vLLM's Hash Chain and Why Prefix Caching Is Still Prefix Caching

Automatic Prefix Caching is designed to improve the efficiency of reusing previously computed data in key-value (KV) caching systems by automatically discovering shared prefixes in requests, but it remains limited by its reliance on reusing only prefix-aligned content. Although techniques such as content hashing and the use of hash chains and radix trees enhance the mechanics of identifying reusable prefixes, they do not expand the scope of what can be reused beyond those prefixes. This results in inefficiencies, especially in workloads where shared content does not align perfectly with prefix boundaries, like in Retrieval-Augmented Generation (RAG) pipelines. While systems like vLLM implement content hashing with fixed-size blocks to facilitate reuse, and SGLang uses a radix tree to match longer prefixes, both approaches remain constrained by the prefix structure. Current research is focused on overcoming these limitations by exploring cache repair and segment-level reuse to recover work that traditional prefix-based systems miss, aiming to enhance KV cache utilization beyond just shared prefixes.