width:revert;min-height:revert;width:revert;margin:16px 0px;padding:8px 16px;border-radius:4px;min-width:200px;border-top:4px solid rgb(144,164,174)"><div id="m_6479905935081366205m_-972133632825488841pfptBannerebho4i4" style="writing-mode:unset;color:unset;font-family:unset;font-feature-settings:unset;font-kerning:unset;font-size:unset;font-size-adjust:unset;font-stretch:unset;font-variant-alternates:unset;font-variant-caps:unset;font-variant-east-asian:unset;font-variant-ligatures:unset;font-variant-numeric:unset;font-weight:unset;text-orientation:unset;zoom:unset;letter-spacing:unset;background:unset;background-blend-mode:unset;border-collapse:unset;border:unset;box-sizing:unset;break-after:unset;break-before:unset;break-inside:unset;caption-side:unset;clear:unset;columns:unset;column-fill:unset;column-gap:unset;column-rule:unset;column-span:unset;empty-cells:unset;float:left;image-orientation:unset;isolation:unset;line-break:unset;line-height:unset;list-style:unset;mix-blend-mode:unset;object-fit:unset;object-position:unset;outline:unset;overflow:unset;quotes:unset;table-layout:unset;text-align:unset;text-align-last:unset;text-decoration:unset;text-emphasis:unset;text-indent:unset;text-justify:unset;text-overflow:unset;text-transform:unset;text-underline-position:unset;vertical-align:unset;white-space:unset;word-break:unset;border-spacing:unset;word-spacing:unset;text-combine-upright:unset;display:block;opacity:unset;border-radius:unset;height:unset;max-height:unset;min-height:unset;min-width:unset;padding:unset;width:unset;margin:0px 0px 1px;max-width:600px"><div id="m_6479905935081366205m_-972133632825488841pfptBannerebho4i4" style="writing-mode:unset;font-family:Arial,sans-serif;font-feature-settings:unset;font-kerning:unset;font-size:14px;font-size-adjust:unset;font-stretch:unset;font-variant-alternates:unset;font-variant-caps:unset;font-variant-east-asian:unset;font-variant-ligatures:unset;font-variant-numeric:unset;font-weight:bold;text-orientation:unset;zoom:unset;letter-spacing:unset;background-blend-mode:unset;background-image:unset;background-position:unset;background-repeat:unset;background-size:unset;border-collapse:unset;border:unset;box-sizing:unset;break-after:unset;break-before:unset;break-inside:unset;caption-side:unset;clear:unset;columns:unset;column-fill:unset;column-gap:unset;column-rule:unset;column-span:unset;empty-cells:unset;float:unset;image-orientation:unset;isolation:unset;line-break:unset;line-height:18px;list-style:unset;mix-blend-mode:unset;object-fit:unset;object-position:unset;outline:unset;overflow:unset;quotes:unset;table-layout:unset;text-align:unset;text-align-last:unset;text-decoration:unset;text-emphasis:unset;text-indent:unset;text-overflow:unset;text-transform:unset;text-underline-position:unset;vertical-align:unset;white-space:unset;word-break:unset;border-spacing:unset;word-spacing:unset;background-clip:unset;background-origin:unset;text-combine-upright:unset;display:block;opacity:unset;background-color:rgb(208,216,220);border-radius:unset;height:unset;margin:unset;max-height:unset;max-width:unset;min-height:unset;min-width:unset;padding:unset;width:unset">This Message Is From an External Sender</div><div id="m_6479905935081366205m_-972133632825488841pfptBannerebho4i4" style="writing-mode:unset;font-family:Arial,sans-serif;font-feature-settings:unset;font-kerning:unset;font-size:12px;font-size-adjust:unset;font-stretch:unset;font-variant-alternates:unset;font-variant-caps:unset;font-variant-east-asian:unset;font-variant-ligatures:unset;font-variant-numeric:unset;font-weight:unset;text-orientation:unset;zoom:unset;letter-spacing:unset;background-blend-mode:unset;background-image:unset;background-position:unset;background-repeat:unset;background-size:unset;border-collapse:unset;border:unset;box-sizing:unset;break-after:unset;break-before:unset;break-inside:unset;caption-side:unset;clear:unset;columns:unset;column-fill:unset;column-gap:unset;column-rule:unset;column-span:unset;empty-cells:unset;float:unset;image-orientation:unset;isolation:unset;line-break:unset;line-height:18px;list-style:unset;mix-blend-mode:unset;object-fit:unset;object-position:unset;outline:unset;overflow:unset;quotes:unset;table-layout:unset;text-align:unset;text-align-last:unset;text-decoration:unset;text-emphasis:unset;text-indent:unset;text-overflow:unset;text-transform:unset;text-underline-position:unset;vertical-align:unset;white-space:unset;word-break:unset;border-spacing:unset;word-spacing:unset;background-clip:unset;background-origin:unset;text-combine-upright:unset;display:block;opacity:unset;background-color:rgb(208,216,220);border-radius:unset;height:unset;margin-bottom:unset;margin-left:unset;margin-right:unset;max-height:unset;max-width:unset;min-height:unset;min-width:unset;padding:unset;width:unset;margin-top:2px">This message came from outside your organization.