Machine learning challenges for computer architecture

Neural networks have become a hot topic in computing and their development is progressing rapidly. They have a long history with some of the first designs proposed in the 1940s. But despite being an active area of research since then, it has not been until the last five to ten years that the field has started to deliver state-of-the-art results, with deep neural network-based algorithms displacing conventional machine-learning and programmed ones in many areas.

The recent developments in neural networks, since around 2010, has coincided with the availability of commodity high-performance GPUs. These devices provide enough memory and compute that networks can be trained with large datasets, in the order of hours or days, to perform classification tasks for practical and interesting problems such as image and speech recognition. Although GPUs have established themselves as the standard way to accelerate neural networks, they have done this by transitioning relatively quickly from applications in traditional HPC, but they are already evolving to meet the needs of machine learning. In this article I want to discuss some of the challenges that neural networks and their development present to GPUs, and indeed more generally to the status quo of computer architecture.

Compute and memory

The fundamental operations of a neural network are floating-point multiplications and additions. These are used to combine input data with the parameters of the network that control the influence of connections between neurons. Modern networks require considerable resources to store millions of parameters and perform billions of operations per input.

Neurons in fully-connected layers take weighted sums of their inputs (a multiplication and an accumulation, MAC, for each input) from every neuron in the previous layer. The number of MACs grows with the square of the layer size, and the number of layers, so even with modest numbers of layers and neurons per layer, the number of MACs can be large. In the AlexNet network, the last three layers are fully connected with 4,096, 4,096 and 1,000 neurons respectively, requiring 58.6 million parameters and, for the forward pass to classify a single input image with a trained network, the same number of MACs.

The use of convolutional layers reduces the number of parameters by sharing a small sets between the neurons. The five convolutional layers preceding the fully-connected layers in AlexNet contain just 2.5 million neurons, but require 655.6 million MACs per input. AlexNet was state of the art in 2009 and networks since then have developed with many more convolutional layers and a smaller fully connected component, resulting in relatively slow growth in the number of parameters but significant increases in the number of MACs. A variant of the VGG network (2014) with 19 layers (three fully connected) has 143.6 million parameters and requires a total of 19.6 million MACs in the forward pass. A variant of the ResNet network (2015) with 50 layers (one fully connected) has 25.5 million parameters and 3.8 billion MACs for the forward pass. More recent work has demonstrated benefits of networks with more than 1,000 layers.

When a network is being trained, more compute is required by an additional backwards pass and and memory requirements increase since intermediate values for each parameter must be maintained from the forward pass.

The challenge for computer architecture here is to deliver the huge number of MACs required for training and inference, whilst minimising the movement of data between fast local memory and slower main memory, or via a communication link. This will of course require corresponding developments in the implementation of neural networks. A recent result demonstrated that when data is kept on chip, much better use of GPU compute resource can be made to achieve an order of magnitude improvement in the depth of network that could be trained. Another has demonstrated that compute can be traded for a logarithmic reduction of memory in the number of layers.

Precision

Reducing the precision of arithmetic reduces the cost of memory and compute since lower-precision floating-point numbers require less bits of storage and require smaller more power-efficient structures in silicon to implement arithmetic operations. Recent research has demonstrated that representations between 8 and 16 bits can deliver similar results to 32-bit precision for inference and training. This has already has an impact on architecture: Google has claimed a 10x increase in efficiency with it’s Tensor Processing Unit (TPU) using 8-bit precision, and Nvidia’s new Pascal architecture supports 16-bit floating-point arithmetic at twice the rate of single precision, and 8-bit integer arithmetic at four times the rate. Intel have also recently announced a variant of their Xeon Phi processor, code named Knights Mill, that will be optimised for deep learning with variable precision floating-point arithmetic.

Structure

There is no single structure for data movement in deep neural networks. The simplest networks have connections between adjacent layers, which are evaluated in sequence, but many more complex structures have been proposed. For example, residual connections provide a pathway between non-adjacent layers, fractal architectures have self-similar structures at different scales and entire neural networks can be used as basic building blocks. There can also by dynamism in the structure; dropout prevents overfitting by randomly removing connections during training to ‘thin’ the network, and networks with stochastic depth randomly exclude subsets of layers during training to make deep networks more shallow.

These neural-network structures contrast with traditional HPC-style programs, which have long been the focus of parallel computing research and development and are characterised by a single structure and algorithm. The challenge here is for computing hardware and the programming models targeting it to support complex, highly-connected and potentially dynamic communication structures.

Programming

There are many languages, frameworks and libraries available for creating deep-learning applications and they are having to evolve quickly though to keep up with the pace of research. This is a strong indication that the means by which we program neural networks need to be general enough to facilitate experimentation but also deliver reasonable performance so that it is practical to explore different designs and hyper parameters.

However, there is a gulf between the high-level representations of neural networks used by researchers and their actual implementation on hardware. For example, Google’s TensorFlow programming framework is written in C++ and interfaces with GPUs via an abstraction layer that calls CUDA library routines. On top of this, Google have released a high-level Python wrapper for TensorFlow, called TensorFlow-Slim. But despite the abstraction and generality of the TensorFlow framework, achieving good computational efficiency on GPUs depends on a heavily-optimised high-level deep neural network library, such as cuDNN or NEON. The problem for all high-level programming approaches is that the performance of neural network designs that cannot exploit an underlying optimised library directly will degrade significantly. Closing the gap between the methods used to build neural networks and their mapping to a machine architecture would deliver more performance for a wider range of programs.

Deployment and portability

Finally, a unique aspect of machine-learning algorithms is the separation between the phase in which they are trained and their subsequent deployment for inference. Since training demands more compute and memory resources and is typically carried out in a data-centre environment where space, power and, to some extent, time are not constraining issues. A trained neural network can be deployed in more constrained environments, such as mobile or robotics, where they may be reacting in real time, to a voice user interface or sensor input for example, with limited memory and power. They may also continue to learn as they are exposed to more data.

The result of training is a set of parameter values and portability to another platform requires the weights to be loaded in an implementation of the same neural network. The implementation may differ in the numerical precision it uses since trained networks are known to be robust to low-precision parameter representations, and doing so takes advantage of the associated memory, performance and power benefits. A portable neural network might therefore need separate implementations for training and inference, optimised for the memory and compute constraints and to be targeted at different machine architectures. A standardised specification of neural networks, including trained parameters, would further improve portability between platforms.

There have been some efforts to try to measure aspects of the implementation, deployment and performance of deep neural networks. In particular Deepmark, which is based on specific networks, and Deepbench, which takes a simpler approach by just looking at important kernels.

In summary

Modern deep neural networks are now state-of-the-art in many application areas of computing but with their unique characteristics, they pose a significant challenge to conventional computer architecture. This challenge however is also an opportunity to build new machines and programming languages that break away from the status quo of sequential shared-memory von Neumann machines.