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Qin:</div>
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PCFactorReorderForNonzeroDiagonal()</div>
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Hong</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> petsc-users <petsc-users-bounces@mcs.anl.gov> on behalf of Qin Lu via petsc-users <petsc-users@mcs.anl.gov><br>
<b>Sent:</b> Thursday, July 9, 2020 10:05 AM<br>
<b>To:</b> Mark Adams <mfadams@lbl.gov>; Barry Smith <bsmith@petsc.dev>; nathaniel.collier@gmail.com <nathaniel.collier@gmail.com><br>
<b>Cc:</b> PETSc Users List <petsc-users@mcs.anl.gov><br>
<b>Subject:</b> Re: [petsc-users] Zero diagonal term for direct solver</font>
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<div dir="ltr">Thanks for all your input!</div>
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<div dir="ltr">How can I set these options in the program instead of as runtime options (in the .petsc file)? For example, which subroutine shall I call to set '<span style="color:rgb(38,40,42)">-pc_factor_nonzeros_along_diagonal", and what is corresponding
argument value?</span></div>
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<div dir="ltr"><span style="color:rgb(38,40,42)">Thanks,</span></div>
<div dir="ltr"><span style="color:rgb(38,40,42)">Qin</span></div>
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<div>On Wednesday, July 8, 2020, 02:15:11 PM CDT, Barry Smith <bsmith@petsc.dev> wrote:
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There is also -pc_factor_nonzeros_along_diagonal which reorders to move the zeros off the diagonal before the numerical factorization
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<div class="x_ydp23091541yiv0981914330"> But if having 0 leads to a factorization without error then it will produce the correct answer (this means that zero got filled in during the factorization)<br clear="none" class="x_ydp23091541yiv0981914330">
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<div class="x_ydp23091541yiv0981914330">On Jul 8, 2020, at 2:11 PM, Mark Adams <<a shape="rect" class="x_ydp23091541yiv0981914330" href="mailto:mfadams@lbl.gov" rel="nofollow" target="_blank">mfadams@lbl.gov</a>> wrote:</div>
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<div class="x_ydp23091541yiv0981914330gmail_attr" dir="ltr">On Wed, Jul 8, 2020 at 2:43 PM Qin Lu via petsc-users <<a shape="rect" class="x_ydp23091541yiv0981914330" href="mailto:petsc-users@mcs.anl.gov" rel="nofollow" target="_blank">petsc-users@mcs.anl.gov</a>>
wrote:<br clear="none" class="x_ydp23091541yiv0981914330">
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<div class="x_ydp23091541yiv0981914330" dir="ltr">Hello,</div>
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<div class="x_ydp23091541yiv0981914330" dir="ltr">I am using the Petsc native direct solver
<font class="x_ydp23091541yiv0981914330" face="Helvetica, Arial, sans-serif">(with KSPPREONLY and PCLU) to solver a small Linear equation system. The matrix has a couple of diagonal terms with value 0, which makes the solver fail. Then I set a very small value
(e.g., 1e-10) at these terms and the solver works. Can this cause noticeable errors in the solution?
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<div class="x_ydp23091541yiv0981914330">Can't say.</div>
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<div class="x_ydp23091541yiv0981914330" dir="ltr"><font class="x_ydp23091541yiv0981914330" face="Helvetica, Arial, sans-serif">Are there any options in Petsc to make the direct solver do row/column pivoting so that all the diagonal terms are non-zero?</font></div>
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<div class="x_ydp23091541yiv0981914330">I see:</div>
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-pc_factor_pivot_in_blocks <true,false>- allow pivoting within the small blocks during factorization (may increase stability of factorization.<br clear="none" class="x_ydp23091541yiv0981914330">
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and<br clear="none" class="x_ydp23091541yiv0981914330">
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-pc_factor_shift_type <shifttype>- Sets shift type or PETSC_DECIDE for the default; use '-help' for a list of available types<br clear="none" class="x_ydp23091541yiv0981914330">
-pc_factor_shift_amount <shiftamount>- Sets shift amount or PETSC_DECIDE for the default<br clear="none" class="x_ydp23091541yiv0981914330">
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<div class="x_ydp23091541yiv0981914330" dir="ltr"><font class="x_ydp23091541yiv0981914330" face="Helvetica, Arial, sans-serif">Thanks for any suggestions.</font></div>
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<div class="x_ydp23091541yiv0981914330" dir="ltr"><font class="x_ydp23091541yiv0981914330" face="Helvetica, Arial, sans-serif">Regards,</font></div>
<div class="x_ydp23091541yiv0981914330" dir="ltr"><font class="x_ydp23091541yiv0981914330" face="Helvetica, Arial, sans-serif">Qin</font></div>
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