[petsc-users] Inquiry about the setup for multigrid as a preconditioner in Petsc.

[Xiaodong Liu]

Thanks. It is very helpful to make me understand the whole process.
Xiaodong Liu, PhD
X: Computational Physics Division
Los Alamos National Laboratory
P.O. Box 1663,
Los Alamos, NM 87544
505-709-0534


On Thu, Mar 12, 2020 at 3:22 PM Dave May <dave.mayhem23 at gmail.com> wrote:

> You want to look at the bottom of each of these web pages
>
>
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMCreateInjection.html
>
>
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMCreateInterpolation.html
>
>
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DMDA/DMCreateInterpolationScale.html
>
> At the bottom you will see URLs to the current set of DM implementations
> which implement Injection, Interpolation.
>
> Thanks
> Dave
>
> On Thu, 12 Mar 2020 at 21:14, Xiaodong Liu <xliu29 at ncsu.edu> wrote:
>
>> Hi, all,
>>
>> I am practising multigrid as a preconditioner in Petsc. From the previous
>> resource, there are 2 main ways to set up the multigrid preconditioner,
>>
>> 1). For general cae,
>> KSPCreate(MPI Comm comm,KSP *ksp);
>> KSPGetPC(KSP ksp,PC *pc);
>> PCSetType(PC pc,PCMG);
>> PCMGSetLevels(pc,int levels,MPI Comm *comms);
>> PCMGSetType(PC pc,PCMGType mode);
>> PCMGSetCycleType(PC pc,PCMGCycleType ctype);
>> ...
>> PCMGSetInterpolation(PC pc,int level,Mat P);
>> PCMGSetRestriction(PC pc,int level,Mat R);
>>
>> The above means that I need to specify a lot details, e.g., cycletype.
>> interpolation and restriction matrix, coarse solver, etc.
>>
>> 2) For the case of structured mesh, (DMDA is enough)
>> Taking the following case as an example,
>>
>> https://www.mcs.anl.gov/petsc/petsc-3.6/src/ksp/ksp/examples/tutorials/ex25.c.html
>>
>>  50:   KSPCreate(PETSC_COMM_WORLD,&ksp);
>>  51:   DMDACreate1d(PETSC_COMM_WORLD,DM_BOUNDARY_NONE,-3,1,1,0,&da);
>>  52:   KSPSetDM(ksp,da);
>>  53:   KSPSetComputeRHS(ksp,ComputeRHS,&user);
>>  54:   KSPSetComputeOperators(ksp,ComputeMatrix,&user);
>>  55:   KSPSetFromOptions(ksp);
>>  56:   KSPSolve(ksp,NULL,NULL);
>>
>> DMDA handles all the multigrid setting automatically, e.g., interpolation
>> and restriction matrix.
>> If my understanding is right, *my question is where to find these source
>> file to define these default interpolation and restriction matrix. *
>>
>> Thanks,S
>> Xiaodong Liu, PhD
>> X: Computational Physics Division
>> Los Alamos National Laboratory
>> P.O. Box 1663,
>> Los Alamos, NM 87544
>> 505-709-0534
>>
>
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