updatePrunables

Load dlnetwork Object

Load a trained dlnetwork object and the corresponding classes.

s = load("digitsCustom.mat");
dlnet_1 = s.dlnet;
classes = s.classes;

Inspect the layers of the dlnetwork object. The network has three convolution layers at locations 2, 5, and 8 of the Layer array.

layers_1 = dlnet_1.Layers

layers_1 = 
  12x1 Layer array with layers:

     1   'input'     Image Input           28x28x1 images with 'zerocenter' normalization
     2   'conv1'     2-D Convolution       20 5x5x1 convolutions with stride [1  1] and padding [0  0  0  0]
     3   'bn1'       Batch Normalization   Batch normalization with 20 channels
     4   'relu1'     ReLU                  ReLU
     5   'conv2'     2-D Convolution       20 3x3x20 convolutions with stride [1  1] and padding [1  1  1  1]
     6   'bn2'       Batch Normalization   Batch normalization with 20 channels
     7   'relu2'     ReLU                  ReLU
     8   'conv3'     2-D Convolution       20 3x3x20 convolutions with stride [1  1] and padding [1  1  1  1]
     9   'bn3'       Batch Normalization   Batch normalization with 20 channels
    10   'relu3'     ReLU                  ReLU
    11   'fc'        Fully Connected       10 fully connected layer
    12   'softmax'   Softmax               softmax

Load Data for Prediction

Load the digits data for prediction.

dataFolder = fullfile(toolboxdir("nnet"),"nndemos","nndatasets","DigitDataset");

imds = imageDatastore(dataFolder, ...
    IncludeSubfolders=true, ...
    LabelSource="foldernames");

Partition the data into pruning and validation sets. Set aside 10% of the data for validation using the splitEachLabel function.

[imdsPrune,imdsValidation] = splitEachLabel(imds,0.9,"randomize");

The network used in this example requires input images of size 28-by-28-by-1. To automatically resize the images, use augmented image datastores.

inputSize = [28 28 1];
augimdsPrune = augmentedImageDatastore(inputSize(1:2),imdsPrune);
augimdsValidation = augmentedImageDatastore(inputSize(1:2),imdsValidation);

Prune dlnetwork Object

Convert the dlnetwork object to a representation that is suitable for pruning by using the taylorPrunableNetwork function. This function returns a TaylorPrunableNetwork object that has the NumPrunables property set to 48. This indicates that 48 filters in the original model are suitable for pruning by using the Taylor pruning algorithm.

prunableNet_1 = taylorPrunableNetwork(dlnet_1)

prunableNet_1 = 
  TaylorPrunableNetwork with properties:

      Learnables: [14x3 table]
           State: [6x3 table]
      InputNames: {'input'}
     OutputNames: {'softmax'}
    NumPrunables: 48

Create a minibatchqueue object that processes and manages mini-batches of images during pruning. For each mini-batch:

miniBatchSize = 128;
imds.ReadSize = miniBatchSize;

mbq = minibatchqueue(augimdsPrune, ...
    MiniBatchSize=miniBatchSize, ...
    MiniBatchFcn=@preprocessMiniBatch, ...
    MiniBatchFormat=["SSCB" ""]);

Calculate Taylor-based importance scores of the prunable filters in the network by looping over the mini-batches of data. For each mini-batch:

while hasdata(mbq)
    [X,T] = next(mbq);
    [~,pruningActivations,pruningGradients] = dlfeval(@modelLoss,prunableNet_1,X,T);
    prunableNet_1 = updateScore(prunableNet_1,pruningActivations,pruningGradients);
end

Finally, remove filters with the lowest importance scores to create a new TaylorPrunableNetwork object by using the updatePrunables function. By default, a single call to this function removes 8 filters. Observe that the new network prunableNet_2 has 40 prunable filters remaining.

prunableNet_2 = updatePrunables(prunableNet_1)

prunableNet_2 = 
  TaylorPrunableNetwork with properties:

      Learnables: [14x3 table]
           State: [6x3 table]
      InputNames: {'input'}
     OutputNames: {'softmax'}
    NumPrunables: 40

To further compress the model, run the custom pruning loop and update prunables again.

Extract Pruned dlnetwork Object

Use the dlnetwork function to extract the pruned dlnetwork object from the pruned TaylorPrunableNetwork object. You can now use this compressed dlnetwork object to perform inference.

dlnet_2 = dlnetwork(prunableNet_2);

Compare the convolution layers of the original and the pruned dlnetwork objects. Observe that the three convolution layers in the pruned network have fewer filters. These counts agree with the fact that, by default, a single call to the updatePrunables function removes 8 filters from the network.

conv_layers_1 = dlnet_1.Layers([2 5 8])

conv_layers_1 = 
  3x1 Convolution2DLayer array with layers:

     1   'conv1'   2-D Convolution   20 5x5x1 convolutions with stride [1  1] and padding [0  0  0  0]
     2   'conv2'   2-D Convolution   20 3x3x20 convolutions with stride [1  1] and padding [1  1  1  1]
     3   'conv3'   2-D Convolution   20 3x3x20 convolutions with stride [1  1] and padding [1  1  1  1]

conv_layers_2 = dlnet_2.Layers([2 5 8])

conv_layers_2 = 
  3x1 Convolution2DLayer array with layers:

     1   'conv1'   2-D Convolution   17 5x5x1 convolutions with stride [1  1] and padding [0  0  0  0]
     2   'conv2'   2-D Convolution   18 3x3x17 convolutions with stride [1  1] and padding [1  1  1  1]
     3   'conv3'   2-D Convolution   17 3x3x18 convolutions with stride [1  1] and padding [1  1  1  1]

Supporting Functions

Model Loss Function

The modelLoss function takes a TaylorPrunableNetwork object net, a mini-batch of input data X with corresponding targets T and returns activations in net and the gradients of the loss with respect to the activations in net. To compute the gradients automatically, this function uses the dlgradient function.

function [loss, pruningActivations, pruningGradients] = modelLoss(net,X,T)

% Calculate network output for training.
[out, ~, pruningActivations] = forward(net,X);

% Calculate loss.
loss = crossentropy(out,T);

% Compute pruning gradients.
pruningGradients = dlgradient(loss,pruningActivations);
end

Mini Batch Preprocessing Function

The preprocessMiniBatch function preprocesses a mini-batch of predictors and labels using the following steps:

function [X,T] = preprocessMiniBatch(dataX,dataT)

% Preprocess predictors.
X = preprocessMiniBatchPredictors(dataX);

% Extract label data from cell and concatenate.
T = cat(2,dataT{1:end});

% One-hot encode labels.
T = onehotencode(T,1);

end

Mini-Batch Predictors Preprocessing Function

The preprocessMiniBatchPredictors function preprocesses a mini-batch of predictors by extracting the image data from the input cell array and concatenating into a numeric array. For grayscale input, concatenating over the fourth dimension adds a third dimension to each image, to use as a singleton channel dimension.

function X = preprocessMiniBatchPredictors(dataX)

% Concatenate.
X = cat(4,dataX{1:end});

% Normalize the images.
X = X/255;

end