[petsc-users] Question with setting up KSP solver parameters.

Fri May 2 17:03:30 CDT 2014

  Your shell matrix is buggy in some way. Whenever the residual norm jumps like crazy at a restart it means that something is wrong with the operator.

   Barry

On May 2, 2014, at 4:41 PM, Song Gao <song.gao2 at mail.mcgill.ca> wrote:

> Dear PETSc users,
> 
> I'm solving a linear system in KSP and trying to setup the solver in codes. But I feel strange because my codes don't converge unless I call KSPGMRESSetRestart twice.
> 
> My codes looks like
> 
> call KSPSetOperators ( pet_solv, pet_mat_mf_shell, pet_matp, DIFFERENT_NONZERO_PATTERN, ierpetsc )
> call KSPSetType ( pet_solv, 'gmres', ierpetsc )
> call KSPGMRESSetRestart ( pet_solv, 30, ierpetsc )
> call KSPGetPC ( pet_solv, pet_precon, ierpetsc )
> call PCSetType ( pet_precon, 'asm', ierpetsc )
> call PCASMSetOverlap ( pet_precon, 1, ierpetsc )
> call KSPSetUp ( pet_solv, ierpetsc )
> call PCASMGetSubKSP ( pet_precon, n_local, first_local, pet_solv_sub, ierpetsc )  ! n_local is one
> call KSPGetPC ( pet_solv_sub(1), pet_precon_sub, ierpetsc )
> call PCSetType ( pet_precon_sub, 'jacobi', ierpetsc )
> call PCJacobiSetUseRowMax ( pet_precon_sub, ierpetsc )
> call KSPSetFromOptions ( pet_solv, ierpetsc )
> call KSPGMRESSetRestart ( pet_solv, 29, ierpetsc )           ! adding this line, the codes converge
> call KSPSolve ( pet_solv, pet_rhsp, pet_solup, ierpetsc )
> 
> runing with 1 CPU  WITHOUT the line with red color and the codes don't converge  
> 
>   runtime options:   -ksp_monitor_true_residual -ksp_view
>   0 KSP preconditioned resid norm 6.585278940829e+00 true resid norm 9.619278462343e-03 ||r(i)||/||b|| 1.000000000000e+00
>   1 KSP preconditioned resid norm 6.585278219510e+00 true resid norm 9.619278462343e-03 ||r(i)||/||b|| 1.000000000000e+00
>   2 KSP preconditioned resid norm 2.198638170622e+00 true resid norm 1.365132713014e-01 ||r(i)||/||b|| 1.419163317039e+01
>   3 KSP preconditioned resid norm 1.599896387215e+00 true resid norm 1.445988845022e-01 ||r(i)||/||b|| 1.503219654865e+01
> .......
>  28 KSP preconditioned resid norm 4.478466011191e-01 true resid norm 1.529879309381e-01 ||r(i)||/||b|| 1.590430420920e+01
>  29 KSP preconditioned resid norm 4.398129572260e-01 true resid norm 1.530132924055e-01 ||r(i)||/||b|| 1.590694073413e+01
>  30 KSP preconditioned resid norm 2.783227613716e+12 true resid norm 1.530369123550e-01 ||r(i)||/||b|| 1.590939621450e+01
> 
> KSP Object: 1 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=10000, initial guess is zero
>   tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
>   left preconditioning
>   using PRECONDITIONED norm type for convergence test
> PC Object: 1 MPI processes
>   type: asm
>     Additive Schwarz: total subdomain blocks = 1, amount of overlap = 1
>     Additive Schwarz: restriction/interpolation type - RESTRICT
>     [0] number of local blocks = 1
>     Local solve info for each block is in the following KSP and PC objects:
>     - - - - - - - - - - - - - - - - - -
>     [0] local block number 0, size = 22905
>     KSP Object:    (sub_)     1 MPI processes
>       type: preonly
>       maximum iterations=10000, initial guess is zero
>       tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
>       left preconditioning
>       using NONE norm type for convergence test
>     PC Object:    (sub_)     1 MPI processes
>       type: jacobi
>       linear system matrix = precond matrix:
>       Matrix Object:       1 MPI processes
>         type: seqbaij
>         rows=22905, cols=22905, bs=5
>         total: nonzeros=785525, allocated nonzeros=785525
>         total number of mallocs used during MatSetValues calls =0
