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<span style="font-family:Menlo;font-size:12px;background-color:rgb(255, 255, 255);display:inline !important" class="ContentPasted0">Karhik,</span><br>
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<span style="font-family:Menlo;font-size:12px;background-color:rgb(255, 255, 255);display:inline !important" class="ContentPasted0">Can you find out the condition number of your matrix?</span></div>
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<span style="font-family:Menlo;font-size:12px;background-color:rgb(255, 255, 255);display:inline !important" class="ContentPasted0">Hong</span></div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> petsc-users <petsc-users-bounces@mcs.anl.gov> on behalf of Karthikeyan Chockalingam - STFC UKRI via petsc-users <petsc-users@mcs.anl.gov><br>
<b>Sent:</b> Wednesday, November 16, 2022 6:04 PM<br>
<b>To:</b> petsc-users@mcs.anl.gov <petsc-users@mcs.anl.gov><br>
<b>Subject:</b> [petsc-users] Different solution while running in parallel</font>
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<p class="x_p1" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> Hello,</span></p>
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<span class="x_apple-converted-space"> </span></p>
<p class="x_p1" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> I tried to solve a (FE discretized) Poisson equation using PCLU. For some reason I am getting different solutions while running the problem on one and two cores. I have attached the output file (out.txt) from both the
runs. I am printing A, b and x from both the runs – while A and b are the same but the solution seems is different.</span></p>
<p class="x_p1" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> </span></p>
<p class="x_p1" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space">I am not sure what I doing wrong.</span></p>
<p class="x_p1" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> </span></p>
<p class="x_p1" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space">Below is my matrix, vector, and solve setup.
</span></p>
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<span class="x_apple-converted-space"> </span></p>
<p class="x_p1" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> </span></p>
<p class="x_p1" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> </span> <span style="color:#3900A0">Mat</span> A;<span style="color:#3900A0"></span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black"> </span><span style="font-size:9.0pt; font-family:Menlo; color:#3900A0">Vec</span><span style="font-size:9.0pt; font-family:Menlo; color:black"> b, x;</span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black"> </span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black"> ierr = </span><span style="font-size:9.0pt; font-family:Menlo; color:#6C36A9">MatCreate</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(</span><span style="font-size:9.0pt; font-family:Menlo; color:#6C36A9">PETSC_COMM_WORLD</span><span style="font-size:9.0pt; font-family:Menlo; color:black">,
&A); </span><span style="font-size:9.0pt; font-family:Menlo; color:#643820">CHKERRQ</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(ierr);</span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black"> ierr = </span><span style="font-size:9.0pt; font-family:Menlo; color:#6C36A9">MatSetType</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(A,
</span><span style="font-size:9.0pt; font-family:Menlo; color:#643820">MATMPIAIJ</span><span style="font-size:9.0pt; font-family:Menlo; color:black">);
</span><span style="font-size:9.0pt; font-family:Menlo; color:#643820">CHKERRQ</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(ierr);</span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black"> ierr = </span><span style="font-size:9.0pt; font-family:Menlo; color:#6C36A9">MatSetSizes</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(A,
</span><span style="font-size:9.0pt; font-family:Menlo; color:#643820">PETSC_DECIDE</span><span style="font-size:9.0pt; font-family:Menlo; color:black">,
</span><span style="font-size:9.0pt; font-family:Menlo; color:#643820">PETSC_DECIDE</span><span style="font-size:9.0pt; font-family:Menlo; color:black">, N, N);
</span><span style="font-size:9.0pt; font-family:Menlo; color:#643820">CHKERRQ</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(ierr);</span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black"> ierr = </span><span style="font-size:9.0pt; font-family:Menlo; color:#6C36A9">MatMPIAIJSetPreallocation</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(A,d_nz,
</span><b><span style="font-size:9.0pt; font-family:Menlo; color:#9B2393">NULL</span></b><span style="font-size:9.0pt; font-family:Menlo; color:black">, o_nz,
</span><b><span style="font-size:9.0pt; font-family:Menlo; color:#9B2393">NULL</span></b><span style="font-size:9.0pt; font-family:Menlo; color:black">);
</span><span style="font-size:9.0pt; font-family:Menlo; color:#643820">CHKERRQ</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(ierr);</span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:#6C36A9"> ierr = MatSetOption(A,MAT_SYMMETRIC,PETSC_TRUE);
</span><span style="font-size:9.0pt; font-family:Menlo; color:#643820">CHKERRQ</span><span style="font-size:9.0pt; font-family:Menlo; color:#6C36A9">(ierr);</span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black"> ierr = </span><span style="font-size:9.0pt; font-family:Menlo; color:#6C36A9">MatCreateVecs</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(A, &b, &x);
</span><span style="font-size:9.0pt; font-family:Menlo; color:#643820">CHKERRQ</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(ierr);</span></p>
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<p class="x_p2" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> </span><span class="x_s1" style="color: rgb(57, 0, 160);">KSP</span>
<span class="x_apple-converted-space"> </span>ksp;</p>
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<span class="x_apple-converted-space"> </span><span class="x_s1" style="color: rgb(57, 0, 160);">PC</span><span class="x_apple-converted-space">
</span>pc;</p>
<p class="x_p3" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: rgb(108, 54, 169);">
<span class="x_apple-converted-space"> </span>KSPCreate<span class="x_s2">(</span>PETSC_COMM_WORLD<span class="x_s2">, &ksp);</span></p>
<p class="x_p2" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> </span><span class="x_s3" style="color: rgb(108, 54, 169);">KSPSetOperators</span>(ksp, A, A);</p>
<p class="x_p2" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> </span>ierr = <span class="x_s3" style="color: rgb(108, 54, 169);">
KSPSetType</span>(ksp,<span class="x_s5" style="color: rgb(100, 56, 32);">KSPPREONLY</span>);<span class="x_s5" style="color: rgb(100, 56, 32);">CHKERRQ</span>(ierr);</p>
<p class="x_p2" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> </span>ierr = <span class="x_s3" style="color: rgb(108, 54, 169);">
KSPGetPC</span>(ksp,&pc);<span class="x_s5" style="color: rgb(100, 56, 32);">CHKERRQ</span>(ierr);</p>
<p class="x_p2" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: black;">
<span class="x_apple-converted-space"> </span>ierr = <span class="x_s3" style="color: rgb(108, 54, 169);">
PCSetType</span>(pc,<span class="x_s5" style="color: rgb(100, 56, 32);">PCLU</span>);<span class="x_s5" style="color: rgb(100, 56, 32);">CHKERRQ</span>(ierr);</p>
<p class="x_p3" style="margin: 0cm; background: white; font-size: 9pt; font-family: Menlo; color: rgb(108, 54, 169);">
<span class="x_apple-converted-space"> </span><span class="x_s2">ierr = </span>
PCFactorSetMatSolverType<span class="x_s2">(pc,</span><span class="x_s5" style="color: rgb(100, 56, 32);">MATSOLVERMUMPS</span><span class="x_s2">);</span><span class="x_s5" style="color: rgb(100, 56, 32);">CHKERRQ</span><span class="x_s2">(ierr);</span></p>
<p class="x_MsoNormal" style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;background:white">
<span style="font-size:9.0pt; font-family:Menlo; color:black"> </span><span style="font-size:9.0pt; font-family:Menlo; color:#6C36A9">KSPSolve</span><span style="font-size:9.0pt; font-family:Menlo; color:black">(ksp, b, x);</span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black"> </span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black">Thank you for your help.</span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black"> </span></p>
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<span style="font-size:9.0pt; font-family:Menlo; color:black">Karhik.</span></p>
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