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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I think I understand; does this mean that it is possible to change the sub PC type to SOR? Or is there a specific reason why the sub type is ILU?
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The reason why I am asking, is that when I look at the residuals coming out of PCMG, they seem to be largest on local domain boundaries (corresponding to the
partitions). Therefore, I’d like to use ASM with overlap instead of SOR as the preconditioner on the coarser levels (to smooth these errors out), but with the ASM/ILU combination this does not work at all (in accordance with your comment on high frequencies).
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks again,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Erik<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Matthew Knepley [mailto:knepley@gmail.com]
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<b>Sent:</b> 17. september 2015 14:12<br>
<b>To:</b> Erik Andreassen<br>
<b>Cc:</b> petsc-users@mcs.anl.gov<br>
<b>Subject:</b> Re: [petsc-users] Preconditioner question ASM vs SOR<o:p></o:p></span></p>
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<p class="MsoNormal">On Thu, Sep 17, 2015 at 7:07 AM, Erik Andreassen <<a href="mailto:erand@mek.dtu.dk" target="_blank">erand@mek.dtu.dk</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I have played a bit with preconditioners for the different levels in PCMG, and among others I have tried additive Schwartz (ASM), which does not seem to work very well – especially
with no overlap. I’m curious to hear if anyone can explain why ASM with no overlap performs so much worse than SOR?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The point of a smoother is to wipe out the high frequencies in the error. SOR does this (provably for the Laplacian), but there is no<o:p></o:p></p>
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<p class="MsoNormal">reason ASM/ILU should do this. If this does not happen, then these parts of the solution remain uncorrected when you go to the<o:p></o:p></p>
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<p class="MsoNormal">coarse grid, and the convergence breaks down.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> Matt<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thanks,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Erik<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">PS: I had a previous question about setting levels in GAMG, and my first impression is (in line with what Mark Adams wrote) that it is better to leave it to figure it out automatically.
Also when using it as the coarse level preconditioner in a PCMG.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-- <o:p></o:p></p>
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<p class="MsoNormal">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<o:p></o:p></p>
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