</div></div><div style="height:0px;clear:both;display:block;line-height:0;font-size:0.01px"></div></div><div dir="ltr" style="font-family:Helvetica;font-size:18px;font-style:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration:none"><div>Hello,</div><div><br></div><div>I am solving the Stokes equations on a MAC grid discretized by finite differences using a DMSTAG object. I have tested the solver quite extensively on manufactured problems and it seems to work well. As I am still just trying to get things working and not yet worried about speed I am using the following solver options:<span> </span><br></div><div>-pc_type fieldsplit</div><div>-pc_fieldsplit_detect_saddle_point</div><div>-fieldsplit_0_pc_type lu</div><div>-fieldsplit_1_ksp_rtol 1.e-8<br></div><div><br></div><div>However I am now using this solver as an inner step of a larger code and have run into issues. The code repeatedly solves the Stokes equations with varying right hand sides coming from changing problem geometry (the solver is a part of an overset grid scheme coupled to a level set method evolving in time). After a couple timesteps I observe the following output when running with -fieldsplit_1_ksp_converged_reason -fieldsplit_1_ksp_monitor_true_residual:<span> </span><br></div><div><br></div><div>Residual norms for back_fieldsplit_1_ solve.<br>    0 KSP preconditioned resid norm 2.826514299465e-02 true resid norm 2.826514299465e-02 ||r(i)||/||b|| 1.000000000000e+00<br>    1 KSP preconditioned resid norm 7.286621865915e-03 true resid norm 7.286621865915e-03 ||r(i)||/||b|| 2.577953300039e-01<br>    2 KSP preconditioned resid norm 1.500598474492e-03 true resid norm 1.500598474492e-03 ||r(i)||/||b|| 5.309007192273e-02<br>    3 KSP preconditioned resid norm 3.796396924978e-04 true resid norm 3.796396924978e-04 ||r(i)||/||b|| 1.343137349666e-02<br>    4 KSP preconditioned resid norm 8.091057439816e-05 true resid norm 8.091057439816e-05 ||r(i)||/||b|| 2.862556697960e-03<br>    5 KSP preconditioned resid norm 3.689113122359e-05 true resid norm 3.689113122359e-05 ||r(i)||/||b|| 1.305181128239e-03<br>    6 KSP preconditioned resid norm 2.116450533352e-05 true resid norm 2.116450533352e-05 ||r(i)||/||b|| 7.487846545662e-04<br>    7 KSP preconditioned resid norm 3.968234031201e-06 true resid norm 3.968234031200e-06 ||r(i)||/||b|| 1.403932055801e-04<br>    8 KSP preconditioned resid norm 6.666949419511e-07 true resid norm 6.666949419506e-07 ||r(i)||/||b|| 2.358717739644e-05<br>    9 KSP preconditioned resid norm 1.941522884928e-07 true resid norm 1.941522884931e-07 ||r(i)||/||b|| 6.868965372998e-06<br>   10 KSP preconditioned resid norm 6.729545258682e-08 true resid norm 6.729545258626e-08 ||r(i)||/||b|| 2.380863687793e-06<br>   11 KSP preconditioned resid norm 3.009070131709e-08 true resid norm 3.009070131735e-08 ||r(i)||/||b|| 1.064586912687e-06<br>   12 KSP preconditioned resid norm 7.849353009588e-09 true resid norm 7.849353009903e-09 ||r(i)||/||b|| 2.777043445840e-07<br>   13 KSP preconditioned resid norm 2.306283345754e-09 true resid norm 2.306283346677e-09 ||r(i)||/||b|| 8.159461097060e-08<br>   14 KSP preconditioned resid norm 9.336302495083e-10 true resid norm 9.336302502503e-10 ||r(i)||/||b|| 3.303115255517e-08<br>   15 KSP preconditioned resid norm 6.537456143401e-10 true resid norm 6.537456141617e-10 ||r(i)||/||b|| 2.312903968982e-08<br>   16 KSP preconditioned resid norm 6.389159552788e-10 true resid norm 6.389159550304e-10 ||r(i)||/||b|| 2.260437724130e-08<br>   17 KSP preconditioned resid norm 6.380905134246e-10 true resid norm 6.380905136023e-10 ||r(i)||/||b|| 2.257517372981e-08<br>   18 KSP preconditioned resid norm 6.380440605992e-10 true resid norm 6.380440604688e-10 ||r(i)||/||b|| 2.257353025207e-08<br>   19 KSP preconditioned resid norm 6.380427156582e-10 true resid norm 6.380427157894e-10 ||r(i)||/||b|| 2.257348267830e-08<br>   20 KSP preconditioned resid norm 6.380426714897e-10 true resid norm 6.380426714004e-10 ||r(i)||/||b|| 2.257348110785e-08<br>   21 KSP preconditioned resid norm 6.380426656970e-10 true resid norm 6.380426658839e-10 ||r(i)||/||b|| 2.257348091268e-08<br>   22 KSP preconditioned resid norm 6.380426650538e-10 true resid norm 6.380426650287e-10 ||r(i)||/||b|| 2.257348088242e-08<br>   23 KSP preconditioned resid norm 6.380426649918e-10 true resid norm 