# Preconditioners quite slow in PDE system with DifferentialEquations.jl

Hi everyone,

I am using DifferentialEquations.jl to solve 2 couple non-linear PDEs used to model a two-phase flow in a porous media:

\begin{align} \frac{\partial \phi}{\partial t} = \nabla \cdot [\phi^n(\nabla P + u_z)] \\ De \frac{\partial P}{\partial t} = \nabla \cdot [\phi^n(\nabla P + u_z)]- \frac{\phi^m}{R - H(P)(R-1)} P \end{align}

The implementation and the equations are similar to this topic (Coupled non-linear PDEs using DifferentialEquations.jl and the MOL), except that I have a new term with R, a constant and H(Pe), the Heaviside function of P on the second equation.

This is the final form of my equation so I would like to have something fast for it because I need to do 2D models. The problem is that adding this new term is slowing down considerably my models (from a few minutes to a few hours for a settings that I could use in 2D). The fastest linear solver is KLUFactorization() but I would like to use KrylovJL_GMRES() with preconditionners (like in this new tutorial Solving Large Stiff Equations · DifferentialEquations.jl) because it should scale really well for lots of nodes.

The problem is that the performance is quite disappointing so I would like to see if someone find it strange or if it cannot really be applied to my equations. I thought first that it was coming from the Heaviside function but after approximating it with a tanh function, it is definitely better but not as good as I would like.

Enough talking, here is the main function in 2D and the benchmarks I did:

function limit_neumann(a, n)
# check if index is on the boundary, if true take value on the opposite for homogeneous neumann boundary, if false don't change the value
a == n+1 ? a = n-1 : a == 0 ? a = 2 : a = a
end

# Approximation of the Heaviside function copied from here: https://discourse.julialang.org/t/handling-instability-when-solving-ode-problems/9019/5
const v1 = 0  # min of the Heaviside function
const v2 = 1.0  # max of the Heaviside function
H(Pe) = (v2-v1)*tanh(50*(Pe-(-0.5)))/2 + (v1 + v2) / 2

# Define the problem in a form of ODEs by discretizing in space
function porosity_wave(du, u, p, t)

ϕ_ad = @view u[:, :, 1]
Pe_ad = @view u[:, :, 2]

# loop over all the points
@inbounds for I in CartesianIndices((nz, nx))
i, j = Tuple(I)
jw, je = limit_neumann(j-1, nx), limit_neumann(j+1, nx)

# bottom boundary with Dirichlet
if i == 1
# porosity

# pressure
du[i, j, 2] = 1 / (ϕ_ad[i, j]^b * De) *

# top boundary with Dirichlet
elseif i == nz
# porosity
du[i, j, 1] = - (((ϕ_top^n + ϕ_ad[i, j]^n) / 2 * ((Pe_top - Pe_ad[i, j]) / Δz_ad + 1) -

# pressure
du[i, j, 2] = 1 / (ϕ_ad[i, j]^b * De) *

# everything else
else
# porosity

# pressure
du[i, j, 2] = 1 /(ϕ_ad[i, j]^b * De) *
end
end
end

TRBDF2 is the fastest method for my case and here are some computational time for 501 nodes and from 0 to 6 dimensionless timestep for the 1D equivalent of that code:

@time sol = solve(prob_sparse, TRBDF2(linsolve=UMFPACKFactorization()), progress = true, progress_steps = 1, save_everystep=false);
# 12.793877 seconds (1.36 M allocations: 9.276 GiB, 1.87% gc time)
@time solve(prob_sparse, TRBDF2(linsolve=KLUFactorization()), progress = true, progress_steps = 1, save_everystep=false);
# 13.084809 seconds (4.52 M allocations: 5.201 GiB, 1.36% gc time, 1.10% compilation time)
@time solve(prob, TRBDF2(linsolve=KrylovJL_GMRES()), progress = true, progress_steps = 1, save_everystep=false);
# 149.106861 seconds (17.02 M allocations: 1.390 GiB, 0.14% gc time, 1.40% compilation time)

