[petsc-dev] Are odd grid numbers intentionally required for structured interpolation/refinement in 3.2.2?

[Aron Ahmadia]

We ended up recording mostly login information into getting into the
supercomputer, but the slides are available here:
http://aron.ahmadia.net/amcs312/petsc-ex50/

What is the PETSc idiom for grabbing the fine level solution after using
SNESGridSequence?  I could not find any documentation about where the
solutions, and the only way I've been able to do it so far is to set a
SNESMonitor and save the grid out to disk when I am at high enough
resolution.

A

On Sun, Oct 9, 2011 at 9:33 PM, Aron Ahmadia <aron.ahmadia at kaust.edu.sa>wrote:

> Thanks guys, I will try to send you pictures/video after the demonstration
> this Wednesday.
>
> -Aron
>
>
> On Sun, Oct 9, 2011 at 5:54 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>>   Note also you can use -da_refine <n>  Then it takes the initial grid
>> sizes and refines n times before creating the DA, then it does multigrid
>> with n+1 levels. If you do it this way you don't have to mess around with
>> figuring what numbers to put in -da_grid_x 127   or whatever.
>>
>>    Barry
>>
>>
>>
>> On Oct 9, 2011, at 8:00 AM, Jed Brown wrote:
>>
>> > On Sun, Oct 9, 2011 at 05:23, Aron Ahmadia <aron.ahmadia at kaust.edu.sa>
>> wrote:
>> > Jed pointed out that I should be demo-ing ex50, not ex19, so I tried to
>> port my stuff over.  Oddly enough, if I am using 2^n grid spacing,
>> everything falls over. 2^n+1 works, though, which seems non-obvious to me.
>> >
>> > --- petsc-ex50/ex50 ‹master*➔ M⁇› » mpirun -n 4 ./ex50 -da_grid_x 65
>> -da_grid_y 65 -pc_type mg -pc_mg_levels 3
>> > lid velocity = 0.000236686, prandtl # = 1, grashof # = 1
>> > Number of Newton iterations = 6
>> >
>> > ./ex50 -da_grid_x 64 -da_grid_y 64 -pc_type mg -pc_mg_levels 2
>> >
>> > lid velocity = 0.000244141, prandtl # = 1, grashof # = 1
>> > [0]PETSC ERROR: --------------------- Error Message
>> ------------------------------------
>> > [0]PETSC ERROR: Arguments are incompatible!
>> > [0]PETSC ERROR: Ratio between levels: (mx - 1)/(Mx - 1) must be integer:
>> mx 64 Mx 32!
>> >
>> > DMMG always started with a coarse problem and refined a given number of
>> times regardless of whether linear or nonlinear multigrid is being used.
>> >
>> > Now we have
>> >
>> > -snes_grid_sequence NLEVELS
>> >
>> > which starts with whatever grid you give it and refines. If you use
>> -snes_grid_sequence N -pc_type mg, then the number of levels in PCMG will be
>> automatically changed inside the grid sequencing so that it references the
>> same coarse level.
>> >
>> > If instead, you only do linear multigrid, then you pass
>> >
>> > -pc_type mg -pc_mg_levels N
>> >
>> > and your specified problem is _coarsened_ to produce the number of
>> levels. That coarsening requires some divisbility in the grid size. The
>> reason it is 2^n+1 instead of 2^n is because this example uses a
>> vertex-centered instead cell-centered discretization. This is natural for
>> finite differences instead of finite volumes, but either method could be
>> used with either centering strategy.
>>
>>
>
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