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<p>Dear Barry,</p>
<p>Thanks for the fast answer. Unfortunately in my case the discrepancy is huge. With the flags</p>
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<p> -ksp_monitor_true_residual -ksp_norm_type unpreconditioned</p>
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<p>this is the output:</p>
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<p> 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</p>
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<p>and this is the output of my own monitor function:</p>
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<p>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>
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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</p>
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<p>Here u is the vector in the KSPSolve. </p>
<p>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.</p>
<p>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?</p>
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<p>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.</p>
<p>Thanks for the help.</p>
<p>Cheers,</p>
<p>Elena</p>
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<div class="moz-cite-prefix">On 11/4/25 03:01, Barry Smith wrote:<br>
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<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>
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