[petsc-users] DIVERGED_NANORING with PC GAMG
Matthew Knepley
knepley at gmail.com
Wed Oct 31 13:38:00 CDT 2018
On Wed, Oct 31, 2018 at 2:13 PM Thibaut Appel <t.appel17 at imperial.ac.uk>
wrote:
> Hi Mark, Matthew,
>
> Thanks for taking the time.
>
> 1) You're not suggesting having -fieldsplit_X_ksp_type *f*gmres for each
> field, are you?
>
> 2) No, the matrix *has* pressure in one of the fields. Here it's a 2D
> problem (but we're also doing 3D), the unknowns are (p,u,v) and those are
> my 3 fields. We are dealing with subsonic/transsonic flows so it is
> convection dominated indeed.
>
> 3) We are in frequency domain with respect to time, i.e.
> \partial{phi}/\partial{t} = -i*omega*phi.
>
> 4) Hypre is unfortunately not an option since we are in complex arithmetic.
>
>
> I'm not sure about "-fieldsplit_pc_type gamg" GAMG should work on one
> block, and hence be a subpc. I'm not up on fieldsplit syntax.
>
> According to the online manual page this syntax applies the suffix to all
> the defined fields?
>
>
> Mark is correct. I wanted you to change the smoother. He shows how to
> change it to Richardson (make sure you add the self-scale option), which is
> probably the best choice.
>
> Thanks,
>
> Matt
>
> You did tell me to set it to GMRES if I'm not mistaken, that's why I tried
> "-fieldsplit_mg_levels_ksp_type gmres" (mentioned in the email). Also, it
> wasn't clear whether these should be applied to each block or the whole
> system, as the online manual pages + .pdf manual barely mention smoothers
> and how to manipulate MG objects with KSP/PC, this especially with
> PCFIELDSPLIT where examples are scarce.
>
> From what I can gather from your suggestions I tried (lines with X are
> repeated for X={0,1,2})
>
> This looks good. How can an identically zero vector produce a 0 residual?
You should always monitor with
-ksp_monitor_true_residual.
Thanks,
Matt
> -ksp_view_pre -ksp_monitor -ksp_converged_reason \
> -ksp_type fgmres -ksp_rtol 1.0e-8 \
> -pc_type fieldsplit \
> -pc_fieldsplit_type multiplicative \
> -pc_fieldsplit_block_size 3 \
> -pc_fieldsplit_0_fields 0 \
> -pc_fieldsplit_1_fields 1 \
> -pc_fieldsplit_2_fields 2 \
> -fieldsplit_X_pc_type gamg \
> -fieldsplit_X_ksp_type gmres \
> -fieldsplit_X_ksp_rtol 1e-10 \
> -fieldsplit_X_mg_levels_ksp_type richardson \
> -fieldsplit_X_mg_levels_pc_type sor \
> -fieldsplit_X_pc_gamg_agg_nsmooths 0 \
> -fieldsplit_X_mg_levels_ksp_richardson_self_scale \
> -log_view
>
> which yields
>
> KSP Object: 1 MPI processes
> type: fgmres
> restart=30, using Classical (unmodified) Gram-Schmidt
> Orthogonalization with no iterative refinement
> happy breakdown tolerance 1e-30
> maximum iterations=10000, initial guess is zero
> tolerances: relative=1e-08, absolute=1e-50, divergence=10000.
> left preconditioning
> using DEFAULT norm type for convergence test
> PC Object: 1 MPI processes
> type: fieldsplit
> PC has not been set up so information may be incomplete
> FieldSplit with MULTIPLICATIVE composition: total splits = 3,
> blocksize = 3
> Solver info for each split is in the following KSP objects:
> Split number 0 Fields 0
> KSP Object: (fieldsplit_0_) 1 MPI processes
> type: preonly
> maximum iterations=10000, initial guess is zero
> tolerances: relative=1e-05, absolute=1e-50, divergence=10000.
> left preconditioning
> using DEFAULT norm type for convergence test
> PC Object: (fieldsplit_0_) 1 MPI processes
> type not yet set
> PC has not been set up so information may be incomplete
> Split number 1 Fields 1
> KSP Object: (fieldsplit_1_) 1 MPI processes
> type: preonly
> maximum iterations=10000, initial guess is zero
> tolerances: relative=1e-05, absolute=1e-50, divergence=10000.
> left preconditioning
> using DEFAULT norm type for convergence test
> PC Object: (fieldsplit_1_) 1 MPI processes
> type not yet set
> PC has not been set up so information may be incomplete
> Split number 2 Fields 2
> KSP Object: (fieldsplit_2_) 1 MPI processes
> type: preonly
> maximum iterations=10000, initial guess is zero
> tolerances: relative=1e-05, absolute=1e-50, divergence=10000.
