<div dir="ltr"><div dir="ltr">On Mon, Mar 14, 2022 at 11:05 AM liluo <<a href="mailto:liluo@um.edu.mo">liluo@um.edu.mo</a>> wrote:<br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p><span style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt">Dear developers,</span></p>
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<p><span style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt">I defined subdomain problems that have some layers of </span><span style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt">ghost points, which are exactly of </span><span style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt">the
"local</span><span style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt">" size of a DA.</span></p>
<p><span style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt">And I want to generate the corresponding submatrix in each subdomain which contains those layers of ghost points.</span></p>
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<p><span style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt">I know that </span><span style="color:rgb(33,33,33);font-family:Calibri,Helvetica,sans-serif;font-size:12pt">MatGetSubMatrices can return a submatrix when determining the
index set like PCASM, but how can I get the index set including those layers of ghost points? When a DA is setup, such IS for creating ltog mapping was already destroyed.</span></p>
<p><span style="color:rgb(33,33,33);font-family:Calibri,Helvetica,sans-serif;font-size:12pt">Or you may have some simpler way to get this?</span></p></div></div></blockquote><div>You can use <a href="https://petsc.org/main/docs/manualpages/IS/ISLocalToGlobalMappingGetIndices.html">https://petsc.org/main/docs/manualpages/IS/ISLocalToGlobalMappingGetIndices.html</a> to get the indices out of the L2G map, and then call MatGetSubmatrix().</div><div><br></div><div> Thanks,</div><div><br></div><div> Matt <br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div><div id="gmail-m_3329168753258936637divtagdefaultwrapper" style="font-size:12pt;color:rgb(0,0,0);font-family:Calibri,Helvetica,sans-serif" dir="ltr">
<p><span style="color:rgb(33,33,33);font-family:Calibri,Helvetica,sans-serif;font-size:12pt">Bests,</span></p>
<p><span style="color:rgb(33,33,33);font-family:Calibri,Helvetica,sans-serif;font-size:12pt">Li Luo</span></p>
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<div id="gmail-m_3329168753258936637divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Mark Adams <<a href="mailto:mfadams@lbl.gov" target="_blank">mfadams@lbl.gov</a>><br>
<b>Sent:</b> Monday, 14 March, 2022 22:49:47<br>
<b>To:</b> liluo<br>
<b>Cc:</b> <a href="mailto:petsc-users@mcs.anl.gov" target="_blank">petsc-users@mcs.anl.gov</a><br>
<b>Subject:</b> Re: [petsc-users] How to obtain the local matrix from a global matrix in DMDA?</font>
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<div dir="ltr">You might want to explain what you want to do.
<div>A local vector has ghost values that you can use to compute local residuals. You don't usually want a matrix that conforms with this local vector.</div>
<div>You could get the global equations of the local vector and use MatCreateSubMatrix, but I think there is a better way to do what you want.</div>
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<div>OK, Matt and Jed beat me</div>
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<div dir="ltr" class="gmail_attr">On Mon, Mar 14, 2022 at 10:35 AM liluo <<a href="mailto:liluo@um.edu.mo" target="_blank">liluo@um.edu.mo</a>> wrote:<br>
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<p>Dear developers,</p>
<p><br>
</p>
<p>I created a DMDA object and obtain the local vector from a global vector by using <span>DMGlobalToLocalBegin/End.</span></p>
<p>How can I obtain the corresponding local matrix from the global matrix? </p>
<p>In my case, the global matrix is of MPIBAIJ type created by <span>DMCreateMatrix(da,MATBAIJ,&A);</span><br>
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<p>Bests,</p>
<p>LI</p>
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</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr" class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><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><div><br></div><div><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br></div></div></div></div></div></div></div></div>