[petsc-users] KSP changes for successive solver

[Barry Smith]

> On Jul 16, 2015, at 5:42 PM, Michele Rosso <mrosso at uci.edu> wrote:
> 
> Barry,
> 
> thanks for your reply. So if I want it fixed, I will have to use the master branch, correct?

  Yes, or edit mg.c and remove the offending lines of code (easy enough). 
> 
> On a side note, what I am trying to achieve is to be able to use how many levels of MG I want, despite the limitation imposed by the local number of grid nodes.

   I assume you are talking about with DMDA? There is no generic limitation for PETSc's multigrid, it is only with the way the DMDA code figures out the interpolation that causes a restriction.

> So far I am using a borrowed code that implements a PC that creates a sub communicator and perform MG on it.
> While reading the documentation I found out that PCMGSetLevels takes in an optional array of communicators. How does this work?

   It doesn't work. It was an idea that never got pursued.

> Can I can simply define my matrix and rhs on the fine grid as I would do normally ( I do not use kspsetoperators and kspsetrhs ) and KSP would take care of it by using the correct communicator for each level?

   No.

   You can use the PCMG geometric multigrid with DMDA for as many levels as it works and then use PCGAMG as the coarse grid solver. PCGAMG automatically uses fewer processes for the coarse level matrices and vectors. You could do this all from the command line without writing code. 

   For example if your code uses a DMDA and calls KSPSetDM() use for example -da_refine 3 -pc_type mg -pc_mg_galerkin -mg_coarse_pc_type gamg  -ksp_view 



  Barry


> 
> Thanks,
> Michele
> 
> 
> 
> 
> On Thu, 2015-07-16 at 17:30 -0500, Barry Smith wrote:
>>    Michel,
>> 
>>     This is a very annoying feature that has been fixed in master 
>> http://www.mcs.anl.gov/petsc/developers/index.html
>>   I would like to have changed it in maint but Jed would have a shit-fit :-) since it changes behavior.
>> 
>>   Barry
>> 
>> 
>> > On Jul 16, 2015, at 4:53 PM, Michele Rosso <mrosso at uci.edu> wrote:
>> > 
>> > Hi,
>> > 
>> > I am performing a series of solves inside a loop. The matrix for each solve changes but not enough to justify a rebuilt of the PC at each solve.
>> > Therefore I am using  KSPSetReusePreconditioner to avoid rebuilding unless necessary. The solver is CG + MG with a custom  PC at the coarse level.
>> > If KSP is not updated each time, everything works as it is supposed to. 
>> > When instead I allow the default PETSc  behavior, i.e. updating PC every time the matrix changes, the coarse level KSP , initially set to PREONLY, is changed into GMRES 
>> > after the first solve. I am not sure where the problem lies (my PC or PETSc), so I would like to have your opinion on this.
>> > I attached the ksp_view for the 2 successive solve and the options stack.
>> > 
>> > Thanks for your help,
>> > Michel
>> > 
>> > 
>> > 
>> > <ksp_view.txt><petsc_options.txt>
>> 
>> 
>> 
>