[petsc-users] DIVERGED_NANORING with PC GAMG

Wed Oct 31 16:05:59 CDT 2018

These are indefinite (bad) Helmholtz problems. Right?

On Wed, Oct 31, 2018 at 2:38 PM Matthew Knepley <knepley at gmail.com> wrote:

> 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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