TAP-CAM: A Tunable Approximate Matching Engine based on Ferroelectric Content Addressable Memory

TL;DR

TAP-CAM introduces a ferroelectric-based ternary CAM that enables tunable approximate matching, significantly improving energy efficiency and accuracy for pattern search tasks like K-nearest neighbor searches.

Contribution

It presents a novel FeFET-based TCAM design capable of both exact and tunable approximate matching, addressing limitations of previous CAMs.

Findings

Achieves 16.95x energy reduction over CMOS CAM

Enhances accuracy by 3.06% in KNN search

Offers flexible approximate matching with robust performance

Abstract

Pattern search is crucial in numerous analytic applications for retrieving data entries akin to the query. Content Addressable Memories (CAMs), an in-memory computing fabric, directly compare input queries with stored entries through embedded comparison logic, facilitating fast parallel pattern search in memory. While conventional CAM designs offer exact match functionality, they are inadequate for meeting the approximate search needs of emerging data-intensive applications. Some recent CAM designs propose approximate matching functions, but they face limitations such as excessively large cell area or the inability to precisely control the degree of approximation. In this paper, we propose TAP-CAM, a novel ferroelectric field effect transistor (FeFET) based ternary CAM (TCAM) capable of both exact and tunable approximate matching. TAP-CAM employs a compact 2FeFET-2R cell structure as the entry storage unit, and similarities in Hamming distances between input queries and stored entries are measured using an evaluation transistor associated with the matchline of CAM array. The operation, robustness and performance of the proposed design at array level have been discussed and evaluated, respectively. We conduct a case study of K-nearest neighbor (KNN) search to benchmark the proposed TAP-CAM at application level. Results demonstrate that compared to 16T CMOS CAM with exact match functionality, TAP-CAM achieves a 16.95x energy improvement, along with a 3.06% accuracy enhancement. Compared to 2FeFET TCAM with approximate match functionality, TAP-CAM achieves a 6.78x energy improvement.