[petsc-users] Hybrid multigrid

[Anton Popov]

Thanks Barry!

Best,

Anton

On 10.04.25 21:17, Barry Smith wrote:
>   Sounds correct.
>
>> On Apr 10, 2025, at 2:04 PM, Anton Popov <popov at uni-mainz.de> wrote:
>>
>>
>> Hi guys,
>>
>> I have a custom multigrid preconditioner that uses Galerkin 
>> coarsening for the staggered grid finite difference. Now I want to 
>> replace a few top level operators with the matrix-free shell matrices.
>>
>> Here is my plan:
>>
>> 1. Set PC_MG_GALERKIN_NONE
>>
>> 2. Call PCMGSetRestriction and PCMGSetInterpolation on all levels as 
>> I do it already
>>
>> 3. Use a combination of PCMGGetSmoother and KSPSetOperators to 
>> communicate my operators with PCMG
>>
>>     a. On the top levels just set the shell matrices minimalistically 
>> equipped withMATOP_GET_DIAGONALand MATOP_MULT.
>>
>>     b. On the bottom levels generate the operators by explicitly 
>> calling MatMatMatMult (R is not the same as P in my case).
>>
>>
>> Use MAT_INITIAL_MATRIX for the first time
>>
>> Use MAT_REUSE_MATRIX for the subsequent calls
>>
>>
>> Is this a proper way to do it, or there is something wrong?
>>
>> If I want the restriction/interpolation matrices to be matrix-free on 
>> the top levels as well, I would need to setMATOP_MULT_ADDfor them. Is 
>> this enough, or do I need something else?
>>
>> Thanks!
>>
>> Anton
>>
>>
>
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