[petsc-users] strange convergence

Sun Apr 23 15:19:55 CDT 2017

> On Apr 23, 2017, at 2:42 PM, Hoang Giang Bui <hgbk2008 at gmail.com> wrote:
> 
> Dear Matt/Barry
> 
> With your options, it results in
> 
>   0 KSP preconditioned resid norm 1.106709687386e+31 true resid norm 9.015150491938e+06 ||r(i)||/||b|| 1.000000000000e+00
>     Residual norms for fieldsplit_u_ solve.
>     0 KSP Residual norm 2.407308987203e+36 
>     1 KSP Residual norm 5.797185652683e+72 

It looks like Matt is right, hypre is seemly producing useless garbage.  

First how do things run on one process. If you have similar problems then debug on one process (debugging any kind of problem is always far easy on one process). 

First run with -fieldsplit_u_type lu (instead of using hypre) to see if that works or also produces something bad. 

What is the operator and the boundary conditions for u? It could be singular.

>     Residual norms for fieldsplit_wp_ solve.
>     0 KSP Residual norm 0.000000000000e+00 
> ...
> 999 KSP preconditioned resid norm 2.920157329174e+12 true resid norm 9.015683504616e+06 ||r(i)||/||b|| 1.000059124102e+00
>     Residual norms for fieldsplit_u_ solve.
>     0 KSP Residual norm 1.533726746719e+36 
>     1 KSP Residual norm 3.692757392261e+72 
>     Residual norms for fieldsplit_wp_ solve.
>     0 KSP Residual norm 0.000000000000e+00 
> 
> Do you suggest that the pastix solver for the "wp" block encounters small pivot? In addition, seem like the "u" block is also singular.
> 
> Giang
> 
> On Sun, Apr 23, 2017 at 7:39 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
>    Huge preconditioned norms but normal unpreconditioned norms almost always come from a very small pivot in an LU or ILU factorization.
> 
>    The first thing to do is monitor the two sub solves. Run with the additional options -fieldsplit_u_ksp_type richardson -fieldsplit_u_ksp_monitor -fieldsplit_u_ksp_max_it 1 -fieldsplit_wp_ksp_type richardson -fieldsplit_wp_ksp_monitor -fieldsplit_wp_ksp_max_it 1
> 
> > On Apr 23, 2017, at 12:22 PM, Hoang Giang Bui <hgbk2008 at gmail.com> wrote:
> >
> > Hello
> >
> > I encountered a strange convergence behavior that I have trouble to understand
> >
> > KSPSetFromOptions completed
> >   0 KSP preconditioned resid norm 1.106709687386e+31 true resid norm 9.015150491938e+06 ||r(i)||/||b|| 1.000000000000e+00
> >   1 KSP preconditioned resid norm 2.933141742664e+29 true resid norm 9.015152282123e+06 ||r(i)||/||b|| 1.000000198575e+00
> >   2 KSP preconditioned resid norm 9.686409637174e+16 true resid norm 9.015354521944e+06 ||r(i)||/||b|| 1.000022631902e+00
> >   3 KSP preconditioned resid norm 4.219243615809e+15 true resid norm 9.017157702420e+06 ||r(i)||/||b|| 1.000222648583e+00
> > .....
> > 999 KSP preconditioned resid norm 3.043754298076e+12 true resid norm 9.015425041089e+06 ||r(i)||/||b|| 1.000030454195e+00
> > 1000 KSP preconditioned resid norm 3.043000287819e+12 true resid norm 9.015424313455e+06 ||r(i)||/||b|| 1.000030373483e+00
> > Linear solve did not converge due to DIVERGED_ITS iterations 1000
> > KSP Object: 4 MPI processes
> >   type: gmres
> >     GMRES: restart=1000, using Modified Gram-Schmidt Orthogonalization
> >     GMRES: happy breakdown tolerance 1e-30
> >   maximum iterations=1000, initial guess is zero
> >   tolerances:  relative=1e-20, absolute=1e-09, divergence=10000
> >   left preconditioning
> >   using PRECONDITIONED norm type for convergence test
> > PC Object: 4 MPI processes
> >   type: fieldsplit
> >     FieldSplit with MULTIPLICATIVE composition: total splits = 2
> >     Solver info for each split is in the following KSP objects:
> >     Split number 0 Defined by IS
> >     KSP Object:    (fieldsplit_u_)     4 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:    (fieldsplit_u_)     4 MPI processes
> >       type: hypre
