[petsc-users] DIVERGED_INDEFINITE_PC in algebraic multigrid

[Michele Rosso]

Thanks Mark! I will try and let you know.

On 05/28/2014 07:54 PM, Mark Adams wrote:
>
>
>
> On Mon, May 26, 2014 at 12:20 PM, Michele Rosso <mrosso at uci.edu 
> <mailto:mrosso at uci.edu>> wrote:
>
>     Mark,
>
>     thank you for your input and sorry my late reply: I saw your email
>     only now.
>     By setting up the solver each time step you mean re-defining the
>     KSP context every time?
>
>
> THe simplest thing is to just delete the object and create it again. 
>  THere are "reset" methods that do the same thing semantically but it 
> is probably just easier to destroy the KSP object and recreate it and 
> redo your setup code.
>
>     Why should this help?
>
>
> AMG methods optimized for a particular operator but "stale" setup data 
> often work well on problems that evolve, at least for a while, and it 
> saves a lot of time to not redo the "setup" every time.  How often you 
> should "refresh" the setup data is problem dependant and the 
> application needs to control that.  There are some hooks to fine tune 
> how much setup data is recomputed each solve, but we are just trying 
> to see if redoing the setup every time helps.  If this fixes the 
> problem then we can think about cost.  If it does not fix the problem 
> then it is more serious.
>
>     I will definitely try that as well as the hypre solution and
>     report back.
>     Again, thank you.
>
>     Michele
>
>
>     On 05/22/2014 09:34 AM, Mark Adams wrote:
>>     If the solver is degrading as the coefficients change, and I
>>     would assume get more nasty, you can try deleting the solver at
>>     each time step.  This will be about 2x more expensive, because it
>>     does the setup each solve, but it might fix your problem.
>>
>>     You also might try:
>>
>>     -pc_type hypre
>>     -pc_hypre_type boomeramg
>>
>>
>>
>>
>>     On Mon, May 19, 2014 at 6:49 PM, Jed Brown <jed at jedbrown.org
>>     <mailto:jed at jedbrown.org>> wrote:
>>
>>         Michele Rosso <mrosso at uci.edu <mailto:mrosso at uci.edu>> writes:
>>
>>         > Jed,
>>         >
>>         > thank you very much!
>>         > I will try with ///-mg_levels_ksp_type chebyshev
>>         -mg_levels_pc_type
>>         > sor/   and report back.
>>         > Yes, I removed the nullspace from both the system matrix
>>         and the rhs.
>>         > Is there a way to have something similar to Dendy's
>>         multigrid or the
>>         > deflated conjugate gradient method with PETSc?
>>
>>         Dendy's MG needs geometry.  The algorithm to produce the
>>         interpolation
>>         operators is not terribly complicated so it could be done,
>>         though DMDA
>>         support for cell-centered is a somewhat awkward.  "Deflated
>>         CG" can mean
>>         lots of things so you'll have to be more precise.  (Most
>>         everything in
>>         the "deflation" world has a clear analogue in the MG world,
>>         but the
>>         deflation community doesn't have a precise language to talk
>>         about their
>>         methods so you always have to read the paper carefully to
>>         find out if
>>         it's completely standard or if there is something new.)
>>
>>
>
>

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