[petsc-users] KSP solver increases the solution time

[Barry Smith]

> System solved in 3664 iterations... 42.683 s

   The preconditioner is simply not up to the task it has been assigned.  This number of iterations is problematic.

    Have you tried -pc_type asm -sub_pc_type lu     If that works well you can try -pc_type asm -sub_pc_type ilu and see if that still works.  

   If the matrix is indeed symmetric positive definite you will want to use -ksp_type cg



    Barry


On Apr 26, 2011, at 1:37 AM, Alejandro Marcos Aragón wrote:

> Hi all,
> 
> I'm using the standard configuration of the KSP solver, but the time it takes to solve a large system of equations is increasing (because of the increasing number of iterations?). These are my timing lines and the log from the KSP solver in two consecutive solves:
> 
> [1]   Solving system... 0.154203 s
> 
> KSP Object:
>  type: gmres
>    GMRES: restart=30, using Classical (unmodified) Gram-Schmidt Orthogonalization with no iterative refinement
>    GMRES: happy breakdown tolerance 1e-30
>  maximum iterations=10000
>  tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
>  left preconditioning
>  using nonzero initial guess
>  using PRECONDITIONED norm type for convergence test
> PC Object:
>  type: bjacobi
>    block Jacobi: number of blocks = 3
>    Local solve is same for all blocks, in the following KSP and PC objects:
>  KSP Object:(sub_)
>    type: preonly
>    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:(sub_)
>    type: ilu
>      ILU: out-of-place factorization
>      0 levels of fill
>      tolerance for zero pivot 1e-12
>      using diagonal shift to prevent zero pivot
>      matrix ordering: natural
>      factor fill ratio given 1, needed 1
>        Factored matrix follows:
>          Matrix Object:
>            type=seqaij, rows=2020, cols=2020
>            package used to perform factorization: petsc
>            total: nonzeros=119396, allocated nonzeros=163620
>              using I-node routines: found 676 nodes, limit used is 5
>    linear system matrix = precond matrix:
>    Matrix Object:
>      type=seqaij, rows=2020, cols=2020
>      total: nonzeros=119396, allocated nonzeros=163620
>        using I-node routines: found 676 nodes, limit used is 5
>  linear system matrix = precond matrix:
>  Matrix Object:
>    type=mpiaij, rows=6058, cols=6058
>    total: nonzeros=365026, allocated nonzeros=509941
>      using I-node (on process 0) routines: found 676 nodes, limit used is 5
> 
> 
> [1]   System solved in 51 iterations... 0.543215 s
> ...
> ...
> ...
> 
> [1]   Solving system... 0.302414 s
> 
> KSP Object:
>  type: gmres
>    GMRES: restart=30, using Classical (unmodified) Gram-Schmidt Orthogonalization with no iterative refinement
>    GMRES: happy breakdown tolerance 1e-30
>  maximum iterations=10000
>  tolerances:  relative=1e-05, absolute=1e-50, divergence=10000
>  left preconditioning
>  using nonzero initial guess
>  using PRECONDITIONED norm type for convergence test
> PC Object:
>  type: bjacobi
>    block Jacobi: number of blocks = 3
>    Local solve is same for all blocks, in the following KSP and PC objects:
>  KSP Object:(sub_)
>    type: preonly
>    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:(sub_)
>    type: ilu
>      ILU: out-of-place factorization
>      0 levels of fill
>      tolerance for zero pivot 1e-12
>      using diagonal shift to prevent zero pivot
>      matrix ordering: natural
>      factor fill ratio given 1, needed 1
>        Factored matrix follows:
>          Matrix Object:
>            type=seqaij, rows=2020, cols=2020
>            package used to perform factorization: petsc
>            total: nonzeros=119396, allocated nonzeros=163620
>              using I-node routines: found 676 nodes, limit used is 5
>    linear system matrix = precond matrix:
>    Matrix Object:
>      type=seqaij, rows=2020, cols=2020
>      total: nonzeros=119396, allocated nonzeros=163620
>        using I-node routines: found 676 nodes, limit used is 5
>  linear system matrix = precond matrix:
>  Matrix Object:
>    type=mpiaij, rows=6058, cols=6058
>    total: nonzeros=365026, allocated nonzeros=509941
>      using I-node (on process 0) routines: found 676 nodes, limit used is 5
> 
> [1]   System solved in 3664 iterations... 42.683 s
> 
> 
> 
> As you can see, the second iteration takes more than 40 seconds to solve. Could some explain why this is happening and why he number of iterations is increasing dramatically between solves? Thank you all,
> 
> Alejandro M. Aragón, Ph.D.