In-Memory Computing based Machine Learning Accelerators: Opportunities and Challenges

Traditional computing systems based on von Neumann architectures are fundamentally bottle-necked by the transfer speeds between memory and processor. With growing computational needs of today's application space, dominated by Machine Learning (ML) workloads, there is a need to design special purpose computing systems operating on the principle of co-located memory and processing units. Such an approach, commonly known as 'In-memory computing', can potentially eliminate expensive data movement costs by computing inside the memory array itself. To that effect, crossbars based on resistive switching Non-Volatile Memory (NVM) devices has shown immense promise in serving as the building blocks of in-memory computing systems, as their high storage density can overcome scaling challenges that plague CMOS technology today. Adding to that, the ability of resistive crossbars to accelerate the main computational kernel of ML workloads by performing massively parallel, in-situ matrix vector multiplication (MVM) operations, makes them a promising candidate for building area and energy-efficient systems. However, the analog computing nature in resistive crossbars introduce approximations in MVM computations due to device and circuit level nonidealities. Further, analog systems pose high cost peripheral circuit requirements for conversions between the analog and digital domain. Thus, there is a need to understand the entire system design stack, from device characteristics to architectures, and perform effective hardware-software co-design to truly realize the potential of resistive crossbars as future computing systems. In this talk, we will present a comprehensive overview of NVM crossbars for accelerating ML workloads. We describe, in detail, the design principles of the basic building blocks, such as the device and associated circuits, that constitute the crossbars. We explore non-idealities arising from the device characteristics and circuit behavior and study their impact on MVM functionality of NVM crossbars for machine learning hardware.