<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><br><div>Begin forwarded message:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;"><span style="font-family:'Helvetica'; font-size:medium; color:rgba(0, 0, 0, 1.0);"><b>From: </b></span><span style="font-family:'Helvetica'; font-size:medium;">"Christon, Mark A" <<a href="mailto:christon@lanl.gov">christon@lanl.gov</a>><br></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;"><span style="font-family:'Helvetica'; font-size:medium; color:rgba(0, 0, 0, 1.0);"><b>Subject: </b></span><span style="font-family:'Helvetica'; font-size:medium;"><b>Re: [petsc-users] Any changes in ML usage between 3.1-p8 -> 3.3-p6?</b><br></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;"><span style="font-family:'Helvetica'; font-size:medium; color:rgba(0, 0, 0, 1.0);"><b>Date: </b></span><span style="font-family:'Helvetica'; font-size:medium;">April 17, 2013 7:06:11 PM EDT<br></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;"><span style="font-family:'Helvetica'; font-size:medium; color:rgba(0, 0, 0, 1.0);"><b>To: </b></span><span style="font-family:'Helvetica'; font-size:medium;">"Mark F. Adams" <<a href="mailto:mark.adams@columbia.edu">mark.adams@columbia.edu</a>>, "Bakosi, Jozsef" <<a href="mailto:jbakosi@lanl.gov">jbakosi@lanl.gov</a>><br></span></div><br>
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<div>Hi Mark,</div>
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<div>Yes, looks like the new version does a little better after 2 iterations, but at the 8th iteration, the residuals increase:(</div>
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<div>I suspect this is why PETSc is whining about an indefinite preconditioner. </div>
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<div>Something definitely changes as we've had about 6-8 regression tests start failing that have been running flawlessly with ML + PETSc 3.1-p8 for almost two years.</div>
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<div>If we can understand what changed, we probably have a fighting chance of correcting it — assuming it's some solver setting for PETSc that we're not currently using.</div>
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<div>- Mark</div>
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<div>-- </div>
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<span style="font-weight:bold">From: </span>"Mark F. Adams" <<a href="mailto:mark.adams@columbia.edu">mark.adams@columbia.edu</a>><br>
<span style="font-weight:bold">Date: </span>Wed, 17 Apr 2013 18:51:02 -0400<br>
<span style="font-weight:bold">To: </span>PETSc users list <<a href="mailto:petsc-users@mcs.anl.gov">petsc-users@mcs.anl.gov</a>><br>
<span style="font-weight:bold">Cc: </span>"Mark A. Christon" <<a href="mailto:christon@lanl.gov">christon@lanl.gov</a>><br>
<span style="font-weight:bold">Subject: </span>Re: [petsc-users] Any changes in ML usage between 3.1-p8 -> 3.3-p6?<br>
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<div>I see you are using icc. Perhaps our icc changed a bit between versions. These results look like both solves are working and the old does a little better (after two iterations).</div>
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<div>Try using jacobi instead of icc.</div>
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<div>On Apr 17, 2013, at 6:21 PM, Jozsef Bakosi <<a href="mailto:jbakosi@lanl.gov">jbakosi@lanl.gov</a>> wrote:</div>
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<div>On 04.17.2013 15:38, Matthew Knepley wrote:</div>
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<div>On 04.17.2013 14:26, Jozsef Bakosi wrote:</div>
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<div>Mark F. Adams mark.adams at <a href="http://columbia.edu">columbia.edu</a></div>
<div>Wed Apr 17 14:25:04 CDT 2013</div>
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<div>2) If you get "Indefinite PC" (I am guessing from using CG) it is because the</div>
<div>preconditioner</div>
<div> really is indefinite (or possible non-symmetric). We improved the checking</div>
<div> for this in one</div>
<div> of those releases.</div>
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<div>AMG does not guarantee an SPD preconditioner so why persist in trying to use</div>
<div>CG?</div>
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<div>AMG is positive if everything is working correctly.</div>
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<div>Are these problems only semidefinite? Singular systems can give erratic</div>
<div>behavior.</div>
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<div>It is a Laplace operator from Galerkin finite elements. And the PC is fine on</div>
<div>ranks 1, 2, 3, and 5 -- indefinite only on 4. I think we can safely say that the</div>
<div>same PC should be positive on 4 as well.</div>
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<div>Why is it safe? Because it sounds plausible? Mathematics is replete with things</div>
<div>that sound plausible and are false. Are there proofs that suggest this? Is there</div>
<div>computational evidence? Why would I believe you?</div>
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<div>Okay, so here is some additional information:</div>
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<div>I tried both old and new PETSc versions again, but now only taking 2 iterations</div>
<div>(both with 4 CPUs) and checked the residuals. I get the same exact PC from ML in</div>
<div>both cases, however, the residuals are different after both iterations:</div>
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<div>Please do a diff on the attached files and you can verify that the ML</div>
<div>diagnostics are exactly the same: same max eigenvalues, nodes aggregated, etc,</div>
<div>while the norm coming out of the solver at the end at both iterations are</div>
<div>different.</div>
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<div>We reproduced the same exact behavior on two different linux platforms.</div>
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<div>Once again: same application source code, same ML source code, different PETSc:</div>
<div>3.1-p8 vs. 3.3-p6.</div>
<div><old.out><new.out></div>
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