Saving and Loading Gluon Models

Training large models take a lot of time and it is a good idea to save the trained models to files to avoid training them again and again. There are a number of reasons to do this. For example, you might want to do inference on a machine that is different from the one where the model was trained. Sometimes model's performance on validation set decreases towards the end of the training because of overfitting. If you saved your model parameters after every epoch, at the end you can decide to use the model that performs best on the validation set. Another reason would be to train your model using one language (like Python that has a lot of tools for training) and run inference using a different language (like Scala probably because your application is built on Scala).

In this tutorial, we will learn ways to save and load Gluon models. There are two ways to save/load Gluon models:

1. Save/load model parameters only

Parameters of any Gluon model can be saved using the save_parameters and load_parameters method. This does not save model architecture. This method is used to save parameters of dynamic (non-hybrid) models. Model architecture cannot be saved for dynamic models because model architecture changes during execution.

2. Save/load model parameters AND architecture

The Model architecture of Hybrid models stays static and don't change during execution. Therefore both model parameters AND architecture can be saved and loaded using export, imports methods.

Let‘s look at the above methods in more detail. Let’s start by importing the modules we'll need.

from __future__ import print_function

import mxnet as mx
import mxnet.ndarray as nd
from mxnet import nd, autograd, gluon
from mxnet.gluon.data.vision import transforms

import numpy as np

Setup: build and train a simple model

We need a trained model before we can save it to a file. So let's go ahead and build a very simple convolutional network and train it on MNIST data.

Let's define a helper function to build a LeNet model and another helper to train LeNet with MNIST.

# Use GPU if one exists, else use CPU
ctx = mx.gpu() if mx.test_utils.list_gpus() else mx.cpu()

# MNIST images are 28x28. Total pixels in input layer is 28x28 = 784
num_inputs = 784
# Clasify the images into one of the 10 digits
num_outputs = 10
# 64 images in a batch
batch_size = 64

# Load the training data
train_data = gluon.data.DataLoader(gluon.data.vision.MNIST(train=True).transform_first(transforms.ToTensor()),
                                   batch_size, shuffle=True)

# Build a simple convolutional network
def build_lenet(net):    
    with net.name_scope():
        # First convolution
        net.add(gluon.nn.Conv2D(channels=20, kernel_size=5, activation='relu'))
        net.add(gluon.nn.MaxPool2D(pool_size=2, strides=2))
        # Second convolution
        net.add(gluon.nn.Conv2D(channels=50, kernel_size=5, activation='relu'))
        net.add(gluon.nn.MaxPool2D(pool_size=2, strides=2))
        # Flatten the output before the fully connected layers
        net.add(gluon.nn.Flatten())
        # First fully connected layers with 512 neurons
        net.add(gluon.nn.Dense(512, activation="relu"))
        # Second fully connected layer with as many neurons as the number of classes
        net.add(gluon.nn.Dense(num_outputs))

        return net

# Train a given model using MNIST data
def train_model(model):
    # Initialize the parameters with Xavier initializer
    model.collect_params().initialize(mx.init.Xavier(), ctx=ctx)
    # Use cross entropy loss
    softmax_cross_entropy = gluon.loss.SoftmaxCrossEntropyLoss()
    # Use Adam optimizer
    trainer = gluon.Trainer(model.collect_params(), 'adam', {'learning_rate': .001})

    # Train for one epoch
    for epoch in range(1):
        # Iterate through the images and labels in the training data
        for batch_num, (data, label) in enumerate(train_data):
            # get the images and labels
            data = data.as_in_context(ctx)
            label = label.as_in_context(ctx)
            # Ask autograd to record the forward pass
            with autograd.record():
                # Run the forward pass
                output = model(data)
                # Compute the loss
                loss = softmax_cross_entropy(output, label)
            # Compute gradients
            loss.backward()
            # Update parameters
            trainer.step(data.shape[0])

            # Print loss once in a while
            if batch_num % 50 == 0:
                curr_loss = nd.mean(loss).asscalar()
                print("Epoch: %d; Batch %d; Loss %f" % (epoch, batch_num, curr_loss))

Let's build a model and train it. After training, we will save and restore this model from a file.

net = build_lenet(gluon.nn.Sequential())
train_model(net)

Saving model parameters to file

Okay, we now have a model (net) that we can save to a file. Let's save the parameters of this model to a file using the save_parameters function.

file_name = "net.params"
net.save_parameters(file_name)

We have successfully saved the parameters of the model into a file.

