LIRS (Low Inter-reference Recency Set)

Goal

Strong scan resistance by using inter-reference recency (distance between repeated touches) rather than simple last-touch recency.

Core Idea (Very High Level)

Classify blocks as:

• LIR: low inter-reference recency (frequently reused) ⇒ protected
• HIR: high inter-reference recency (infrequently reused) ⇒ candidates for eviction

Maintain:

• S stack: tracks recency information to estimate inter-reference recency
• Q queue: resident HIR blocks; eviction occurs from Q

Core Data Structures (Typical)

• Hash index K -> EntryMeta
• S: stack (often an intrusive list) with pruning rules to keep it meaningful
• Q: queue (often an intrusive list) for resident HIR entries

Complexity & Overhead

• More complex than LRU/2Q; requires careful maintenance of S pruning invariants.

References

• Jiang, Zhang (2002): “LIRS: An Efficient Low Inter-reference Recency Set Replacement Policy…”.
• Wikipedia: https://en.wikipedia.org/wiki/Cache_replacement_policies