PD-Swap: Prefill-Decode Logic Swapping for End-to-End LLM Inference on Edge FPGAs via Dynamic Partial Reconfiguration

Abstract

Aggressively quantized large language models (LLMs), such as BitNet-style 1.58-bit Transformers with ternary weights, make it feasible to deploy generative AI on low-power edge FPGAs. However, as prompts grow to tens of thousands of tokens, edge hardware performance drops sharply with sequence length due to quadratic prefill cost and rapidly increasing KV-cache bandwidth demands, making inference latency of longer context length a first-order system concern. Recent studies on LLMs expose a fundamental prefill-decode asymmetry: prefill is compute-bound and dominated by dense matrix-matrix operations, whereas decoding is memory-bandwidth-bound and dominated by KV-cache traffic. A static accelerator must provision resources and a single dataflow for both regimes, leading to duplicated attention logic, underutilized fabric, and tight LUT/URAM limits that cap model size and usable context. We propose a prefill--decode disaggregated LLM accelerator, PD-Swap, that uses Dynamic Partial Reconfiguration (DPR) to time-multiplex the attention module on edge FPGAs. The core table-lookup ternary matrix multiplication and weight-buffering engines remain static, while the attention subsystem is a reconfigurable partition with two phase-specialized architectures: a compute-heavy, token-parallel prefill engine and a bandwidth-optimized, KV-cache-centric decoding engine. A roofline-inspired model and design space exploration jointly optimize reconfigurable-region size, parallelism under reconfiguration and routability constraints, and reconfiguration latency is hidden by computation latency. PD-Swap achieves up to 27~tokens/s decoding throughput, outperforming prior state-of-the-art works by 1.3x--2.1x (larger gains at longer context lengths), without extra area cost.