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<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Dear Matt and Dear Barry,</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
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<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">I have some follow up questions regarding FieldSplit. </span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Let's assume that I solve again the 3D Stokes flow but now I have also a global constraint
that controls the flow rate at the inlet. Now, the matrix has the same unknowns as before, i.e. ux0,uy0,uz0,p0//ux1,uy1,uz1,p1//..., but the last line (and the last column) corresponds to the contribution of the global constraint equation. I want to incorporate
the last line (and last column) into the local block of velocities (split 0) and the pressure. The problem is how I do that. I have two questions:</span></div>
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<span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Now, the block size should be 5 in the matrix and vector creation for this problem?</span></li><li style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0); list-style-type: "2) ";">
<span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">I have to rely entirely on PCFieldSplitSetIS to create the two blocks? Can I augment simply the previously defined
block 0 with the last line of the matrix?</span></li></ol>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Up to this moment, I use the following commands to create the Field split:</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">ufields(3) = [0, 1, 2]<br>
pfields(1) = [3]</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">call PCSetType(pc, PCFIELDSPLIT, ierr)</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">call PCFieldSplitSetBlockSize(pc, 4,ierr)</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"> call PCFieldSplitSetFields(pc, "0", 3, ufields, ufields,ierr)</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"> call PCFieldSplitSetFields(pc, "1", 1, pfields, pfields,ierr)</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Thanks,</span></div>
<div class="elementToProof"><span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Pantelis</span></div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>Από:</b> Matthew Knepley <knepley@gmail.com><br>
<b>Στάλθηκε:</b> Παρασκευή, 19 Ιανουαρίου 2024 11:31 μμ<br>
<b>Προς:</b> Barry Smith <bsmith@petsc.dev><br>
<b>Κοιν.:</b> Pantelis Moschopoulos <pmoschopoulos@outlook.com>; petsc-users@mcs.anl.gov <petsc-users@mcs.anl.gov><br>
<b>Θέμα:</b> Re: [petsc-users] Question about a parallel implementation of PCFIELDSPLIT</font>
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<div dir="ltr">On Fri, Jan 19, 2024 at 4:25 PM Barry Smith <<a href="mailto:bsmith@petsc.dev">bsmith@petsc.dev</a>> wrote:<br>
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Generally fieldsplit is used on problems that have a natural "split" of the variables into two or more subsets. For example u0,v0,u1,v1,u2,v2,u3,v4 This is often indicated in the vectors and matrices with the "blocksize" argument, 2 in this case. DM also
often provides this information.
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<div> When laying out a vector/matrix with a blocksize one must ensure that an equal number of of the subsets appears on each MPI process. So, for example, if the above vector is distributed over 3 MPI processes one could use u0,v0,u1,v1 u2,v2
u3,v3 but one cannot use u0,v0,u1 v1,u2,v2 u3,v3. Another way to think about it is that one must split up the vector as indexed by block among the processes. For most multicomponent problems this type of decomposition is very natural in the logic of
the code.</div>
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<div>This blocking is only convenient, not necessary. You can specify your own field division using PCFieldSplitSetIS().</div>
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<div> Thanks,</div>
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<div> Matt</div>
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<div> Barry</div>
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<div>On Jan 19, 2024, at 3:19 AM, Pantelis Moschopoulos <<a href="mailto:pmoschopoulos@outlook.com" target="_blank">pmoschopoulos@outlook.com</a>> wrote:</div>
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Dear all,</div>
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<span style="font-family:Aptos,Aptos_EmbeddedFont,Aptos_MSFontService,Calibri,Helvetica,sans-serif; font-size:12pt">When I am using PCFIELDSPLIT and pc type "schur" in serial mode everything works fine. When I turn now to parallel, I observe that the number
of ranks that I can use must divide the number of N without any remainder, where N is the number of unknowns. Otherwise, an error of the following form emerges: "Local columns of A10 3473 do not equal local rows of A00 3471".</span></div>
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<span style="font-family:Aptos,Aptos_EmbeddedFont,Aptos_MSFontService,Calibri,Helvetica,sans-serif; font-size:12pt">Can I do something to overcome this?</span></div>
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</span></div>
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<span style="font-family:Aptos,Aptos_EmbeddedFont,Aptos_MSFontService,Calibri,Helvetica,sans-serif; font-size:12pt">Thanks,<br>
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<span style="font-family:Aptos,Aptos_EmbeddedFont,Aptos_MSFontService,Calibri,Helvetica,sans-serif; font-size:12pt">Pantelis</span></div>
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<span class="x_gmail_signature_prefix">-- </span><br>
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<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>
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<div><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br>
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