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>
> 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.
