Optimized k-Nearest neighbors search implementation on resource-constrained FPGA platforms

The k-Nearest Neighbors (kNN) algorithm is a fundamental machine learning classification technique with wide-ranging applications. Among various kNN implementation choices, FPGA-based heterogeneous systems have gained popularity due to FPGA's inherent parallelism, energy efficiency, and reconfigurability. However, implementing the kNN algorithm on resource-constrained embedded FPGA platforms, typically characterized by constrained programmable resources shared among various application-specific hardware units, necessitates a kNN accelerator architecture that balances high performance, hardware efficiency, and flexibility. To address this challenge, in this paper, we present a kNN hardware accelerator unit designed to optimize resource utilization by utilizing sequential, i.e. accumulation-based, instead of pipelined/parallel distance computations. The proposed architecture incorporates two key algorithmic optimizations to reduce the iteration count of the sequential distance computation loop: a dynamic lower bound enabling early termination of the distance computation and an online element selection that maximizes partial distance growth per iteration. We further enhance the accelerator's performance by incorporating multiple optimized sequential distance computation units, each dedicated to processing a segment of the training dataset. Our experiments demonstrate that the proposed approach is scalable, making it applicable to various hardware platforms and resource constraints. In particular, when implemented on an AMD Zynq device, the proposed single-core kNN accelerator occupies a mere 5 % of the FPGA's resources while delivering a speedup of 3 – 5 times compared to the kNN software implementation running on the accompanying ARM A9 processor. For the 8-core kNN accelerator, the resource utilization stands at 30 $\%$, while the speedup factor ranges between 25 and 35.