Low-memory convolutional neural networks through incremental depth-first processing

Low-memory convolutional neural networks through incremental depth-first processing

Jonathan Binas, Yoshua Bengio
July 15, 2019
Abstract

We introduce an incremental processing scheme for convolutional neural network (CNN) inference, targeted at embedded applications with limited memory budgets. Instead of processing layers one by one, individual input pixels are propagated through all parts of the network they can influence under the given structural constraints. This depth-first updating scheme comes with hard bounds on the memory footprint: the memory required is constant in the case of 1D input and proportional to the square root of the input dimension in the case of 2D input.

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1.2

1 Introduction

Convolutional neural networks (CNNs) deliver state of the art results in most computer vision tasks, such as image segmentation, classification, and object recognition [11, 13]. Due to their versatile fields of application, CNN models are being integrated into embedded systems, such as smartphones, internet of things (IoT) endpoints, robotics, or hearing aids. However, the extensive memory requirements of such models pose serious challenges to the system designer, and have led researchers to explore various approaches for increasing the efficiency of CNN model implementations. Promising ways of reducing memory requirements and/or lowering the number of operations required to run the network include model compression techniques [17, 16, 1, 5, 4, 7], pruning [15, 12, 19, 2], parameter and variable quantization [10, 18, 9, 14, 21, 6], or architectural optimizations [8, 20].

In this work, we achieve a substantial reduction in memory occupation while not altering the model itself, but rather the way updates are computed. Thus, our method is applicable to any CNN model, whereby the computed output is identical to the original output. In addition to its low memory properties, our technique naturally leads to a sparse update scheme, where no unnecessary computation is carried out if values are zero. This sparsity, which arises from an event-based processing style employed here, can be exploited, for instance, in conjunction with low-precision parameters and activations, which typically encourage a sparse activation vector.

2 Asynchronous incremental updates

Rather than computing complete feature maps layer by layer, we propose to process inputs in a depth-first fashion. The advantages of our approach arise from processing small segments, or even single components of the input individually, while keeping in memory only a fraction of the whole network state: only those components of the network state which are non-zero and can be influenced by the current or future input segments (or components) need to be stored.

As an illustration of our method, consider a multilayer neural network (we will add convolutions later), where the activation of a unit of layer is given by

(1)

where is a non-linearity, are the weight parameters, is a bias term, and is the input layer. Rather than computing whole layers one by one for a given input sample, we can process individual components of the input separately. To do so, we introduce a stateful equivalent of the neuron eq. 1, given by

(2)

where is a state variable, which is retained throughout the presentation of an input sample. Furthermore, the neuron emits changes of its activation in terms of “events”,

(3)

at times where such events would be non-zero. Without loss of generality, we assume the initial state of the network to be zero, , , and . An event is transmitted to a connected neuron via a weight and added to its state variable,

(4)
(5)

Note that a potential change of the target neuron, , is now transmitted in the same way to connected neurons of higher layers. It is now straight-forward to show that the states and are equal given that both networks have received the same input at their input layers (see [3] for details).

The same event-based update scheme can be applied to CNNs, whereby each neuron only connects to a small number of neurons of the next higher layer, as specified by the convolution kernel. Thus, each neuron can only influence a few neurons of the next higher layer, typically leading to a cone-shaped dependency structure. Consequently, a single component of the input only influences a cone-shaped subspace of the whole state space, and thus, it is sufficient to keep only the respective cone in memory when the input component is processed. In the one-dimensional case, where the input is a vector, consecutive components can be presented one by one, and their contribution to the state cone and the output be computed individually. Thereby, the memory requirements are constant, regardless of the size of the input vector. This is illustrated in fig. 1. Conversely, if the input is two-dimensional, a whole row of cones needs to be stored, as their states are still relevant during the presentation of the next row. Thus, the memory requirements scale with the square-root of the input dimension, in this case.

Figure 1: Illustration of the update scheme applied to a one-dimensional model with two hidden layers (each described by a convolutional kernel) and one (averaging) output neuron. The top panels (a-c) show three consecutive update steps of a traditional CNN, where complete layers are computed one by one. The bottom panels (d-f) show three steps of the incremental update procedure for the same network, where in each step the effects of one of three non-zero input pixels on the whole network are computed. Only the shaded regions need to be kept in memory. Panel e shows the maximum memory that will ever be required by this model (7 activations to be stored,) regardless of the input size.

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