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3.3. Concise Implementation of Linear Regression

Broad and intense interest in deep learning for the past several years has inspired both companies, academics, and hobbyists to develop a variety of mature open source frameworks for automating the repetitive work of implementing gradient-based learning algorithms. In the previous section, we relied only on (i) NDArray for data storage and linear algebra; and (ii) GradientCollector for calculating derivatives. In practice, because data iterators, loss functions, optimizers, and neural network layers (and some whole architectures) are so common, modern libraries implement these components for us as well.

In this section, we will show you how to implement the linear regression model from Section 3.2 concisely by using DJL.

3.3.1. Generating the Dataset

To start, we will generate the same dataset as in the previous section.

%load ../utils/djl-imports
%load ../utils/
%load ../utils/
NDManager manager = NDManager.newBaseManager();

NDArray trueW = manager.create(new float[]{2, -3.4f});
float trueB = 4.2f;

DataPoints dp = DataPoints.syntheticData(manager, trueW, trueB, 1000);
NDArray features = dp.getX();
NDArray labels = dp.getY();

3.3.2. Reading the Dataset

Just like in the last section, we can call upon DJL’s dataset package to read data. The first step will be to instantiate an ArrayDataset. Here, we set the features and labels as parameters. We also specify a batchSize and specify a Boolean value shuffle indicating whether or not we want the ArrayDataset to randomly sample the data.

// Saved in the utils file for later use
public ArrayDataset loadArray(NDArray features, NDArray labels, int batchSize, boolean shuffle) {
    return new ArrayDataset.Builder()
                  .setData(features) // set the features
                  .optLabels(labels) // set the labels
                  .setSampling(batchSize, shuffle) // set the batch size and random sampling

int batchSize = 10;
ArrayDataset dataset = loadArray(features, labels, batchSize, false);

To verify that it is working, we can read and print the first minibatch of instances.

Batch batch = dataset.getData(manager).iterator().next();
NDArray X = batch.getData().head();
NDArray y = batch.getLabels().head();
ND: (10, 2) gpu(0) float32
[[ 0.2925, -0.7184],
 [ 0.1   , -0.3932],
 [ 2.547 , -0.0034],
 [ 0.0083, -0.251 ],
 [ 0.129 ,  0.3728],
 [ 1.0822, -0.665 ],
 [ 0.5434, -0.7168],
 [-1.4913,  1.4805],
 [ 0.1374, -1.2208],
 [ 0.3072,  1.1135],

ND: (10) gpu(0) float32
[ 7.2342,  5.7411,  9.3138,  5.0536,  3.1772,  8.6284,  7.7434, -3.808 ,  8.6185,  1.0259]

3.3.3. Defining the Model

When we implemented linear regression from scratch (in Section 3.2), we defined our model parameters explicitly and coded up the calculations to produce output using basic linear algebra operations. You should know how to do this. But once your models get more complex, and once you have to do this nearly every day, you will be glad for the assistance. The situation is similar to coding up your own blog from scratch. Doing it once or twice is rewarding and instructive, but you would be a lousy web developer if every time you needed a blog you spent a month reinventing the wheel.

For standard operations, we can use DJL’s predefined blocks, which allow us to focus especially on the layers used to construct the model rather than having to focus on the implementation. To define a linear model, we first import the Model class, which defines a lot of useful methods to interact with our model. We will first define a model variable model. We will then instantiate a SequentialBlock variable net, which will refer to an instance of the SequentialBlock class. The SequentialBlock class defines a container for several layers that will be chained together. Given input data, a SequentialBlock passes it through the first layer, in turn passing the output as the second layer’s input and so forth. In the following example, our model consists of only one layer, so we do not really need SequentialBlock. But since nearly all of our future models will involve multiple layers, we will use it anyway just to familiarize you with the most standard workflow.

Recall the architecture of a single-layer network as shown in Section 3.3.3. The layer is said to be fully-connected because each of its inputs are connected to each of its outputs by means of a matrix-vector multiplication. In DJL, we can use a Linear block to apply a linear transformation. We simply set the number of outputs (in our case its set to 1) and choose if we want to include a bias(yes).

Linear regression is a single-layer neural network.

Model model = Model.newInstance("lin-reg");

SequentialBlock net = new SequentialBlock();
Linear linearBlock = Linear.builder().optBias(true).setUnits(1).build();


3.3.4. Defining the Loss Function

In DJL, the Loss class defines various loss functions. We will use the imported class Loss. In this example, we will use the DJL implementation of squared loss (L2Loss).

(3.3.1)\[L2Loss = \sum_{i = 1}^{n}(y_i - \hat{y_i})^2\]

L2 Loss or ‘Mean Squared Error’ is the sum of the squared difference between the true y value and the predicted y value.

Loss l2loss = Loss.l2Loss();

3.3.5. Defining the Optimization Algorithm

Minibatch SGD and related variants are standard tools for optimizing neural networks and thus DJL supports SGD alongside a number of variations on this algorithm through its Optimizer class. When we instantiate the Optimizer, we will specify the optimization algorithm we wish to use (sgd). We can also manually set hyper-parameters. SGD just requires learningRate, here we set it to a fixed rate of 0.03.