KrylovJL_GMRES beeing slower without a preconditionner is fine for me. But here is with preconditionners:

# multigrid
@time solve(prob_sparse, TRBDF2(linsolve=KrylovJL_GMRES(),precs=algebraicmultigrid,concrete_jac=true),progress = true, progress_steps = 1, save_everystep=false);
# more than 1h for multigrid
# jacobian smoother
@time solve(prob_sparse, TRBDF2(linsolve=KrylovJL_GMRES(),precs=algebraicmultigrid2,concrete_jac=true),progress = true, progress_steps = 1, save_everystep=false);
# 2399.434492 seconds (73.80 M allocations: 72.087 GiB, 0.41% gc time, 0.21% compilation time)

for incompletelu, I had to use a very big Tau to see a result but it is not really good (better than without at least):

# τ = 50.0
@time solve(prob_sparse, TRBDF2(linsolve=KrylovJL_GMRES(),precs=incompletelu,concrete_jac=true),progress = true, progress_steps = 1, save_everystep=false);
# 369.047623 seconds (19.37 M allocations: 1.705 GiB, 0.08% gc time, 1.26% compilation time)
# τ = 5
@time solve(prob_sparse, TRBDF2(linsolve=KrylovJL_GMRES(),precs=incompletelu,concrete_jac=true),progress = true, progress_steps = 1, save_everystep=false);
# 371.915629 seconds (10.18 M allocations: 1.069 GiB, 0.05% gc time, 0.28% compilation time)
# τ = 100
@time solve(prob_sparse, TRBDF2(linsolve=KrylovJL_GMRES(),precs=incompletelu,concrete_jac=true),progress = true, progress_steps = 1, save_everystep=false);
# 363.336921 seconds (10.03 M allocations: 1.018 GiB, 0.04% gc time, 0.26% compilation time)
# τ = 500
@time solve(prob_sparse, TRBDF2(linsolve=KrylovJL_GMRES(),precs=incompletelu,concrete_jac=true),progress = true, progress_steps = 1, save_everystep=false);
# 388.538630 seconds (10.34 M allocations: 1.030 GiB, 0.04% gc time, 0.24% compilation time)
# τ = 1000
@time solve(prob_sparse, TRBDF2(linsolve=KrylovJL_GMRES(),precs=incompletelu,concrete_jac=true),progress = true, progress_steps = 1, save_everystep=false);
# 380.170013 seconds (10.33 M allocations: 1.032 GiB, 0.19% gc time, 0.23% compilation time)
# τ = 5e5
@time solve(prob_sparse, TRBDF2(linsolve=KrylovJL_GMRES(),precs=incompletelu,concrete_jac=true),progress = true, progress_steps = 1, save_everystep=false);
# 62.048754 seconds (3.94 M allocations: 346.247 MiB, 0.06% gc time, 1.31% compilation time)
# τ = 5e4
@time solve(prob_sparse, TRBDF2(linsolve=KrylovJL_GMRES(),precs=incompletelu,concrete_jac=true),progress = true, progress_steps = 1, save_everystep=false);
# 60.994903 seconds (2.36 M allocations: 219.073 MiB, 0.10% gc t

Around tau=5e4 is the best I could do, so 1 min. Is there some hope to speed this up, or is the new non-linearity just too hard to solve?

Can you share a profile and annotate it? I’d like to know where it’s being locked up. I noted when doing this Krylov.jl optimizations for ODE usage · Issue #1563 · SciML/OrdinaryDiffEq.jl · GitHub that there’s probably a few optimizations that can be done in Krylov.jl and the implementation of the IncompleteLU backsolve. I’d need to see the profile to know which of those would be useful to your case.

Yes, AlgebraicMultigrid.jl’s Gauss-Seidel smoother is not efficient I think. I mentioned that to @ranjan the other day and it’s something that will need follow-up.

Compared to? Is there a faster C++/Fortran reference code for this case that I should know about? What technique did it use and how fast did it go?