Profiling Lux Training Loops

Only for Reactant

This tutorial is applicable iff you are using Reactant.jl (AutoEnzyme with ReactantDevice) for training.

To profile the training loop, wrap the training loop with Reactant.with_profiler and pass the path to the directory where the traces should be saved. Note that this will have some overhead and hence should be used only for debugging purposes.

A simple example is shown below:

using Reactant, Lux, Random, MLUtils, Optimisers

dev = reactant_device()

x_data = rand(Float32, 32, 1024)
y_data = x_data .^ 2 .- 1

dl = DataLoader((x_data, y_data); batchsize=32, shuffle=true) |> dev;

model = Chain(Dense(32 => 64, relu), Dense(64 => 32))
ps, st = Lux.setup(Random.default_rng(), model) |> dev;

Reactant.with_profiler(joinpath(tempdir(), "lux_training_trace")) do
    train_state = Training.TrainState(model, ps, st, Adam(0.001))
    for epoch in 1:10
        for (x, y) in dl
            _, loss, _, train_state = Training.single_train_step!(
                AutoEnzyme(), MSELoss(), (x, y), train_state; return_gradients=Val(false)
            )
        end
    end
end

WARNING: All log messages before absl::InitializeLog() is called are written to STDERR
I0000 00:00:1768092621.983476    9445 profiler_session.cc:103] Profiler session initializing.
I0000 00:00:1768092621.983537    9445 profiler_session.cc:118] Profiler session started.
I0000 00:00:1768092665.756963    9445 profiler_session.cc:68] Profiler session collecting data.
I0000 00:00:1768092665.835163    9445 save_profile.cc:150] Collecting XSpace to repository: /tmp/lux_training_trace/plugins/profile/2026_01_11_00_51_05/runnervmi13qx.xplane.pb
I0000 00:00:1768092665.899648    9445 save_profile.cc:123] Creating directory: /tmp/lux_training_trace/plugins/profile/2026_01_11_00_51_05

I0000 00:00:1768092665.948137    9445 save_profile.cc:129] Dumped gzipped tool data for trace.json.gz to /tmp/lux_training_trace/plugins/profile/2026_01_11_00_51_05/runnervmi13qx.trace.json.gz
I0000 00:00:1768092665.974837    9445 profiler_session.cc:136] Profiler session tear down.

Once the run is completed, you can use xprof to analyze the traces. An example of the output is shown below: