<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div dir="ltr"></div><div dir="ltr"><br></div><div dir="ltr"><br><blockquote type="cite">On 16 Feb 2023, at 8:43 AM, user_gong Kim <ksi2443@gmail.com> wrote:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr"><div dir="ltr"><p class="MsoNormal" style="margin:0cm 0cm 8pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US"> </span></p>
<p class="MsoNormal" style="margin:0cm 0cm 8pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">Hello,</span></p>
<p class="MsoNormal" style="margin:0cm 0cm 8pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US"> </span></p>
<p class="MsoNormal" style="margin:0cm 0cm 8pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">There are some questions about some preconditioners.</span></p>
<p class="MsoNormal" style="margin:0cm 0cm 8pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">The questions are from problem Au=b. The global
matrix A has zero value diagonal terms.</span></p>
<p class="gmail-MsoListParagraph" style="margin:0cm 0cm 8pt 38pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">1.<span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-stretch:normal;font-size:7pt;line-height:normal;font-family:"Times New Roman"">
</span></span><span lang="EN-US">Which preconditioner is preferred
for matrix A which has zero value in diagonal terms?<br></span></p></div></div></blockquote><div><br></div><div>This question has not a single answer. It all depends on where your A and b are coming from.</div><br><blockquote type="cite"><div dir="ltr"><div dir="ltr"><p class="gmail-MsoListParagraph" style="margin:0cm 0cm 8pt 38pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">
The most frequently used basic 2 preconditioners are jacobi and SOR (gauss seidel).
</span></p></div></div></blockquote><div>They are not the most frequently used. And rightfully so, as they very often can’t handle non-trivial systems.</div><br><blockquote type="cite"><div dir="ltr"><div dir="ltr"><p class="gmail-MsoListParagraph" style="margin:0cm 0cm 8pt 38pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">As people knows both methods should have non zero diagonal terms. Although the
improved method is applied in PETSc, jacobi can also solve the case with zero
diagonal term, but I ask because I know that it is not recommended.</span></p>
<p class="gmail-MsoListParagraph" style="margin:0cm 0cm 8pt 38pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">2.<span style="font-variant-numeric:normal;font-variant-east-asian:normal;font-stretch:normal;font-size:7pt;line-height:normal;font-family:"Times New Roman"">
</span></span><span lang="EN-US">Second question is about running
code with the two command options below in a single process.<br>
1<sup>st</sup> command : -ksp_type gmres -pc_type bjacobi -sub_pc_type jacobi<br>
2<sup>nd</sup> command : -ksp_type gmres -pc_type hpddm -sub_pc_type jacobi<br>
When domain decomposition methods such as bjacobi or hpddm are parallel, the
global matrix is divided for each process. As far as I know, running it in a
single process should eventually produce the same result if the sub pc type is
the same. However, in the second option, ksp did not converge.<br></span></p></div></div></blockquote><div>1st command: it’s pointless to couple PCBJACOBI with PCJABOCI, it’s equivalent to only using PCJABOBI.</div><div>2nd command: it’s pointless to use PCHPDDM if you don’t specify in some way how to coarsen your problem (either algebraically or via an auxiliary operator). You just have a single level (equivalent to PCBJACOBI), but its options are prefixed by -pc_hpddm_coarse_ instead of -sub_</div><div>Again, both sets of options do not make sense.</div><div>If you want, you could share your A and b (or tell us what you are discretizing) and we will be able to provide a better feedback.</div><div><br></div><div>Thanks,</div><div>Pierre</div><br><blockquote type="cite"><div dir="ltr"><div dir="ltr"><p class="gmail-MsoListParagraph" style="margin:0cm 0cm 8pt 38pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">
In this case, I wonder how to analyze the situation.<br>
How can I monitor and see the difference between the two?</span></p>
<p class="MsoNormal" style="margin:0cm 0cm 8pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US"> </span></p>
<p class="MsoNormal" style="margin:0cm 0cm 8pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US"> </span></p>
<p class="MsoNormal" style="margin:0cm 0cm 8pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">Thanks,</span></p>
<p class="MsoNormal" style="margin:0cm 0cm 8pt;text-align:justify;line-height:107%;font-size:10pt;font-family:"\00b9d1\00c740 \00ace0\00b515""><span lang="EN-US">Hyung Kim</span></p></div>
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