Note: Block.collect_params().save() is not a recommended way to save parameters of a Gluon network if you plan to load the parameters back into a Gluon network using Block.load_parameters().

Loading model parameters from file

Let's now create a network with the parameters we saved into the file. We build the network again using the helper first and then load the weights from the file we saved using the load_parameters function.

new_net = build_lenet(gluon.nn.Sequential())
new_net.load_parameters(file_name, ctx=ctx)

Note that to do this, we need the definition of the network as Python code. If we want to recreate this network on a different machine using the saved weights, we need the same Python code (build_lenet) that created the network to create the new_net object shown above. This means Python code needs to be copied over to any machine where we want to run this network.

If our network is Hybrid, we can even save the network architecture into files and we won‘t need the network definition in a Python file to load the network. We’ll see how to do it in the next section.

Let's test the model we just loaded from file.

import matplotlib.pyplot as plt

def verify_loaded_model(net):
    """Run inference using ten random images.
    Print both input and output of the model"""

    def transform(data, label):
        return data.astype(np.float32)/255, label.astype(np.float32)

    # Load ten random images from the test dataset
    sample_data = mx.gluon.data.DataLoader(mx.gluon.data.vision.MNIST(train=False, transform=transform),
                                  10, shuffle=True)

    for data, label in sample_data:

        # Display the images
        img = nd.transpose(data, (1,0,2,3))
        img = nd.reshape(img, (28,10*28,1))
        imtiles = nd.tile(img, (1,1,3))
        plt.imshow(imtiles.asnumpy())
        plt.show()

        # Display the predictions
        data = nd.transpose(data, (0, 3, 1, 2))
        out = net(data.as_in_context(ctx))
        predictions = nd.argmax(out, axis=1)
        print('Model predictions: ', predictions.asnumpy())

        break

verify_loaded_model(new_net)

Model inputs

Model predictions: [1. 1. 4. 5. 0. 5. 7. 0. 3. 6.]

Saving model parameters AND architecture to file

Hybrid models can be serialized as JSON files using the export function. Once serialized, these models can be loaded from other language bindings like C++ or Scala for faster inference or inference in different environments.

Note that the network we created above is not a Hybrid network and therefore cannot be serialized into a JSON file. So, let's create a Hybrid version of the same network and train it.

net = build_lenet(gluon.nn.HybridSequential())
net.hybridize()
train_model(net)

We now have a trained hybrid network. This can be exported into files using the export function. The export function will export the model architecture into a .json file and model parameters into a .params file.

net.export("lenet", epoch=1)

export in this case creates lenet-symbol.json and lenet-0001.params in the current directory.

Loading model parameters AND architecture from file

From a different frontend

One of the main reasons to serialize model architecture into a JSON file is to load it from a different frontend like C, C++ or Scala. Here is a couple of examples:

  1. Loading serialized Hybrid networks from C
  2. Loading serialized Hybrid networks from Scala

From Python

Serialized Hybrid networks (saved as .JSON and .params file) can be loaded and used inside Python frontend using gluon.nn.SymbolBlock. To demonstrate that, let's load the network we serialized above.

deserialized_net = gluon.nn.SymbolBlock.imports("lenet-symbol.json", ['data'], "lenet-0001.params")

deserialized_net now contains the network we deserialized from files. Let's test the deserialized network to make sure it works.

verify_loaded_model(deserialized_net)

Model inputs

Model predictions: [4. 8. 0. 1. 5. 5. 8. 8. 1. 9.]

That's all! We learned how to save and load Gluon networks from files. Parameters of any Gluon network can be persisted into files. For hybrid networks, both the architecture of the network and the parameters can be saved to and loaded from files.