6.380426645888e-10 ||r(i)||/||b|| 2.257348086686e-08<br>   24 KSP preconditioned resid norm 6.380426649803e-10 true resid norm 6.380426644294e-10 ||r(i)||/||b|| 2.257348086122e-08<br>   25 KSP preconditioned resid norm 6.380426649796e-10 true resid norm 6.380426649774e-10 ||r(i)||/||b|| 2.257348088061e-08<br>   26 KSP preconditioned resid norm 6.380426649795e-10 true resid norm 6.380426653788e-10 ||r(i)||/||b|| 2.257348089481e-08<br>   27 KSP preconditioned resid norm 6.380426649795e-10 true resid norm 6.380426646744e-10 ||r(i)||/||b|| 2.257348086989e-08<br>   28 KSP preconditioned resid norm 6.380426649795e-10 true resid norm 6.380426650818e-10 ||r(i)||/||b|| 2.257348088430e-08<br>   29 KSP preconditioned resid norm 6.380426649795e-10 true resid norm 6.380426649518e-10 ||r(i)||/||b|| 2.257348087970e-08<br>   30 KSP preconditioned resid norm 6.380426652142e-10 true resid norm 6.380426652142e-10 ||r(i)||/||b|| 2.257348088898e-08<br>   31 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426646799e-10 ||r(i)||/||b|| 2.257348087008e-08<br>   32 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426648077e-10 ||r(i)||/||b|| 2.257348087460e-08<br>   33 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426649048e-10 ||r(i)||/||b|| 2.257348087804e-08<br>   34 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426648142e-10 ||r(i)||/||b|| 2.257348087483e-08<br>   35 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651079e-10 ||r(i)||/||b|| 2.257348088522e-08<br>   36 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650433e-10 ||r(i)||/||b|| 2.257348088294e-08<br>   37 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426649765e-10 ||r(i)||/||b|| 2.257348088057e-08<br>   38 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650364e-10 ||r(i)||/||b|| 2.257348088269e-08<br>   39 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650051e-10 ||r(i)||/||b|| 2.257348088159e-08<br>   40 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651154e-10 ||r(i)||/||b|| 2.257348088549e-08<br>   41 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650246e-10 ||r(i)||/||b|| 2.257348088227e-08<br>   42 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650702e-10 ||r(i)||/||b|| 2.257348088389e-08<br>   43 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651686e-10 ||r(i)||/||b|| 2.257348088737e-08<br>   44 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650870e-10 ||r(i)||/||b|| 2.257348088448e-08<br>   45 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651208e-10 ||r(i)||/||b|| 2.257348088568e-08<br>   46 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651441e-10 ||r(i)||/||b|| 2.257348088650e-08<br>   47 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650955e-10 ||r(i)||/||b|| 2.257348088478e-08<br>   48 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650877e-10 ||r(i)||/||b|| 2.257348088451e-08<br>   49 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426651240e-10 ||r(i)||/||b|| 2.257348088579e-08<br>   50 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426650534e-10 ||r(i)||/||b|| 2.257348088329e-08<br>   51 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426648615e-10 ||r(i)||/||b|| 2.257348087651e-08<br>   52 KSP preconditioned resid norm 6.380426652141e-10 true resid norm 6.380426649523e-10 ||r(i)||/||b|| 2.257348087972e-08<br>   53 KSP preconditioned resid norm 6.380426652140e-10 true resid norm 6.380426652601e-10 ||r(i)||/||b|| 2.257348089061e-08<br>   54 KSP preconditioned resid norm 6.380426652125e-10 true resid norm 6.380427512852e-10 ||r(i)||/||b|| 2.257348393411e-08<br>   55 KSP preconditioned resid norm 6.380426651849e-10 true resid norm 6.380603444402e-10 ||r(i)||/||b|| 2.257410636701e-08<br>   56 KSP preconditioned resid norm 6.380426646751e-10 true resid norm 6.439925413105e-10 ||r(i)||/||b|| 2.278398313542e-08<br>   57 KSP preconditioned resid norm 6.380426514019e-10 true resid norm 2.674218007058e-09 ||r(i)||/||b|| 9.461186902765e-08<br>   58 KSP preconditioned resid norm 6.380425077384e-10 true resid norm 2.406759314486e-08 ||r(i)||/||b|| 8.514937691775e-07<br>   59 KSP preconditioned resid norm 6.380406171326e-10 true resid norm 3.100137288622e-07 ||r(i)||/||b|| 1.096805803957e-05<br>    Linear back_fieldsplit_1_ solve did not converge due to DIVERGED_BREAKDOWN iterations 60<br></div><div><br></div><div>Any advice on steps I could take to elucidate the issue would be greatly appreciated. Thanks so much for any help in advance!</div><div><br></div><div>Best,</div><div>Colton Bryant<span> </span></div></div></div></blockquote></div><br></div></div></blockquote></div>