> Initial residual: 462.13689530272404
> 0 SNES Function norm 4.621368953027e+02
> Residual norms for firedrake_0_ solve.
> 0 KSP Residual norm 4.621368953027e+02
> Residual norms for firedrake_0_fieldsplit_0_ solve.
> 0 KSP Residual norm 1.000000000000e+00
> 1 KSP Residual norm 3.501082228626e-15
> Residual norms for firedrake_0_fieldsplit_1_ solve.
> 0 KSP Residual norm 0.000000000000e+00
> 1 KSP Residual norm 0.000000000000e+00
> 1 KSP Residual norm 1.612167203819e-12
> 1 SNES Function norm 1.599350481360e-12
> ```
>
> Using the fieldsplit additive preconditioner, the problem converges in a
> single KSP iteration, as expected. However, I do not understand why the
> residual of fieldsplit_0 (1e+0) does not coincide with the outer residual
> (462.13689530272404). It should be the case given that only fieldsplit_0 has
> a non-zero residual contribution. The fact that it is just 1 is suspicious.
> Is there something about how the fieldsplit works that I am missing?
In my reading, it says
> Residual norms for firedrake_0_ solve.
> 0 KSP Residual norm 4.621368953027e+02
so the residual in the first (0th) split does correspond to the
outer-most KSP residual norm (not printed but equal to the SNES at the first
iteration of Newton.
It is the residual norm of the first field of the first field that is 1
> Residual norms for firedrake_0_fieldsplit_0_ solve.
> 0 KSP Residual norm 1.000000000000e+00
Or am I missing something
Barry
> Initial residual: 462.13689530272404
> 0 SNES Function norm 4.621368953027e+02
> Residual norms for firedrake_0_ solve.
> 0 KSP Residual norm 4.621368953027e+02
> Residual norms for firedrake_0_ solve.
> 0 KSP unpreconditioned resid norm 4.621368953027e+02 true resid norm
> 4.621368953027e+02 ||r(i)||/||b|| 1.000000000000e+00
> Residual norms for firedrake_0_fieldsplit_0_ solve.
> 0 KSP Residual norm 1.000000000000e+00
> Residual norms for firedrake_0_fieldsplit_0_ solve.
> 0 KSP none resid norm 1.000000000000e+00 true resid norm
> 3.501082228626e-15 ||r(i)||/||b|| 3.501082228626e-15
> 1 KSP Residual norm 3.501082228626e-15
> 1 KSP none resid norm 3.501082228626e-15 true resid norm
> 3.501082228626e-15 ||r(i)||/||b|| 3.501082228626e-15
> Residual norms for firedrake_0_fieldsplit_1_ solve.
> 0 KSP Residual norm 0.000000000000e+00
> Residual norms for firedrake_0_fieldsplit_1_ solve.
> 0 KSP none resid norm 0.000000000000e+00 true resid norm
> 0.000000000000e+00 ||r(i)||/||b|| inf
> 1 KSP Residual norm 0.000000000000e+00
> 1 KSP none resid norm 0.000000000000e+00 true resid norm
> 0.000000000000e+00 ||r(i)||/||b|| inf
> 1 KSP Residual norm 1.612167203819e-12
> 1 KSP unpreconditioned resid norm 1.612167203819e-12 true resid norm
> 1.589286585800e-12 ||r(i)||/||b|| 3.438995245681e-15
> 1 SNES Function norm 1.599350481360e-12
> On Sep 17, 2024, at 1:04 AM, miguel.salazar <miguel.sala...@corintis.com>
> wrote:
>
> These are the new options (with ksp_monitor_true_residual)
>
> solver_mumps_assembled = {
> "ksp_type": "preonly",
> "ksp_monitor": None,
> "ksp_monitor_true_residual": None,
> "pc_type": "python",
> "pc_python_type": "firedrake.AssembledPC",
> "assembled_pc_type": "lu",
> "assembled_pc_factor_mat_solver_type": "mumps",
> "assembled_mat_mumps_icntl_14": 200,
> "assembled_mat_mumps_icntl_24": 1,
> }
> solver_fieldsplit = {
> "mat_type": "matfree",
> "snes_monitor": None,
> "snes_type": "newtonls",
> "ksp_type": "fgmres",
> "ksp_monitor_true_residual": None,
> "ksp_rtol": 1e-1,
> "ksp_monitor": None,
> "pc_type": "fieldsplit",
> "pc_fieldsplit_type": "additive",
> "fieldsplit_0": solver_mumps_assembled,
> "fieldsplit_1": solver_mumps_assembled,
> }
>
> And this is the output
>
> Initial residual: 462.13689530272404
> 0 SNES Function norm 4.621368953027e+02
> Residual norms for firedrake_0_ solve.
