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See <a href="https://petsc.org/release/docs/manualpages/Mat/MatMPIAIJSetPreallocation.html#MatMPIAIJSetPreallocation" id="LPlnkOWALinkPreview">https://petsc.org/release/docs/manualpages/Mat/MatMPIAIJSetPreallocation.html#MatMPIAIJSetPreallocation</a></div>
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<a target="_blank" id="LPUrlAnchor175907" href="https://petsc.org/release/docs/manualpages/Mat/MatMPIAIJSetPreallocation.html#MatMPIAIJSetPreallocation" style="text-decoration: none; color: var(--themePrimary);">MatMPIAIJSetPreallocation - PETSc</a></div>
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MatMPIAIJSetPreallocation Preallocates memory for a sparse parallel matrix in AIJ format (the default parallel PETSc format). For good matrix assembly performance the user should preallocate the matrix storage by setting the parameters d_nz (or d_nnz) and o_nz
(or o_nnz).</div>
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petsc.org</div>
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if you use mpiaij matrix.</div>
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Hong</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> petsc-users <petsc-users-bounces@mcs.anl.gov> on behalf of Patrick Alken <patrick.alken@geomag.info><br>
<b>Sent:</b> Monday, August 22, 2022 10:56 AM<br>
<b>To:</b> petsc-users <petsc-users@mcs.anl.gov><br>
<b>Subject:</b> [petsc-users] Preallocating a matrix with the d_nnz and o_nnz arrays</font>
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<div class="PlainText">I am looping through all the non-zero elements of my matrix to count the
<br>
diagonal and off-diagonal terms in order to pre-allocate the matrix. <br>
However, what is the criteria for knowing if a given element is in the <br>
diagonal portion or off-diagonal portion? It seems that it will depend <br>
on row, column, and also processor. I was doing something like this:<br>
<br>
MatGetLocalSize(A, &m, &n);<br>
<br>
MatGetOwnershipRange(A, &first, &last);<br>
<br>
for (i = first; i < last; ++i) {<br>
<br>
for (j = 0; j < N; ++j) {<br>
<br>
if (abs(i - j) < m)<br>
<br>
++d_nnz[i - first];<br>
<br>
else<br>
<br>
++o_nnz[i - first];<br>
<br>
}<br>
<br>
}<br>
<br>
However, the criteria: abs(i - j) < m is not correct. I think the <br>
correct criteria will include the processor number also. Does anyone <br>
know the correct criteria to use?<br>
<br>
</div>
</span></font></div>
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