<html aria-label="message body"><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><br id="lineBreakAtBeginningOfMessage"><div><br><blockquote type="cite"><div>On Nov 4, 2025, at 4:06 AM, Moral Sanchez, Elena <Elena.Moral.Sanchez@ipp.mpg.de> wrote:</div><br class="Apple-interchange-newline"><div><meta charset="UTF-8"><div id="divtagdefaultwrapper" dir="ltr" style="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; -webkit-text-stroke-width: 0px; text-decoration: none; font-size: 12pt; font-family: Calibri, Helvetica, sans-serif;"><p style="margin-top: 0px; margin-bottom: 0px;"></p><div><div style="margin-top: 0px; margin-bottom: 0px;">Dear Barry,</div><div style="margin-top: 0px; margin-bottom: 0px;">Thanks for the fast answer. Unfortunately in my case the discrepancy is huge. With the flags</div><blockquote><div style="margin-top: 0px; margin-bottom: 0px;"> -ksp_monitor_true_residual -ksp_norm_type unpreconditioned</div></blockquote><div style="margin-top: 0px; margin-bottom: 0px;">this is the output:</div><blockquote><div style="margin-top: 0px; margin-bottom: 0px;"> 0 KSP unpreconditioned resid norm 5.568889644229e-01 true resid norm 5.568889644229e-01 ||r(i)||/||b|| 1.000000000000e+00<br> 1 KSP unpreconditioned resid norm 2.831772665189e-01 true resid norm 2.831772665189e-01 ||r(i)||/||b|| 5.084986139245e-01<br> 2 KSP unpreconditioned resid norm 1.875950094147e-01 true resid norm 1.875950094147e-01 ||r(i)||/||b|| 3.368625011435e-01</div></blockquote><div style="margin-top: 0px; margin-bottom: 0px;">and this is the output of my own monitor function:</div><blockquote><div style="margin-top: 0px; margin-bottom: 0px;">Iter 0/10 | res = 5.57e-01/1.00e-08 | 0.0 s<br>difference KSPBuildSolution and u: 0.0<br>UNPRECONDITIONED norm: 0.5568889644229376<br>PRECONDITIONED norm: 2.049041078011257<br>KSPBuildResidual 2-norm: 0.5568889644229299<br>difference KSPBuildResidual and b-A(KSPBuildSolution): 6.573603152700697e-13<br><br>Iter 1/10 | res = 2.83e-01/1.00e-08 | 0.0 s<br>difference KSPBuildSolution and u: 0.0<br>UNPRECONDITIONED norm: 0.7661983589104541<br>PRECONDITIONED norm: 2.7387602134717137<br>KSPBuildResidual 2-norm: 0.2831772665189212<br>difference KSPBuildResidual and b-A(KSPBuildSolution): 0.1700718741085172<br><br>Iter 2/10 | res = 1.88e-01/1.00e-08 | 0.0 s<br>difference KSPBuildSolution and u: 0.0<br>UNPRECONDITIONED norm: 0.7050518160900253<br>PRECONDITIONED norm: 2.421773833445645<br>KSPBuildResidual 2-norm: 0.18759500941469456<br>difference KSPBuildResidual and b-A(KSPBuildSolution): 0.19327058976599623</div></blockquote><div style="margin-top: 0px; margin-bottom: 0px;">Here u is the vector in the KSPSolve. </div><div style="margin-top: 0px; margin-bottom: 0px;">After the first iteration, the residual computed from KSPBuildSolution and the residual from KSPBuildResidual diverge. They are the same when I run the same code without preconditioner.</div><div style="margin-top: 0px; margin-bottom: 0px;">Another observation is that after convergence (wrt. unpreconditioned norm == 2-norm of KSPBuildResidual) the solution with and without preconditioner looks quite different. How is this possible if my preconditioner is SPD?</div><div style="margin-top: 0px; margin-bottom: 0px;"><br></div><div style="margin-top: 0px; margin-bottom: 0px;">By the way, where can I find your implementation of "My monitor" in src/snes/tutorials/ex5.c? I tried to look at the Gitlab repository but could not find it.</div></div></div></div></blockquote><div><br></div>I thought I attached it to the email.</div><div><br></div><div><br><blockquote type="cite"><div><div id="divtagdefaultwrapper" dir="ltr" style="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; -webkit-text-stroke-width: 0px; text-decoration: none; font-size: 12pt; font-family: Calibri, Helvetica, sans-serif;"><div><div style="margin-top: 0px; margin-bottom: 0px;">Thanks for the help.</div><div style="margin-top: 0px; margin-bottom: 0px;">Cheers,</div><div style="margin-top: 0px; margin-bottom: 0px;">Elena</div></div><p style="margin-top: 0px; margin-bottom: 0px;"></p><div style="margin-top: 0px; margin-bottom: 0px;"><br></div><div style="margin-top: 0px; margin-bottom: 0px;"><br></div><div style="margin-top: 0px; margin-bottom: 0px;"><br></div><div style="margin-top: 0px; margin-bottom: 0px;"><br></div><div style="margin-top: 0px; margin-bottom: 0px;"><br></div><div class="moz-cite-prefix">On 11/4/25 03:01, Barry Smith wrote:<br></div><blockquote type="cite"><div> 0 KSP unpreconditioned resid norm 1.265943996096e+00 true resid norm 1.265943996096e+00 ||r(i)||/||b|| 1.000000000000e+00</div><div>My monitor 0 1.265943996096e+00</div></blockquote></div></div></blockquote></div><br></body></html>