Begin forwarded message:
> From: "Christon, Mark A" <christon at lanl.gov> > Subject: Re: [petsc-users] Any changes in ML usage between 3.1-p8 -> 3.3-p6? > Date: April 17, 2013 7:06:11 PM EDT > To: "Mark F. Adams" <mark.adams at columbia.edu>, "Bakosi, Jozsef" <jbakosi > at lanl.gov> > > Hi Mark, > > Yes, looks like the new version does a little better after 2 iterations, but > at the 8th iteration, the residuals increase:( > > I suspect this is why PETSc is whining about an indefinite preconditioner. > > 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. > > 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. > > - Mark > > -- > Mark A. Christon > Computational Physics Group (CCS-2) > Computer, Computational and Statistical Sciences Division > Los Alamos National Laboratory > MS D413, P.O. Box 1663 > Los Alamos, NM 87545 > > E-mail: christon at lanl.gov > Phone: (505) 663-5124 > Mobile: (505) 695-5649 (voice mail) > > International Journal for Numerical Methods in Fluids > > From: "Mark F. Adams" <mark.adams at columbia.edu> > Date: Wed, 17 Apr 2013 18:51:02 -0400 > To: PETSc users list <petsc-users at mcs.anl.gov> > Cc: "Mark A. Christon" <christon at lanl.gov> > Subject: Re: [petsc-users] Any changes in ML usage between 3.1-p8 -> 3.3-p6? > >> 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). >> >> Try using jacobi instead of icc. >> >> >> On Apr 17, 2013, at 6:21 PM, Jozsef Bakosi <jbakosi at lanl.gov> wrote: >> >>>> On 04.17.2013 15:38, Matthew Knepley wrote: >>>>> On 04.17.2013 14:26, Jozsef Bakosi wrote: >>>>>> Mark F. Adams mark.adams at columbia.edu >>>>>> Wed Apr 17 14:25:04 CDT 2013 >>>>>> 2) If you get "Indefinite PC" (I am guessing from using CG) it is >>>>>> because the >>>>>> preconditioner >>>>>> really is indefinite (or possible non-symmetric). We improved the >>>>>> checking >>>>>> for this in one >>>>>> of those releases. >>>>>> AMG does not guarantee an SPD preconditioner so why persist in trying to >>>>>> use >>>>>> CG? >>>>>> AMG is positive if everything is working correctly. >>>>>> Are these problems only semidefinite? Singular systems can give erratic >>>>>> behavior. >>>>> It is a Laplace operator from Galerkin finite elements. And the PC is >>>>> fine on >>>>> ranks 1, 2, 3, and 5 -- indefinite only on 4. I think we can safely say >>>>> that the >>>>> same PC should be positive on 4 as well. >>>> Why is it safe? Because it sounds plausible? Mathematics is replete with >>>> things >>>> that sound plausible and are false. Are there proofs that suggest this? Is >>>> there >>>> computational evidence? Why would I believe you? >>> Okay, so here is some additional information: >>> I tried both old and new PETSc versions again, but now only taking 2 >>> iterations >>> (both with 4 CPUs) and checked the residuals. I get the same exact PC from >>> ML in >>> both cases, however, the residuals are different after both iterations: >>> Please do a diff on the attached files and you can verify that the ML >>> diagnostics are exactly the same: same max eigenvalues, nodes aggregated, >>> etc, >>> while the norm coming out of the solver at the end at both iterations are >>> different. >>> We reproduced the same exact behavior on two different linux platforms. >>> Once again: same application source code, same ML source code, different >>> PETSc: >>> 3.1-p8 vs. 3.3-p6. >>> <old.out><new.out> >> >> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130417/8a50f9b7/attachment-0001.html>
