Hello,

I have recently posted some questions related to the use of Real function spaces for some FSI problems I'm trying to solve.

The problem I'm facing now is that the solving phase is consuming absurd amount of time. It does not seem to be the PETSc resolution itself, but the assembly phase, since, once the KSP iterations begin they proceed reasonably fast.

The piece of code I believe is relevant is:

#------------------------------------------------------------------------

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EU = VectorElement("Lagrange", 'triangle', orderU)
EP = FiniteElement("Lagrange", 'triangle', orderP)
EL = FiniteElement("Real", 'triangle', 0)
ES = []

ndofs = 20

for i in range(ndofs):
   ES.append(FiniteElement("Real", 'triangle', 0))
EM = MixedElement([EU,EP,EL,ES])

W_f = FunctionSpace(mesh, EM)  # mesh has a few hundred nodes

sol = Function(W_f)

auxs = split(sol)
u = auxs[0]
p = auxs[1]
l = auxs[2]
s = []
for i in range(ndofs):
    s.append(auxs[i+3])
   
auxT = TestFunctions(W_f)
v = auxT[0]
q = auxT[1]
r = auxT[2]
d = []
for i in range(ndofs):
    d.append(auxT[i+3])

uT = u
for i in range(ndofs):
     uT = uT + s[i]*phifluid[i]

#  ( phifluid[:] are functions belonging to W_f that are zero everywhere except on some boundary nodes )

print('   |>          - Fluid part')
Res  = momentum_residual(uT, p, l, s, v, mesh) # Stokes operator
Res += mass_residual(uT, p, l, s, q, r, mesh) # Incompressibility restriction

print('   |>          - Beam part')
aream1 = 1/assemble(Constant(1) * dx(mesh))
 for i in range(ndofs):
    Res += momentum_residual(uT, p, l, s, d[i]*phifluid[i], mesh)
    force_b = Constant(force_beam(vphi, dphibeam[i]))
    Res -= d[i]*force_b*aream1*dx

# Until this point the code runs reasonably fast

# Here starts the bottleneck

idsplit = '2'

for i in range(ndofs):
      idsplit = idsplit+','+str(i+3) # idsplit ends up being 2,3,4,5,...

   sp = {
      "mat_type": "matfree",
      "snes_type": "newtonls",
      "snes_monitor": None,
      "snes_converged_reason": None,
      "snes_linesearch_type": "basic",
      "snes_max_it": 8,
      "snes_rtol": 1e-10,
      "ksp_type": "fgmres",
      "ksp_monitor_true_residual": None,
      "ksp_max_it": 10,
      "pc_type": "fieldsplit",
      "pc_fieldsplit_type": "schur",
      "pc_fieldsplit_schur_fact_type": "full",
      "pc_fieldsplit_0_fields": "0,1",
      "pc_fieldsplit_1_fields": idsplit,
      "fieldsplit_0_ksp_type": "preonly",
      "fieldsplit_0_pc_type": "python",
      "fieldsplit_0_ksp_monitor_true_residual": None,
      "fieldsplit_0_pc_python_type": "firedrake.AssembledPC",
      "fieldsplit_0_assembled_pc_type": "lu",
      "fieldsplit_0_assembled_pc_factor_mat_solver_type": "mumps",
      "fieldsplit_0_assembled_mat_mumps_icntl_14": 200,
      "mat_mumps_icntl_14": 200,
      "fieldsplit_1_ksp_type": "gmres",
      "fieldsplit_1_ksp_monitor_true_residual": None,
      "fieldsplit_1_ksp_max_it": ndofs+1,
      "fieldsplit_1_ksp_convergence_test": "skip",
      "fieldsplit_1_pc_type": "none"
   }

solve(Res == 0, sol, bcs=bcs, J=Jac, solver_parameters=sp)

#------------------------------------------------------------------------

Any tip on what could be going on would be welcome.

Regards

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