arXiv subjects

Buhe Han

Publications and source records attributed to Buhe Han.

2 recordsLinked to original sources

arXiv2026

PDD: Unleashing Economical and Flexible Heterogeneous LLM Inference via Cross-Datacenter Prefill-Decode Disaggregation

Interconnecting geographically dispersed clusters over wide-area Ethernet provides a scalable and cost-effective alternative to dedicated intra-datacenter heterogeneous clusters for Large Language Model (LLM) inference. However, this cross-datacenter disaggregation imposes heavy KV-cache transfers between clusters, causing substantial Time-to-First-Token (TTFT) latency, which is especially detrimental for agentic workloads characterized by long contexts, high cache hit rates, and short outputs. We propose PDD, a three-tier disaggregation architecture built upon prefill-decode (PD) disaggregation, consisting of Prefill, RelayDecode (RLD), and MainDecode (MD) instances. On Cluster A, Prefill instances perform the prefill computation, while RLD instances immediately receive the KV cache via high-speed RDMA and begin decoding, thereby overlapping the KV transfer to Cluster B over TCP-based Ethernet. MD instances on Cluster B receive the KV cache along with the tokens produced by RLD, and decoding is then seamlessly handed off from RLD to MD for completion. To maximize overall efficiency, PDD employs three core mechanisms: Decode-side RadixCache to alleviate bandwidth bottlenecks, an Extend-Decode Handoff mechanism for smooth control migration between RLD and MD, and multi-stage pipeline orchestration to manage complex inter-tier dependencies under high concurrency and long-term serving. We further design a low-cost, fine-grained heterogeneous deployment scheme that maximizes latency-masking efficiency at marginal cost. Compared to the intra-DC homogeneous PD baseline, PDD's cross-datacenter mapping of compute-intensive H100s and memory-bandwidth-optimized H200s achieves a Benefit-Cost Ratio (BCR) up to 37.5% higher in SLA-compliant goodput.

cs.AR↗

arXiv2025

semi-PD: Towards Efficient LLM Serving via Phase-Wise Disaggregated Computation and Unified Storage

Existing large language model (LLM) serving systems fall into two categories: 1) a unified system where prefill phase and decode phase are co-located on the same GPU, sharing the unified computational resource and storage, and 2) a disaggregated system where the two phases are disaggregated to different GPUs. The design of the disaggregated system addresses the latency interference and sophisticated scheduling issues in the unified system but leads to storage challenges including 1) replicated weights for both phases that prevent flexible deployment, 2) KV cache transfer overhead between the two phases, 3) storage imbalance that causes substantial wasted space of the GPU capacity, and 4) suboptimal resource adjustment arising from the difficulties in migrating KV cache. Such storage inefficiency delivers poor serving performance under high request rates. In this paper, we identify that the advantage of the disaggregated system lies in the disaggregated computation, i.e., partitioning the computational resource to enable the asynchronous computation of two phases. Thus, we propose a novel LLM serving system, semi-PD, characterized by disaggregated computation and unified storage. In semi-PD, we introduce a computation resource controller to achieve disaggregated computation at the streaming multi-processor (SM) level, and a unified memory manager to manage the asynchronous memory access from both phases. semi-PD has a low-overhead resource adjustment mechanism between the two phases, and a service-level objective (SLO) aware dynamic partitioning algorithm to optimize the SLO attainment. Compared to state-of-the-art systems, semi-PD maintains lower latency at higher request rates, reducing the average end-to-end latency per request by 1.27-2.58x on DeepSeek series models, and serves 1.55-1.72x more requests adhering to latency constraints on Llama series models.

cs.CL↗