> 0 KSP Residual norm 4.621368953027e+02
> Residual norms for firedrake_0_ solve.
> 0 KSP unpreconditioned resid norm 4.621368953027e+02 true resid norm
> 4.621368953027e+02 ||r(i)||/||b|| 1.000000000000e+00
> Residual norms for firedrake_0_fieldsplit_0_ solve.
> 0 KSP Residual norm 1.000000000000e+00
> Residual norms for firedrake_0_fieldsplit_0_ solve.
> 0 KSP none resid norm 1.000000000000e+00 true resid norm
> 3.501082228626e-15 ||r(i)||/||b|| 3.501082228626e-15
> 1 KSP Residual norm 3.501082228626e-15
> 1 KSP none resid norm 3.501082228626e-15 true resid norm
> 3.501082228626e-15 ||r(i)||/||b|| 3.501082228626e-15
> Residual norms for firedrake_0_fieldsplit_1_ solve.
> 0 KSP Residual norm 0.000000000000e+00
> Residual norms for firedrake_0_fieldsplit_1_ solve.
> 0 KSP none resid norm 0.000000000000e+00 true resid norm
> 0.000000000000e+00 ||r(i)||/||b|| inf
> 1 KSP Residual norm 0.000000000000e+00
> 1 KSP none resid norm 0.000000000000e+00 true resid norm
> 0.000000000000e+00 ||r(i)||/||b|| inf
> 1 KSP Residual norm 1.612167203819e-12
> 1 KSP unpreconditioned resid norm 1.612167203819e-12 true resid norm
> 1.589286585800e-12 ||r(i)||/||b|| 3.438995245681e-15
> 1 SNES Function norm 1.599350481360e-12
>
> The true residual is very low. (3.501082228626e-15)
>
>
>
>
> MIGUEL ANGEL SALAZAR DE TROYA
> Head of Software Engineering
> miguel.sala...@corintis.com <mailto:miguel.sala...@corintis.com>
> Corintis SA
> EPFL Innovation Park Building C
> 1015 Lausanne
> <2024-08-1609_02_11-re_mailmigrationfromgoogletooffice365-sebastien.go...@corintis.com-co_4456a1bf-dd3e-46b7-85f9-21f278e66a79.png>
> Here at Corintis we care for your privacy. That is why we have taken
> appropriate measures to ensure that the data you have provided to us is
> always secure.
> From: Barry Smith <bsm...@petsc.dev <mailto:bsm...@petsc.dev>>
> Date: Monday, 16 September 2024 at 19:34
> To: miguel.salazar <miguel.sala...@corintis.com
> <mailto:miguel.sala...@corintis.com>>
> Cc: petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov>
> <petsc-users@mcs.anl.gov <mailto:petsc-users@mcs.anl.gov>>
> Subject: Re: [petsc-users] Residual at the fieldsplit level
>
>
> Try ksp_monitor_true_residual. Let's see if it is an issue of
> preconditioned vs unpreconditioned residual.
>
>
>
>
> On Sep 16, 2024, at 1:05 PM, miguel.salazar <miguel.sala...@corintis.com
> <mailto:miguel.sala...@corintis.com>> wrote:
>
> Hello,
>
> I have this simple example on Firedrake to illustrate my point. I am solving
> for a two-component poisson equation (uncoupled). Only the first component
> has a non-zero residual.