>             block size is 5
>     - - - - - - - - - - - - - - - - - -
>   linear system matrix followed by preconditioner matrix:
>   Matrix Object:   1 MPI processes
>     type: shell
>     rows=22905, cols=22905
>   Matrix Object:   1 MPI processes
>     type: seqbaij
>     rows=22905, cols=22905, bs=5
>     total: nonzeros=785525, allocated nonzeros=785525
>     total number of mallocs used during MatSetValues calls =0
>         block size is 5
>      WARNING: zero iteration in iterative solver
> 
> runing with 1 CPU  WITH  the line with red color and the codes converge  
> 
> runtime options:   -ksp_monitor_true_residual -ksp_view
>   0 KSP preconditioned resid norm 6.585278940829e+00 true resid norm 9.619278462343e-03 ||r(i)||/||b|| 1.000000000000e+00
>   1 KSP preconditioned resid norm 2.566248171026e+00 true resid norm 4.841043870812e-03 ||r(i)||/||b|| 5.032647604250e-01
>   2 KSP preconditioned resid norm 1.410418402651e+00 true resid norm 3.347509391208e-03 ||r(i)||/||b|| 3.480000505561e-01
>   3 KSP preconditioned resid norm 9.665409287757e-01 true resid norm 2.289877121679e-03 ||r(i)||/||b|| 2.380508195748e-01
>   4 KSP preconditioned resid norm 4.469486152454e-01 true resid norm 1.283813398084e-03 ||r(i)||/||b|| 1.334625463968e-01
>   5 KSP preconditioned resid norm 2.474889829653e-01 true resid norm 7.956009139680e-04 ||r(i)||/||b|| 8.270900120862e-02
>   ............
>  24 KSP preconditioned resid norm 9.518780877620e-05 true resid norm 6.273993696172e-07 ||r(i)||/||b|| 6.522312167937e-05
>  25 KSP preconditioned resid norm 6.837876679998e-05 true resid norm 4.612861071815e-07 ||r(i)||/||b|| 4.795433555514e-05
>  26 KSP preconditioned resid norm 4.864361942316e-05 true resid norm 3.394754589076e-07 ||r(i)||/||b|| 3.529115621682e-05
> KSP Object: 1 MPI processes
>   type: gmres
>     GMRES: restart=29, using Classical (unmodified) Gram-Schmidt Orthogonalization with no iterative refinement
>     GMRES: happy breakdown tolerance 1e-30
>   maximum iterations=10000, initial guess is zero
>   tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
>   left preconditioning
>   using PRECONDITIONED norm type for convergence test
> PC Object: 1 MPI processes
>   type: asm
>     Additive Schwarz: total subdomain blocks = 1, amount of overlap = 1
>     Additive Schwarz: restriction/interpolation type - RESTRICT
>     [0] number of local blocks = 1
>     Local solve info for each block is in the following KSP and PC objects:
>     - - - - - - - - - - - - - - - - - -
>     [0] local block number 0, size = 22905
>     KSP Object:    (sub_)     1 MPI processes
>       type: preonly
>       maximum iterations=10000, initial guess is zero
>       tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
>       left preconditioning
>       using NONE norm type for convergence test
>     PC Object:    (sub_)     1 MPI processes
>       type: jacobi
>       linear system matrix = precond matrix:
>       Matrix Object:       1 MPI processes
>         type: seqbaij
>         rows=22905, cols=22905, bs=5
>         total: nonzeros=785525, allocated nonzeros=785525
>         total number of mallocs used during MatSetValues calls =0
>             block size is 5
>     - - - - - - - - - - - - - - - - - -
>   linear system matrix followed by preconditioner matrix:
>   Matrix Object:   1 MPI processes
>     type: shell
>     rows=22905, cols=22905
>   Matrix Object:   1 MPI processes
>     type: seqbaij
>     rows=22905, cols=22905, bs=5
>     total: nonzeros=785525, allocated nonzeros=785525
>     total number of mallocs used during MatSetValues calls =0
>         block size is 5
>      WARNING: zero iteration in iterative solver
> 
> 
> What would be my error here? Thank you.