PersistentKV: Page-Aware Decode Scheduling for Long-Context LLM Serving on Commodity GPUs
About
Autoregressive large language model (LLM) serving is increasingly limited by key-value (KV) cache movement rather than dense matrix multiplication. Modern paged-attention systems reduce fragmentation, and mature kernels like FlashInfer provide highly optimized decode attention. However, the best single-kernel implementation is not always the best serving schedule: low-active long-context decode can under-utilize GPUs, while mixed sequence lengths introduce tension between many exact-length launches and coarse padded batches. We present PersistentKV, a native block-table decode attention engine and page-aware scheduling study for grouped-query attention (GQA). PersistentKV maps work by KV-head group, executes directly over native page tables, and adds a compact workqueue schedule executing only non-empty row-KV-head-sequence-split tasks. On an RTX 3060 (FP16, page size 16, Hq=32, Hkv=8, d=128), a calibrated roofline-style policy selects FlashInfer for small active batches, PersistentKV sequence splitting for batch size 1 (B1) long-context steps, and PersistentKV workqueue scheduling for supported B8 long-context GQA steps. With cost-model constants fixed on calibration traces, five held-out seeds improve mean wall decode-token throughput by 1.04x to 1.08x on B8 bimodal, uniform, and Zipf-like workloads, and by 1.40x on a B1 bucketed trace. For the B4 boundary case and uncalibrated GQA ratios, the policy avoids regressions by routing to FlashInfer. We also report an attention-plus-MLP timing proxy and workload counters showing workqueue scheduling reduces launch fan-out from 16.00 to 2.00 launches per step on held-out bimodal B8. These results show that work assignment is a decisive serving-system variable.