In-Memory Nearest Neighbor Search with FeFET Multi-Bit Content-Addressable Memories
This work addresses the need for fast and low-energy hardware support for accurate nearest neighbor search in applications such as image classification, representing an incremental improvement over existing TCAM-based approaches.
The paper tackles the problem of achieving software-comparable accuracy in nearest neighbor search for tasks like one/few-shot learning by proposing a novel distance function implemented with FeFET multi-bit content-addressable memories, resulting in a 98.34% accuracy for a 5-way, 5-shot classification on the Omniglot dataset, which is only 0.8% lower than software and a 13% improvement over prior TCAM-based methods at iso-energy and iso-delay.
Nearest neighbor (NN) search is an essential operation in many applications, such as one/few-shot learning and image classification. As such, fast and low-energy hardware support for accurate NN search is highly desirable. Ternary content-addressable memories (TCAMs) have been proposed to accelerate NN search for few-shot learning tasks by implementing $L_\infty$ and Hamming distance metrics, but they cannot achieve software-comparable accuracies. This paper proposes a novel distance function that can be natively evaluated with multi-bit content-addressable memories (MCAMs) based on ferroelectric FETs (FeFETs) to perform a single-step, in-memory NN search. Moreover, this approach achieves accuracies comparable to floating-point precision implementations in software for NN classification and one/few-shot learning tasks. As an example, the proposed method achieves a 98.34% accuracy for a 5-way, 5-shot classification task for the Omniglot dataset (only 0.8% lower than software-based implementations) with a 3-bit MCAM. This represents a 13% accuracy improvement over state-of-the-art TCAM-based implementations at iso-energy and iso-delay. The presented distance function is resilient to the effects of FeFET device-to-device variations. Furthermore, this work experimentally demonstrates a 2-bit implementation of FeFET MCAM using AND arrays from GLOBALFOUNDRIES to further validate proof of concept.