---
title: "RadixAttention, Eviction & Cache Churn"
description: "Prefix reuse costs memory: how a radix tree and block hashing answer the same question, when a cached block stops being findable, and cache churn."
canonical_url: "https://fanout.sh/inference-eng/curriculum/10-07"
md_url: "https://fanout.sh/inference-eng/curriculum/10-07.md"
access: "pro-preview"
---

# RadixAttention, Eviction & Cache Churn

Prefix reuse costs memory: how a radix tree and block hashing answer the same question, when a cached block stops being findable, and cache churn.

- Access: Fanout Pro (public preview)

## Public preview

Keeping a block because someone might send its tokens again costs the space that block sits in, and some other request wanted that space. Nothing in this module has charged that cost yet. Both halves of reuse are on the page now, the lookup the automatic prefix caching lesson opened and the traffic the shared prompt reuse lesson shaped for…

## In this lesson

- Two structures for one question
- When a block stops being findable
- The operator's version, and the gate in front of it
- Churn

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