Thank you Barry, your suggestion really helped speed up the program. The
maximum number of iterations is 48. I still don't know what the asm
pre-conditioner is but I guess I just need to read the manual. I'm trying to
add code to do what you suggested automatically, and I found that I can add:
PC pc;
ierr = KSPGetPC(ksp_,&pc); CHKERR(ierr);
ierr = PCSetType(pc, PCASM); CHKERR(ierr);
However, I cannot find the function to replace the -sub_pc_type. Can you point
me where to look? My system may not be symmetric so I can't use the cg option.
Thanks again for your response.
a?
On Apr 26, 2011, at 2:23 PM, Barry Smith wrote:
>
>> 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 Arag?n 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. Arag?n, Ph.D.
>
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