>
> ```
> import firedrake as fd
> from firedrake import inner, grad, dx, sin, pi
>
> N = 10
> mesh = fd.UnitSquareMesh(N, N)
> V = fd.FunctionSpace(mesh, "CG", 1)
> W = V * V
> u = fd.Function(W)
> v = fd.TestFunction(W)
> a = inner(grad(u[0]), grad(v[0])) * dx + inner(grad(u[1]), grad(v[1])) * dx
> x = fd.SpatialCoordinate(mesh)
> f = fd.Function(V)
> f.interpolate(fd.Constant(1e4) * sin(x[0] * pi) * sin(2 * x[1] * pi))
> L = f * v[0] * dx
> F = a - L
> bcs = [fd.DirichletBC(W.sub(0), fd.Constant(2.0), (1,))]
>
>
> def snes_firedrake_residual(F, u, bcs):
> for bcs_ in bcs:
> bcs_.apply(u)
> residual = fd.assemble(F, bcs=bcs, zero_bc_nodes=True)
> with residual.dat.vec_ro as r:
> print("Initial residual:", r.norm())
>
>
> snes_firedrake_residual(F, u, bcs)
>
>
> problem = fd.NonlinearVariationalProblem(F, u, bcs=bcs)
> solver_mumps_assembled = {
> "ksp_type": "preonly",
> "ksp_monitor": None,
> "pc_type": "python",
> "pc_python_type": "firedrake.AssembledPC",
> "assembled_pc_type": "lu",
> "assembled_pc_factor_mat_solver_type": "mumps",
> "assembled_mat_mumps_icntl_14": 200,
> "assembled_mat_mumps_icntl_24": 1,
> }
> solver_fieldsplit = {
> "mat_type": "matfree",
> "snes_type": "newtonls",
> "ksp_type": "fgmres",
> "ksp_rtol": 1e-1,
> "ksp_monitor": None,
> "pc_type": "fieldsplit",
> "pc_fieldsplit_type": "additive",
> "fieldsplit_0": solver_mumps_assembled,
> "fieldsplit_1": solver_mumps_assembled,
> }
>
> solver = fd.NonlinearVariationalSolver(problem,
> solver_parameters=solver_fieldsplit)
> solver.solve()
> ```
>
> The PETSc output is as follow
>
> ```
> Initial residual: 462.13689530272404
> 0 SNES Function norm 4.621368953027e+02
> Residual norms for firedrake_0_ solve.
> 0 KSP Residual norm 4.621368953027e+02
> Residual norms for firedrake_0_fieldsplit_0_ solve.
> 0 KSP Residual norm 1.000000000000e+00
> 1 KSP Residual norm 3.501082228626e-15
> Residual norms for firedrake_0_fieldsplit_1_ solve.
> 0 KSP Residual norm 0.000000000000e+00
> 1 KSP Residual norm 0.000000000000e+00
> 1 KSP Residual norm 1.612167203819e-12
> 1 SNES Function norm 1.599350481360e-12
> ```
>
> Using the fieldsplit additive preconditioner, the problem converges in a
> single KSP iteration, as expected. However, I do not understand why the
> residual of fieldsplit_0 (1e+0) does not coincide with the outer residual
> (462.13689530272404). It should be the case given that only fieldsplit_0 has
> a non-zero residual contribution. The fact that it is just 1 is suspicious.
> Is there something about how the fieldsplit works that I am missing?
>
> Thanks,
> Miguel
>
> MIGUEL ANGEL SALAZAR DE TROYA
> Head of Software Engineering
> miguel.sala...@corintis.com <mailto:miguel.sala...@corintis.com>
> Corintis SA
> EPFL Innovation Park Building C
> 1015 Lausanne
>
> <2024-08-1609_02_11-re_mailmigrationfromgoogletooffice365-sebastien.go...@corintis.com-co_4456a1bf-dd3e-46b7-85f9-21f278e66a79.png>
> Here at Corintis we care for your privacy. That is why we have taken
> appropriate measures to ensure that the data you have provided to us is
> always secure.
