[petsc-users] DIVERGED_INDEFINITE_PC in algebraic multigrid

[Mark Adams]

On Mon, May 26, 2014 at 12:20 PM, Michele Rosso <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> wrote:
>
>> Michele Rosso <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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