Dear Matt,
thanks for your prompt response.
You are were right (KSPView output below): I was trying to precondition the
off-diagonal blocks (I messed up the ordering of the blocks, stored as ((00),
(01), (11, 10)) where I should have simply used row-major ordering).
One question: is PCFieldSplitSetIS is most effective way to go about this? Or
is there a way to pass the DMDAs directly rather than extracting the ISs from A?
Thanks a lot,
Vincent
KSP Object: 8 MPI processes
type: cg
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: 8 MPI processes
type: fieldsplit
FieldSplit with MULTIPLICATIVE composition: total splits = 3
Solver info for each split is in the following KSP objects:
Split number 0 Defined by IS
KSP Object: (fieldsplit_0_) 8 MPI processes
type: cg
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_) 8 MPI processes
type: jacobi
PC has not been set up so information may be incomplete
linear system matrix = precond matrix:
Mat Object: (fieldsplit_0_) 8 MPI processes
type: mpiaij
rows=262144, cols=262144
total: nonzeros=7.07789e+06, allocated nonzeros=7.07789e+06
total number of mallocs used during MatSetValues calls =0
Split number 1 Defined by IS
KSP Object: (fieldsplit_1_) 8 MPI processes
type: cg
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_) 8 MPI processes
type: jacobi
PC has not been set up so information may be incomplete
linear system matrix = precond matrix:
Mat Object: (fieldsplit_1_) 8 MPI processes
type: shell
rows=262144, cols=262144
Split number 2 Defined by IS
KSP Object: (fieldsplit_2_) 8 MPI processes
type: cg
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_) 8 MPI processes
type: jacobi
PC has not been set up so information may be incomplete
linear system matrix = precond matrix:
Mat Object: (fieldsplit_2_) 8 MPI processes
type: shell
rows=262144, cols=262144
linear system matrix = precond matrix:
Mat Object: 8 MPI processes
type: nest
rows=786432, cols=786432
Matrix object:
type=nest, rows=3, cols=3
MatNest structure:
(0,0) : prefix="fieldsplit_0_", type=mpiaij, rows=262144, cols=262144
(0,1) : type=shell, rows=262144, cols=262144
(0,2) : type=shell, rows=262144, cols=262144
(1,0) : type=mpiaij, rows=262144, cols=262144
(1,1) : prefix="fieldsplit_1_", type=shell, rows=262144, cols=262144
(1,2) : type=shell, rows=262144, cols=262144
(2,0) : type=mpiaij, rows=262144, cols=262144
(2,1) : type=shell, rows=262144, cols=262144
(2,2) : prefix="fieldsplit_2_", type=shell, rows=262144, cols=262144
________________________________
From: Matthew Knepley [[email protected]]
Sent: Friday, April 29, 2016 7:42 AM
To: Le Chenadec, Vincent
Cc: [email protected]
Subject: Re: [petsc-users] PCFIELDSPLIT on a MATNEST with MATSHELL off-diagonal
blocks
On Fri, Apr 29, 2016 at 7:34 AM, Le Chenadec, Vincent
<[email protected]<mailto:[email protected]>> wrote:
Dear all,
I'm trying to use PCFIELDSPLIT as a preconditioner on a block matrix A,
[A_{00} A_{01}]
[A_{10} A_{11}]
where the diagonal blocks (A_{00} and A_{11}) are diagonally dominant. I would
like to use either one of the block preconditioners (not even based on the
Schur complement, simply either one of the Jacobi/Gauss-Seidel ones).
For any questions about solvers, we need to see the output of -ksp_view.
Below, it seems clear that either a) One of your diagonal blocks is a MatShell,
or b) you are trying to invert
an off-diagonal block.
Matt
The issue I'm running into concerns the formats I would like to use, so I'll
start with that:
1. A_{00} and A_{11} are both in MATMPIAIJ format (created using
DMCreateMatrix on 2D DMDAs of different sizes),
2. A_{10} and A_{01} are both in MATSHELL format (created using
DMCreateShell). The reason why I used this format is that it seemed convenient,
since the 2D DMDAs are of different sizes and I assumed I had to provide the
MATOP_MULT operation only,
3. Finally the blocks are stored in MATNEST format (MatCreateNest).
With this choice, matrix-vector multiplications work fine (the DMDAs being
packed in a DMComposite), which makes me think that so far the code is ok.
Also, when I attempt to invert the matrix without preconditioner, I get the
correct result.
Now here is where things do not work: if I try to use a preconditioner, I get
the error pasted below (for a simple PCJACOBI)
[0]PETSC ERROR: No support for this operation for this object type
[0]PETSC ERROR: Mat type shell
[0]PETSC ERROR: #1 MatGetDiagonal() line 4306 in
petsc/3.6.3_dbg/src/mat/interface/matrix.c
[0]PETSC ERROR: #2 PCSetUp_Jacobi() line 180 in
petsc/3.6.3_dbg/src/ksp/pc/impls/jacobi/jacobi.c
[0]PETSC ERROR: #3 PCSetUp_Jacobi_NonSymmetric() line 261 in
petsc/3.6.3_dbg/src/ksp/pc/impls/jacobi/jacobi.c
[0]PETSC ERROR: #4 PCApply_Jacobi() line 286 in
petsc/3.6.3_dbg/src/ksp/pc/impls/jacobi/jacobi.c
[0]PETSC ERROR: #5 PCApply() line 483 in
petsc/3.6.3_dbg/src/ksp/pc/interface/precon.c
[0]PETSC ERROR: #6 KSP_PCApply() line 242 in
petsc/3.6.3_dbg/include/petsc/private/kspimpl.h
[0]PETSC ERROR: #7 KSPSolve_CG() line 139 in
petsc/3.6.3_dbg/src/ksp/ksp/impls/cg/cg.c
[0]PETSC ERROR: #8 KSPSolve() line 604 in
petsc/3.6.3_dbg/src/ksp/ksp/interface/itfunc.c
[0]PETSC ERROR: #9 PCApply_FieldSplit() line 988 in
petsc/3.6.3_dbg/src/ksp/pc/impls/fieldsplit/fieldsplit.c
[0]PETSC ERROR: #10 PCApply() line 483 in
petsc/3.6.3_dbg/src/ksp/pc/interface/precon.c
[0]PETSC ERROR: #11 KSP_PCApply() line 242 in
petsc/3.6.3_dbg/include/petsc/private/kspimpl.h
[0]PETSC ERROR: #12 KSPSolve_CG() line 139 in
petsc/3.6.3_dbg/src/ksp/ksp/impls/cg/cg.c
[0]PETSC ERROR: #13 KSPSolve() line 604 in
petsc/3.6.3_dbg/src/ksp/ksp/interface/itfunc.c
I may not be calling the appropriate routines, here is what I'm basically doing:
1. KSP type set to KSPCG (A is symmetric), PC type to PCFIELDSPLIT,
2. I then use MatNestGetISs on A to supply the ISs to PCFieldSplitSetIS,
3. Finally I use PCFieldSplitGetSubKSP to use set the subKSP for each
diagonal block.
As I mentioned, as long as the subKSPs don't have any preconditioning (PCNONE),
KSPSolve does not complain, only when I try to using one (even PCJACOBI) do I
get similar errors). That makes me suspect that I'm doing something wrong, but
I could not find an example that was doing the same thing...
Any pointers would be much appreciated!
Thanks,
Vincent
--
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
