[petsc-users] GMRES -> PCMG -> PCASM pre- post- smoother

[Barry Smith]

   What you describe is not the expected behavior. I expected exactly the result that you expected.

Do you perhaps have some PETSc options around that may be changing the post-smoother? On the command line or in the file petscrc or in the environmental variable PETSC_OPTIONS?  Can you send us some code that we could run that reproduces the problem?

  Barry

> On Aug 19, 2015, at 9:26 PM, Aulisa, Eugenio <eugenio.aulisa at ttu.edu> wrote:
> 
> Hi,
> 
> I am solving an iteration of
> 
> GMRES -> PCMG -> PCASM 
> 
> where I build my particular ASM domain decomposition.
> 
> In setting the PCMG I would like at each level 
> to use the same pre- and post-smoother
> and for this reason I am using 
> ...
> PCMGGetSmoother ( pcMG, level , &subksp );
> 
> to extract and set at each level the ksp object.
> 
> In setting PCASM then I use
> ...
> KSPGetPC ( subksp, &subpc );
> PCSetType ( subpc, PCASM );
> ...
> and then set my own decomposition
> ...
> PCASMSetLocalSubdomains(subpc,_is_loc_idx.size(),&_is_ovl[0],&_is_loc[0]);
> ...
> 
> Now everything compiles, and runs with no memory leakage, 
> but I do not get the expected convergence.
> 
> When I checked the output of -ksp_view, I saw something that puzzled me:
> at each level >0, while in the MG pre-smoother the ASM domain decomposition
> is the one that I set, for example with 4 processes  I get
> 
>>>>>>>>>>>>>>>>>>>> 
> ...
> Down solver (pre-smoother) on level 2 -------------------------------
>    KSP Object:    (level-2)     4 MPI processes
>      type: gmres
>        GMRES: restart=30, using Classical (unmodified) Gram-Schmidt Orthogonalization with no iterative refinement
>        GMRES: happy breakdown tolerance 1e-30
>      maximum iterations=1
>      using preconditioner applied to right hand side for initial guess
>      tolerances:  relative=1e-12, absolute=1e-20, divergence=1e+50
>      left preconditioning
>      using nonzero initial guess
>      using NONE norm type for convergence test
>    PC Object:    (level-2)     4 MPI processes
>      type: asm
>        Additive Schwarz: total subdomain blocks = 198, amount of overlap = 0
>        Additive Schwarz: restriction/interpolation type - RESTRICT
>        [0] number of local blocks = 52
>        [1] number of local blocks = 48
>        [2] number of local blocks = 48
>        [3] number of local blocks = 50
>        Local solve info for each block is in the following KSP and PC objects:
>        - - - - - - - - - - - - - - - - - -
> ...
>>>>>>>>>>>> 
> 
> 
> in the post-smoother I have the default ASM  decomposition with overlapping 1:
> 
> 
>>>>>>>>>>>> 
> ...
> Up solver (post-smoother) on level 2 -------------------------------
>    KSP Object:    (level-2)     4 MPI processes
>      type: gmres
>        GMRES: restart=30, using Classical (unmodified) Gram-Schmidt Orthogonalization with no iterative refinement
>        GMRES: happy breakdown tolerance 1e-30
>      maximum iterations=2
>      tolerances:  relative=1e-12, absolute=1e-20, divergence=1e+50
>      left preconditioning
>      using nonzero initial guess
>      using NONE norm type for convergence test
>    PC Object:    (level-2)     4 MPI processes
>      type: asm
>        Additive Schwarz: total subdomain blocks = 4, amount of overlap = 1
>        Additive Schwarz: restriction/interpolation type - RESTRICT
>        Local solve is same for all blocks, in the following KSP and PC objects:
>      KSP Object:      (level-2sub_)       1 MPI processes
>        type: preonly
>        maximum iterations=10000, initial guess is zero
>        tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
>        left preconditioning
> ...
>>>>>>>>>>>>>> 
> %%%%%%%%%%%%%%%%%%%%%%%%
> 
> So it seams that by using
> 
> PCMGGetSmoother ( pcMG, level , &subksp );
> 
> I was capable  to set both the pre- and post- smoothers to be PCASM
> but everything I did after that applied only to the
> pre-smoother, while the post-smoother got the default PCASM options.
> 
> I know that I can use
> PCMGGetSmootherDown and PCMGGetSmootherUp, but that would 
> probably double the memory allocation and the computational time in the ASM.
> 
> Is there any way I can just use PCMGGetSmoother 
> and use the same PCASM in the pre- and post- smoother?
> 
> I hope I was clear enough.
> 
> Thanks a lot for your help,
> Eugenio
> 
> 
>