Hi,

I normally try to be very specific in titles, but as good practice in some ML communities I am catchy this time.

I want to learn more about how ML works in Julia and I worked through some of the examples in Flux’s ModelZoo. Now, I want to implement a Graph NN, which takes in a (small) graph, with parameters on each vertex and predicts another graph with the same, lets call it topology, but different parameters.

BACKGROUND: I want to teach a graph NN to predict the movement of physical, classical entities with forces between them, e.g. planets with gravitational force, classical charged particles with coulomb forces, etc. The idea comes from this paper http://arxiv.org/abs/2006.11287. The NN’s input is a state of the system, e.g. position and velocity, and mass/charge of particles and the desired output is the next state of the system, according to a small discretization of time.

The graph NN consists of two MLP (multi-layer perceptron) NNs, one NN which predicts messages/edges between vetices and the other predicts the new state/parameters of a vertex given this vertix previous state and incoming messages/connecting edges. Please refer to the mentioned paper for detailed information. I guess, in the ML community graph NN networks are (broadly?) known.

I am aware of the Julia package `GeometricFlux.jl`

, and I got some inspiration there. The reason why I do not use it (yet), is that I want to learn how ML in Julia works.

A minimal working example, with random data, looks like this

```
N = 5 # number of vertices
g = complete_graph(N) # this comes from SimpleGraph.jl
nvp = 5 # number of vertex parameters (e.g. 2d position + velocity + mass)
nep = 100 # number of edge parameters
# MLP predicting vertices
v_nn = Chain(
Dense(nvp + nep, 300, Flux.relu),
Dense(300, 300, Flux.relu),
Dense(300, nvp)
) |> gpu
# MLP predicting edges
e_nn = Chain(
Dense(2 * nvp, 300, Flux.relu),
Dense(300, 300, Flux.relu),
Dense(300, nep)
) |> gpu
# input
# s_vp source vertex parameters
# d_vp destination vertex parameters
function message(s_vp, d_vp)
nn_input = vcat(s_vp, d_vp)
e_nn(nn_input)
end
# input
# vps vertex parameters, outer array are vertices, inner array are the corresponding parameters
function predict_edges(vps::AbstractVector{<:AbstractVector})
new_edge_parameters = Dict()
for e in edges(g)
e = Tuple(e)
s = e[1] # source
d = e[2] # destination
push!(new_edge_parameters, e => message(vps[s], vps[d]))
end
new_edge_parameters
end
aggregate_edge_parameters(eps) = sum(eps)
# input
# v predict the parameters of vertex v
# vps vertex parameters
# eps messages/edge parameters
function predict_vertex(v, vpsAbstractVector{<:AbstractVector}, eps::Dict)
neighbor_edge_parameters = [eps[vn < v ? (vn, v) : (v,vn)] for vn in neighbors(g, v)]
aggr_ep = aggregate_edge_parameters(neighbor_edge_parameters)
nn_input = vcat(vps[v], aggr_ep)
v_nn(nn_input)
end
function loss(vps::AbstractVector{<:AbstractVector}, vps_next::AbstractVector{<:AbstractVector}
)
eps = predict_edges(vps)
l_v = zero(Float32)
for v in vertices(g)
vp_nn = predict_vertex(v, vps, eps)
l_v += norm(vps_next[v] .- vp_nn, 1)
end
l_v /= nv(g)
l_v
#l_e = norm(values(eps), 1)
l_v #+ l_e
end
data = [([rand(Float32, nvp) for _ in 1:nv(g)], [rand(Float32, nvp) for _ in 1:nv(g)])] |> gpu
Flux.train!(loss, params(v_nn, e_nn), data, Flux.ADAM(0.05))
```

I got this working and working on a GPU, which I had difficulties with. The main difficulty was creating the buffer structure for the edges. `Zygote.jl`

does not like arrays (except Zygote.Buffer) and `CUDA.jl`

wants `CuArrays`

.

Training with ~10 000 data points, i.e. `length(data) = 10 000`

in the MWE, takes a long time (I did not run in global scope ofc). The memory fills up quickly, I guess, because I create a lot of `CuArrays`

, everytime I calculate the messages, and the GPU load is at ~10-15% measured with GPU-Z. I do not know how accurate that is. Further problems with this code

- not fast
- the dictionary is created in main memory, not on the GPU
- only works with
`scalarindex(false)`

, because of calculating the norm in the loss function - errors when outcommenting the
`l_e`

… lines. I already filed an issue at`Zygote.jl`

about that, see https://github.com/FluxML/Zygote.jl/issues/760.

I would like to address the above issues, especially how to correctly create buffer structures, which work with CUDA and Zygote, and I’d like to get some feedback/input on how to efficiently write more complicated loss functions than `mse(model(x), y)`

.

I know that this is a very broad question. If there is any good resource, where I can read about this, please point me to it. If something is not clear, I will further explain.