[petsc-users] GAMG Indefinite

[Barry Smith]

   We see this occasionally, there is nothing in the definition of GAMG that guarantees a positive definite preconditioner even if the operator was positive definite so we don't think this is a bug in the code. We've found using a slightly stronger smoother, like one more smoothing step seems to remove the problem.

   Barry

> On May 19, 2016, at 1:07 PM, Sanjay Govindjee <s_g at berkeley.edu> wrote:
> 
> I am trying to solve a very ordinary nonlinear elasticity problem
> using -ksp_type cg -pc_type gamg in PETSc 3.7.0, which worked fine 
> in PETSc 3.5.3.
> 
> The problem I am seeing is on my first Newton iteration, the Ax=b
> solve returns with and Indefinite Preconditioner error (KSPGetConvergedReason == -8):
> (log_view.txt output also attached)
> 
>   0 KSP Residual norm 8.411630828687e-02 
>   1 KSP Residual norm 2.852209578900e-02 
>   NO CONVERGENCE REASON:  Indefinite Preconditioner
>   NO CONVERGENCE REASON:  Indefinite Preconditioner
> 
> On the next and subsequent Newton iterations, I see perfectly normal
> behavior and the problem converges quadratically.  The results look fine.
> 
> I tried the same problem with -pc_type jacobi as well as super-lu, and mumps 
> and they all work without complaint. 
> 
> My run line for GAMG is: 
> -ksp_type cg -ksp_monitor -log_view -pc_type gamg -pc_gamg_type agg -pc_gamg_agg_nsmooths 1 -options_left
> 
> The code flow looks like:
> 
> ! If no matrix allocation yet
> if(Kmat.eq.0) then   
>   call MatCreate(PETSC_COMM_WORLD,Kmat,ierr)
>   call MatSetSizes(Kmat,numpeq,numpeq,PETSC_DETERMINE,PETSC_DETERMINE,ierr)
>   call MatSetBlockSize(Kmat,nsbk,ierr)
>   call MatSetFromOptions(Kmat, ierr)
>   call MatSetType(Kmat,MATAIJ,ierr)
>   call MatMPIAIJSetPreallocation(Kmat,PETSC_NULL_INTEGER,mr(np(246)),PETSC_NULL_INTEGER,mr(np(247)),ierr)
>   call MatSeqAIJSetPreallocation(Kmat,PETSC_NULL_INTEGER,mr(np(246)),ierr)
> endif
> 
> call MatZeroEntries(Kmat,ierr)
> 
> ! Code to set values in matrix
> 
> call MatAssemblyBegin(Kmat, MAT_FINAL_ASSEMBLY, ierr)
> call MatAssemblyEnd(Kmat, MAT_FINAL_ASSEMBLY, ierr)
> call MatSetOption(Kmat,MAT_NEW_NONZERO_LOCATIONS,PETSC_TRUE,ierr)
> 
> ! If no rhs allocation yet
> if(rhs.eq.0) then
>   call VecCreate        (PETSC_COMM_WORLD, rhs, ierr)
>   call VecSetSizes      (rhs, numpeq, PETSC_DECIDE, ierr)
>   call VecSetFromOptions(rhs, ierr)
> endif
> 
> ! Code to set values in RHS
> 
> call VecAssemblyBegin(rhs, ierr)
> call VecAssemblyEnd(rhs, ierr)
> 
> if(kspsol_exists) then
>   call KSPDestroy(kspsol,ierr)
> endif
> 
> call KSPCreate(PETSC_COMM_WORLD, kspsol   ,ierr)
> call KSPSetOperators(kspsol, Kmat, Kmat, ierr)
> call KSPSetFromOptions(kspsol,ierr)
> call KSPGetPC(kspsol, pc ,      ierr)
> 
> call PCSetCoordinates(pc,ndm,numpn,hr(np(43)),ierr)
> 
> call KSPSolve(kspsol, rhs, sol, ierr)
> call KSPGetConvergedReason(kspsol,reason,ierr)
> 
> ! update solution, go back to the top
> 
> reason is coming back as -8 on my first Ax=b solve and 2 or 3 after that
> (with gamg).  With the other solvers it is coming back as 2 or 3 for
> iterative options and 4 if I use one of the direct solvers.
> 
> Any ideas on what is causing the Indefinite PC on the first iteration with GAMG?
> 
> Thanks in advance,
> -sanjay
> 
> -- 
> -----------------------------------------------
> Sanjay Govindjee, PhD, PE
> Professor of Civil Engineering
> 
> 779 Davis Hall
> University of California
> Berkeley, CA 94720-1710
> 
> Voice:  +1 510 642 6060
> FAX:    +1 510 643 5264
> 
> s_g at berkeley.edu
> http://www.ce.berkeley.edu/~sanjay
> 
> -----------------------------------------------
> 
> Books:  
> 
> Engineering Mechanics of Deformable 
> Solids: A Presentation with Exercises
> 
> http://www.oup.com/us/catalog/general/subject/Physics/MaterialsScience/?view=usa&ci=9780199651641
> http://ukcatalogue.oup.com/product/9780199651641.do
> http://amzn.com/0199651647
> 
> 
> Engineering Mechanics 3 (Dynamics) 2nd Edition
> 
> http://www.springer.com/978-3-642-53711-0
> http://amzn.com/3642537111
> 
> 
> Engineering Mechanics 3, Supplementary Problems: Dynamics 
> 
> http://www.amzn.com/B00SOXN8JU
> 
> 
> -----------------------------------------------
> 
> <log_view.txt>