In-Memory Computing with Cmos and Emerging Memory Technologies

Shubham Jain, Purdue University

Abstract

Modern computing workloads such as machine learning and data analytics perform simple computations on large amounts of data. Traditional von Neumann computing systems, which consist of separate processor and memory subsystems, are inefficient in realizing modern computing workloads due to frequent data transfers between these subsystems that incur significant time and energy costs. In-memory computing embeds computational capabilities within the memory subsystem to alleviate the fundamental processor-memory bottleneck, thereby achieving substantial system-level performance and energy benefits. In this dissertation, we explore a new generation of in-memory computing architectures that are enabled by emerging memory technologies and new CMOS-based memory cells. The proposed designs realize Boolean and non-Boolean computations natively within memory arrays.For Boolean computing, we leverage the unique characteristics of emerging memories that allow multiple word lines within an array to be simultaneously enabled, opening up the possibility of directly sensing functions of the values stored in multiple rows using single access. We propose Spin-Transfer Torque Compute-in-Memory (STT-CiM), a design for in-memory computing with modifications to peripheral circuits that leverage this principle to perform logic, arithmetic, and complex vector operations. We address the challenge of reliable in-memory computing under process variations utilizing error detecting and correcting codes to control errors during CiM operations. We demonstrate how STT-CiM can be integrated within a generalpurpose computing system and propose architectural enhancements to processor instruction sets and on-chip buses for in-memory computing.For non-Boolean computing, we explore crossbar arrays of resistive memory elements, which are known to compactly and efficiently realize a key primitive operation involved in machine learning algorithms, i.e., vector-matrix multiplication. We highlight a key challenge involved in this approach - the actual function computed by a resistive crossbar can deviate substantially from the desired vector-matrix multiplication operation due to a range of device and circuit level non-idealities. It is essential to evaluate the impact of the errors introduced by these non-idealities at the application level. There has been no study of the impact of non-idealities on the accuracy of large-scale workloads (e.g., Deep Neural Networks [DNNs] with millions of neurons and billions of synaptic connections), in part because existing device and circuit models are too slow to use in application-level evaluation. We propose a Fast Crossbar Model (FCM) to accurately capture the errors arising due to crossbar nonidealities while being four-to-five orders of magnitude faster than circuit simulation. We also develop RxNN, a software framework to evaluate DNN inference on resistive crossbar systems. Using RxNN, we evaluate a suite of large-scale DNNs developed for the ImageNet Challenge (ILSVRC). Our evaluations reveal that the errors due to resistive crossbar non-idealities can degrade the overall accuracy of DNNs considerably, motivating the need for compensation techniques. Subsequently, we propose CxDNN, a hardware-software methodology that enables the realization of large-scale DNNs on crossbar systems with minimal degradation in accuracy by compensating for errors due to non-idealities. CxDNN comprises of (i) an optimized mapping technique to convert floating-point weights and activations to crossbar conductances and input voltages, (ii) a fast re-training method to recover accuracy loss due to this conversion, and (iii) low-overhead compensation hardware to mitigate dynamic and hardware-instance-specific errors. Unlike previous efforts that are limited to small networks and require the training and deployment of hardware-instance-specific models, CxDNN presents a scalable compensation methodology that can address large DNNs (e.g., ResNet-50 on ImageNet), and enables a common model to be trained and deployed on many devices.

Degree

Ph.D.

Advisors

Raghunathan, Purdue University.

Subject Area

Energy|Design|Architecture|Artificial intelligence|Computer science

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