vLLM’s Hash Chain, SGLang’s Radix Tree

vLLM's hash chain and SGLang's radix tree offer distinct methods for improving prefix caching in data processing systems. vLLM utilizes a hash chain via Automatic Prefix Caching (APC) that leverages content hashing to detect shared prefixes without explicit tracking, using fixed-size blocks that facilitate efficient lookups and LRU eviction. However, this method is limited to prefix-bound content and does not support shared suffixes or segments beyond prefix positions. In contrast, SGLang employs a radix tree structure for KV cache entries, allowing prefix matching at any token boundary, which is beneficial in multi-turn conversations with variable-length shared contexts. While vLLM's approach is optimized for templated workloads and provides a flat, mmap-friendly memory layout ideal for shared or persistent cache stores, SGLang excels in scenarios with multi-turn conversations, offering higher effective hit rates and cache-aware request routing. Both systems converge effectively for agentic workloads with stable prefixes, though their architectural differences become influential in high-concurrency, variable-length scenarios.

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