> left preconditioning
> using DEFAULT norm type for convergence test
> PC Object: (fieldsplit_2_) 1 MPI processes
> type not yet set
> PC has not been set up so information may be incomplete
> linear system matrix = precond matrix:
> Mat Object: 1 MPI processes
> type: seqaij
> rows=52500, cols=52500
> total: nonzeros=1127079, allocated nonzeros=1128624
> total number of mallocs used during MatSetValues calls =0
> not using I-node routines
> 0 KSP Residual norm 3.583290589961e+00
> 1 KSP Residual norm 0.000000000000e+00
> Linear solve converged due to CONVERGED_ATOL iterations 1
>
> so something must not be set correctly. The solution is identically zero
> everywhere.
>
> Is that option list what you meant? If you could let me know what should
> be corrected.
>
>
> Thanks for your support,
>
>
> Thibaut
>
>
> On 31/10/2018 16:43, Mark Adams wrote:
>
>
>
> On Tue, Oct 30, 2018 at 5:23 PM Appel, Thibaut via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
>
>> Dear users,
>>
>> Following a suggestion from Matthew Knepley I’ve been trying to apply
>> fieldsplit/gamg for my set of PDEs but I’m still encountering issues
>> despite various tests. pc_gamg simply won’t start.
>> Note that direct solvers always yield the correct, physical result.
>> Removing the fieldsplit to focus on the gamg bit and trying to solve the
>> linear system on a modest size problem still gives, with
>>
>> '-ksp_monitor -ksp_rtol 1.0e-10 -ksp_gmres_restart 300 -ksp_type gmres
>> -pc_type gamg'
>>
>> [3]PETSC ERROR: --------------------- Error Message
>> --------------------------------------------------------------
>> [3]PETSC ERROR: Petsc has generated inconsistent data
>> [3]PETSC ERROR: Have un-symmetric graph (apparently). Use
>> '-(null)pc_gamg_sym_graph true' to symetrize the graph or
>> '-(null)pc_gamg_threshold -1' if the matrix is structurally symmetric.
>>
>> And since then, after adding '-pc_gamg_sym_graph true' I have been getting
>> [0]PETSC ERROR: --------------------- Error Message
>> --------------------------------------------------------------
>> [0]PETSC ERROR: Petsc has generated inconsistent data
>> [0]PETSC ERROR: Eigen estimator failed: DIVERGED_NANORINF at iteration
>>
>> -ksp_chebyshev_esteig_noisy 0/1 does not change anything
>>
>> Knowing that Chebyshev eigen estimator needs a positive spectrum I tried
>> ‘-mg_levels_ksp_type gmres’ but iterations would just go on endlessly.
>>
>
> This is OK, but you need to use '-ksp_type *f*gmres' (this could be why
> it is failing ...).
>
> It looks like your matrix is 1) just the velocity field and 2) very
> unsymmetric (eg, convection dominated). I would start with
> ‘-mg_levels_ksp_type richardson -mg_levels_pc_type sor’.
>
> I would also start with unsmoothed aggregation: '-pc_gamg_nsmooths 0'
>
>
>>
>> It seems that I have indeed eigenvalues of rather high magnitude in the
>> spectrum of my operator without being able to determine the reason.
>> The eigenvectors look like small artifacts at the wall-inflow or
>> wall-outflow corners with zero anywhere else but I do not know how to
>> interpret this.
>> Equations are time-harmonic linearized Navier-Stokes to which a forcing
>> is applied, there’s no time-marching.
>>
>
> You mean you are in frequency domain?
>
>
>>
>> Matrix is formed with a MPIAIJ type. The formulation is incompressible,
>> in complex arithmetic and the 2D physical domain is mapped to a logically
>> rectangular,
>
>
> This kind of messes up the null space that AMG depends on but AMG theory
> is gone for NS anyway.
>
>
>> regular collocated grid with a high-order finite difference method.
>> I determine the ownership of the rows/degrees of freedom of the matrix
>> with PetscSplitOwnership and I’m not using DMDA.
>>
>
> Our iterative solvers are probably not going to work well on this but you
> should test hypre also (-pc_type hypre -pc_hypre_type boomeramg). You need
> to configure PETSc to download hypre.
>
> Mark
>
>
>>
>> The Fortran application code is memory-leak free and has undergone a
>> strict verification/validation procedure for different variations of the
>> PDEs.
>>
>> If there’s any problem with the matrix what could help for the
>> diagnostic? At this point I’m running out of ideas so I would really
>> appreciate additional suggestions and discussions.
>>
>> Thanks for your continued support,
>>
>>
>> Thibaut
>
>
--
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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