Tracker lrt = Tracker.fixed(0.03f);
Optimizer sgd = Optimizer.sgd().setLearningRateTracker(lrt).build();

3.3.6. Instantiate Configuration and Trainer

Now we’ll create a training configuration that describes how we want to train our model. We will then initialize a trainer to do the training for us.

DefaultTrainingConfig config = new DefaultTrainingConfig(l2loss)
    .optOptimizer(sgd) // Optimizer (loss function)
    .optDevices(manager.getEngine().getDevices(1)) // single GPU
    .addTrainingListeners(TrainingListener.Defaults.logging()); // Logging

Trainer trainer = model.newTrainer(config);
INFO Training on: 1 GPUs.
INFO Load MXNet Engine Version 1.9.0 in 0.112 ms.

3.3.7. Initializing Model Parameters

Before training our model, we need to initialize the model parameters, such as the weights and biases in the linear regression model. We simply call the initialize function with the shape of the model we are training.

// First axis is batch size - won't impact parameter initialization
// Second axis is the input size
trainer.initialize(new Shape(batchSize, 2));

3.3.8. Metrics

Normally, DJL doesn’t record metrics unless explicitly told to as recording metrics impacts the execution flow optimizations. To record metrics, we must instantiate metrics from outside the trainer object and then pass it in.

Metrics metrics = new Metrics();

3.3.9. Training

You might have noticed that expressing our model through DJL requires comparatively few lines of code. We did not have to individually allocate parameters, define our loss function, or implement stochastic gradient descent. Once we start working with much more complex models, DJL’s advantages will grow considerably. However, once we have all the basic pieces in place, the training loop itself is strikingly similar to what we did when implementing everything from scratch.

To refresh your memory: for some number of epochs, we will make a complete pass over the dataset (train_data), iteratively grabbing one minibatch of inputs and the corresponding ground-truth labels. For each minibatch, we go through the following ritual:

  • Generate predictions, calculate loss, and calculate gradients by calling trainBatch(batch) (forward pass and backward pass).

  • Update the model parameters by invoking the step function.

Logging will automatically print the evaluators we are watching during each epoch.

int numEpochs = 3;

for (int epoch = 1; epoch <= numEpochs; epoch++) {
    System.out.printf("Epoch %d\n", epoch);
    // Iterate over dataset
    for (Batch batch : trainer.iterateDataset(dataset)) {
        // Update loss and evaulator
        EasyTrain.trainBatch(trainer, batch);

        // Update parameters

    // reset training and validation evaluators at end of epoch
    trainer.notifyListeners(listener -> listener.onEpoch(trainer));
Epoch 1
Training:    100% |████████████████████████████████████████| L2Loss: 4.99
INFO Epoch 1 finished.
INFO Train: L2Loss: 4.99
Epoch 2
Training:    100% |████████████████████████████████████████| L2Loss: 0.01
INFO Epoch 2 finished.
INFO Train: L2Loss: 0.01
Epoch 3
Training:    100% |████████████████████████████████████████| L2Loss: 8.58E-05
INFO Epoch 3 finished.
INFO Train: L2Loss: 8.58E-05

Below, we compare the model parameters learned by training on finite data and the actual parameters that generated our dataset. To access parameters with DJL, we first access the layer that we need from model and then access that layer’s weight and bias through its parameter list by calling getParameters(). We then simply get each param with valueAt(). Here, valueAt(0) and valueAt(1) returns the weights and bias respectively. As in our from-scratch implementation, note that our estimated parameters are close to their ground truth counterparts.

Block layer = model.getBlock();
ParameterList params = layer.getParameters();
NDArray wParam = params.valueAt(0).getArray();
NDArray bParam = params.valueAt(1).getArray();

float[] w = trueW.sub(wParam.reshape(trueW.getShape())).toFloatArray();
System.out.printf("Error in estimating w: [%f %f]\n", w[0], w[1]);
System.out.printf("Error in estimating b: %f\n", trueB - bParam.getFloat());
Error in estimating w: [-0.000193 -0.000954]
Error in estimating b: 0.000900

3.3.10. Saving Your Model

Now that you have trained your model, you probably want to save it for future use. Additionally, you probably also want to add metadata such as training accuracy and epochs trained. You can do this easily. Simply point to a file location with Paths.get. Metadata can be saved with the setProperty method. Then call the save method on the model to save it!

Path modelDir = Paths.get("../models/lin-reg");

model.setProperty("Epoch", Integer.toString(numEpochs)); // save epochs trained as metadata, "lin-reg");

Model (
    Name: lin-reg
    Model location: /codebuild/output/src863413548/src/
    Data Type: float32
    Epoch: 3

3.3.11. Summary

  • Using DJL, we can implement models much more succinctly.

  • In DJL, the training.dataset package provides tools for data processing, the nn package defines a large number of neural network layers, and the Loss class defines many common loss functions.

  • DJL’s training.initializer package provides various methods for model parameter initialization.

3.3.12. Exercises

  1. Review the DJL documentation to see what loss functions and initialization methods are provided in the class Loss and Trainer. Replace the loss with L1 Loss.

  2. How do you access the parameters during training?