Simultaneous Multi Layer Access: A High Bandwidth and Low Cost 3D-Stacked Memory Interface

TL;DR

This paper introduces SMLA, a novel architecture that enhances 3D-stacked DRAM bandwidth by simultaneously accessing multiple layers through existing global bitlines, achieving higher throughput without additional cost.

Contribution

The paper proposes SMLA, a new multi-layer access method that aggregates internal bandwidth and operates at higher IO frequencies, significantly improving performance and energy efficiency.

Findings

SMLA achieves 4x bandwidth increase for four-layer DRAM.

SMLA reduces energy consumption by 18%.

SMLA improves multi-programmed workload performance by 55%.

Abstract

Limited memory bandwidth is a critical bottleneck in modern systems. 3D-stacked DRAM enables higher bandwidth by leveraging wider Through-Silicon-Via (TSV) channels, but today's systems cannot fully exploit them due to the limited internal bandwidth of DRAM. DRAM reads a whole row simultaneously from the cell array to a row buffer, but can transfer only a fraction of the data from the row buffer to peripheral IO circuit, through a limited and expensive set of wires referred to as global bitlines. In presence of wider memory channels, the major bottleneck becomes the limited data transfer capacity through these global bitlines. Our goal in this work is to enable higher bandwidth in 3D-stacked DRAM without the increased cost of adding more global bitlines. We instead exploit otherwise-idle resources, such as global bitlines, already existing within the multiple DRAM layers by accessing the layers simultaneously. Our architecture, Simultaneous Multi Layer Access (SMLA), provides higher bandwidth by aggregating the internal bandwidth of multiple layers and transferring the available data at a higher IO frequency. To implement SMLA, simultaneous data transfer from multiple layers through the same IO TSVs requires coordination between layers to avoid channel conflict. We first study coordination by static partitioning, which we call Dedicated-IO, that assigns groups of TSVs to each layer. We then provide a simple, yet sophisticated mechanism, called Cascaded-IO, which enables simultaneous access to each layer by time-multiplexing the IOs. By operating at a frequency proportional to the number of layers, SMLA provides a higher bandwidth (4X for a four-layer stacked DRAM). Our evaluations show that SMLA provides significant performance improvement and energy reduction (55%/18% on average for multi-programmed workloads, respectively) over a baseline 3D-stacked DRAM with very low area overhead.