> >         HYPRE BoomerAMG preconditioning
> >         HYPRE BoomerAMG: Cycle type V
> >         HYPRE BoomerAMG: Maximum number of levels 25
> >         HYPRE BoomerAMG: Maximum number of iterations PER hypre call 1
> >         HYPRE BoomerAMG: Convergence tolerance PER hypre call 0
> >         HYPRE BoomerAMG: Threshold for strong coupling 0.6
> >         HYPRE BoomerAMG: Interpolation truncation factor 0
> >         HYPRE BoomerAMG: Interpolation: max elements per row 0
> >         HYPRE BoomerAMG: Number of levels of aggressive coarsening 0
> >         HYPRE BoomerAMG: Number of paths for aggressive coarsening 1
> >         HYPRE BoomerAMG: Maximum row sums 0.9
> >         HYPRE BoomerAMG: Sweeps down         1
> >         HYPRE BoomerAMG: Sweeps up           1
> >         HYPRE BoomerAMG: Sweeps on coarse    1
> >         HYPRE BoomerAMG: Relax down          symmetric-SOR/Jacobi
> >         HYPRE BoomerAMG: Relax up            symmetric-SOR/Jacobi
> >         HYPRE BoomerAMG: Relax on coarse     Gaussian-elimination
> >         HYPRE BoomerAMG: Relax weight  (all)      1
> >         HYPRE BoomerAMG: Outer relax weight (all) 1
> >         HYPRE BoomerAMG: Using CF-relaxation
> >         HYPRE BoomerAMG: Measure type        local
> >         HYPRE BoomerAMG: Coarsen type        PMIS
> >         HYPRE BoomerAMG: Interpolation type  classical
> >       linear system matrix = precond matrix:
> >       Mat Object:      (fieldsplit_u_)       4 MPI processes
> >         type: mpiaij
> >         rows=938910, cols=938910, bs=3
> >         total: nonzeros=8.60906e+07, allocated nonzeros=8.60906e+07
> >         total number of mallocs used during MatSetValues calls =0
> >           using I-node (on process 0) routines: found 78749 nodes, limit used is 5
> >     Split number 1 Defined by IS
> >     KSP Object:    (fieldsplit_wp_)     4 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:    (fieldsplit_wp_)     4 MPI processes
> >       type: lu
> >         LU: out-of-place factorization
> >         tolerance for zero pivot 2.22045e-14
> >         matrix ordering: natural
> >         factor fill ratio given 0, needed 0
> >           Factored matrix follows:
> >             Mat Object:             4 MPI processes
> >               type: mpiaij
> >               rows=34141, cols=34141
> >               package used to perform factorization: pastix
> >             Error :                        -nan
> >   Error :                        -nan
> >     total: nonzeros=0, allocated nonzeros=0
> >             Error :                        -nan
> >     total number of mallocs used during MatSetValues calls =0
> >                 PaStiX run parameters:
> >                   Matrix type :                      Symmetric
> >                   Level of printing (0,1,2):         0
> >                   Number of refinements iterations : 0
> >   Error :                        -nan
> >       linear system matrix = precond matrix:
> >       Mat Object:      (fieldsplit_wp_)       4 MPI processes
> >         type: mpiaij
> >         rows=34141, cols=34141
> >         total: nonzeros=485655, allocated nonzeros=485655
> >         total number of mallocs used during MatSetValues calls =0
> >           not using I-node (on process 0) routines
> >   linear system matrix = precond matrix:
> >   Mat Object:   4 MPI processes
> >     type: mpiaij
> >     rows=973051, cols=973051
> >     total: nonzeros=9.90037e+07, allocated nonzeros=9.90037e+07
> >     total number of mallocs used during MatSetValues calls =0
> >       using I-node (on process 0) routines: found 78749 nodes, limit used is 5
> >
> > The pattern of convergence gives a hint that this system is somehow bad/singular. But I don't know why the preconditioned error goes up too high. Anyone has an idea?
> >
> > Best regards
> > Giang Bui
> >
> 
> 