<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div><br></div> If your matrix has the form <div><br></div><div>( A B )</div><div>( C 0 )</div><div><br></div><div>then often PCFIELDSPLIT can be a useful preconditioner with the option -pc_fieldsplit_detect_saddle_point</div><div><br></div><div><br><div><br><blockquote type="cite"><div>On Feb 16, 2023, at 2:42 AM, user_gong Kim <ksi2443@gmail.com> wrote:</div><br class="Apple-interchange-newline"><div><div dir="ltr"><div style="margin: 0cm 0cm 8pt; text-align: justify; line-height: 107%; font-size: 10pt; font-family: "맑은 고딕";"><span lang="EN-US"> </span><br class="webkit-block-placeholder"></div><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><div style="margin: 0cm 0cm 8pt; text-align: justify; line-height: 107%; font-size: 10pt; font-family: "맑은 고딕";"><span lang="EN-US"> </span><br class="webkit-block-placeholder"></div><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>
The most frequently used basic 2 preconditioners are jacobi and SOR (gauss seidel).
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>
In this case, I wonder how to analyze the situation.<br>
How can I monitor and see the difference between the two?</span></p><div style="margin: 0cm 0cm 8pt; text-align: justify; line-height: 107%; font-size: 10pt; font-family: "맑은 고딕";"><span lang="EN-US"> </span><br class="webkit-block-placeholder"></div><div style="margin: 0cm 0cm 8pt; text-align: justify; line-height: 107%; font-size: 10pt; font-family: "맑은 고딕";"><span lang="EN-US"> </span><br class="webkit-block-placeholder"></div><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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