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How does prefix caching work internally in an LLM server, and when does it actually help?

A shared system prompt gets re-prefilled on every request unless the server remembers it. Prefix caching skips that work, but only when the blocks line up exactly. Here is the hashing and eviction machinery underneath.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

A shared system prompt gets re-prefilled on every request unless the server remembers it. Prefix caching skips that work, but only when the blocks line up exactly. Here is the hashing and eviction machinery underneath.

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Next in this trackDesign the request router and scheduler for a multi-replica LLM serving cluster.Next in this trackCompare static, dynamic, and continuous batching for LLM serving and state the tradeoffs.Next in this trackHow do you offload the KV cache to CPU or NVMe, and when is it worth the bandwidth hit?
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