getindex cost memory

[kongdd]

using StructArrays
using BenchmarkTools

abstract type AbstractSoilParam{T} end

@kwdef mutable struct ParamCampbell{T} <: AbstractSoilParam{T}
  θ_sat::T = 0.287       # [m3 m-3]
  # θ_res::T = 0.075     # [m3 m-3]
  ψ_sat::T = -10.0       # [cm]
  Ksat::T = 34 / 3600    # [cm s-1]
  b::T = 4.0             # [-]
end

mutable struct SoilParam{T}
  par::StructVector{ParamCampbell{T}} # multiple layers
end

begin
  N = 10
  θ_sat = fill(0.287, N)       # [m3 m-3]
  ψ_sat = fill(-10.0, N)       # [cm]
  Ksat = fill(34 / 3600, N)    # [cm s-1]
  b = fill(4.0, N)
  par = StructVector{ParamCampbell{Float64}}((θ_sat, ψ_sat, Ksat, b))
  param = SoilParam{Float64}(par)
end

function test_param(param::SoilParam{Float64}, N::Int)
  @inbounds for k in 1:100_00
    for i in 1:N
      _par = param.par[i]
    end
  end
end

julia> @btime test_param($param, N)
  2.558 ms (100000 allocations: 4.58 MiB)

[aplavin]

Your field type in SoilParam{T},

StructVector{ParamCampbell{T}}

Is not concrete: StructArrays have more type parameters than just the eltype.