Abstract

Analog hardware accelerators, which perform computation within a dense memory array, have the potential to overcome the major bottlenecks faced by digital hardware for data-heavy workloads such as deep learning. Exploiting the intrinsic computational advantages of memory arrays, however, has proven to be challenging principally due to the overhead imposed by the peripheral circuitry and due to the non-ideal properties of memory devices that play the role of the synapse. We review the existing implementations of these accelerators for deep supervised learning, organizing our discussion around the different levels of the accelerator design hierarchy, with an emphasis on circuits and architecture. We explore and consolidate the various approaches that have been proposed to address the critical challenges faced by analog accelerators, for both neural network inference and training, and highlight the key design trade-offs underlying these techniques.

Highlights

  • Processor performance has advanced at an inexorable pace by riding on continued increases in transistor density, enabled by Dennard scaling, and, more recently, by running many processor cores in parallel

  • By embedding neural network computations directly inside the memory elements that store the weights, analog neuromorphic accelerators based on non-volatile memory (NVM) arrays can greatly reduce the energy and latency costs associated with data movement

  • To be useful for neuromorphic computing, non-volatile memory devices must meet a number of requirements that are considerably more stringent than those for storage-class memory,[69] if these devices are to be used for training

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Summary

INTRODUCTION

Processor performance has advanced at an inexorable pace by riding on continued increases in transistor density, enabled by Dennard scaling, and, more recently, by running many processor cores in parallel. The so-called memory wall, called the von-Neumann bottleneck, presents an opportunity for neuromorphic accelerators that can perform computations directly inside the memory array where the network’s parameters are stored Analog processing inside such an array can inherently parallelize the matrix algebra computational primitives that underlie many machine learning algorithms. Somewhat differently from recent surveys[14,15,16] of neural network accelerators based on emerging devices, we organize this review around the basic components and ideas that make crossbar-based architectures work. These ideas are partially but not entirely agnostic of the specific choice of memory device. VIII, we survey some known approaches to combat device- and array-level non-ideal effects using architectural and algorithmic techniques

COMPUTATIONAL PRIMITIVES IN DEEP LEARNING
Inference and training of deep neural networks
Recurrent and convolutional neural networks
Processing large neural networks
ANALOG IN-MEMORY ACCELERATION OF NEURAL NETWORKS
Device requirements for inference and training
Emerging non-volatile memories
Floating-gate and charge-trap memory
VMM with volatile capacitive memories
Access devices
Limitations on crossbar size
PERIPHERAL CIRCUITS IN ANALOG ACCELERATORS
Analog vs digital routing
Analog voltage levels
Analog temporal encoding
Digital temporal encoding
Input bit slicing
Analog-to-digital converters
The neuron function
ARCHITECTURES FOR INFERENCE
Synaptic bit slicing
Reducing ADC overhead
Signed computation
Hierarchical organization
Convolutional neural network inference acceleration
Flexibility and reconfigurability
Performance comparison of inference accelerators
ARCHITECTURES FOR TRAINING
Supporting backpropagation
Training convolutional neural networks
Compensation for device imprecision and asymmetry
VIII. MITIGATING ARRAY- AND DEVICE-LEVEL NONIDEALITIES
Parasitic resistance
Handling sparse neural networks
Device-aware neural network training
Findings
CONCLUSION
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