Variable namings within MPIInterfaceContainer2D, MPIL2MortarContainer2D

[DanielDoehring]

I think local_neighbor_ids in MPIInterfaceContainer2D and MPIL2MortarContainer2D is a bit of a misnomer, as the global element ids are stored in those datastructures:

Trixi.jl/src/solvers/dgsem_tree/containers_2d.jl

Line 910 in 4357fe9

mpi_interfaces.local_neighbor_ids[count] = element

Trixi.jl/src/solvers/dgsem_tree/containers_2d.jl

Lines 1216 to 1231 in 4357fe9

	local_neighbor_ids = Vector{Int}()
	local_neighbor_positions = Vector{Int}()
	if is_own_cell(mesh.tree, lower_cell_id)
	push!(local_neighbor_ids, c2e[lower_cell_id])
	push!(local_neighbor_positions, 1)
	end
	if is_own_cell(mesh.tree, upper_cell_id)
	push!(local_neighbor_ids, c2e[upper_cell_id])
	push!(local_neighbor_positions, 2)
	end
	if is_own_cell(mesh.tree, large_cell_id)
	push!(local_neighbor_ids, c2e[large_cell_id])
	push!(local_neighbor_positions, 3)
	end
	mpi_mortars.local_neighbor_ids[count] = local_neighbor_ids

In contrast, the stored quantities for the P4est version seem truly local:

Trixi.jl/src/solvers/dgsem_p4est/containers_parallel.jl

Line 365 in 4357fe9

mpi_interfaces.local_neighbor_ids[mpi_interface_id] = local_quad_id + 1

Trixi.jl/src/solvers/dgsem_p4est/containers_parallel.jl

Lines 420 to 428 in 4357fe9

	local_neighbor_ids = local_small_quad_ids .+ 1
	local_neighbor_positions = local_small_quad_positions
	# Add large element information if it is locally available
	if local_large_quad_id > -1
	push!(local_neighbor_ids, local_large_quad_id + 1) # convert to 1-based index
	push!(local_neighbor_positions, 2^(ndims(mesh)-1) + 1)
	end
	mpi_mortars.local_neighbor_ids[mpi_mortar_id] = local_neighbor_ids

Xref: https://github.com/trixi-framework/Trixi.jl/pull/1097/files

[ranocha]

Could you please add this to the agenda of one of our next developer meetings? Ideally, when @sloede is back