Combining memory and a controller with photonics through 3D-stacking to enable scalable and energy-efficient systems

It is well-known that memory latency, energy, capacity, bandwidth, and scalability will be critical bottlenecks in future large-scale systems. This paper addresses these problems, focusing on the interface between the compute cores and memory, comprising the physical interconnect and the memory access protocol. For the physical interconnect, we study the prudent use of emerging silicon-photonic technology to reduce energy consumption and improve capacity scaling. We conclude that photonics are effective primarily to improve socket-edge bandwidth by breaking the pin barrier, and for use on heavily utilized links. For the access protocol, we propose a novel packet based interface that relinquishes most of the tight control that the memory controller holds in current systems and allows the memory modules to be more autonomous, improving flexibility and interoperability. The key enabler here is the introduction of a 3D-stacked interface die that allows both these optimizations without modifying commodity memory dies. The interface die handles all conversion between optics and electronics, as well as all low-level memory device control functionality. Communication beyond the interface die is fully electrical, with TSVs between dies and low-swing wires on-die. We show that such an approach results in substantially lowered energy consumption, reduced latency, better scalability to large capacities, and better support for heterogeneity and interoperability.