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})
-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@mcs.anl.gov <mailto:petsc-users@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
