Buffers & memorysenior🔒 Pro concept

Clock-sweep buffer eviction, usage_count & the BAS_BULKREAD ring

Simple terms

Memory is full and Postgres needs somewhere to put a page. Which one goes? There is no proper least-recently-used list, keeping one accurate under load would cost more than it saves. Instead every cached page carries a small popularity counter. A hand sweeps around the buffers knocking each counter down by one, and evicts the first page it finds already at zero. Pages you keep touching keep topping themselves back up, so the hot ones survive sweep after sweep.

You might be asked

How does PostgreSQL decide which shared buffer to evict when the cache is full, what is usage_count, and why doesn't one big sequential scan flush the whole buffer cache?

TopicShared buffers / buffer manager / VACUUM-independent caching
PostgreSQL14, 15, 16, 17, 18
Tools usedpg_buffercache (per-buffer usagecount + pg_buffercache_summary), EXPLAIN (ANALYZE, BUFFERS), a dedicated shared_buffers=2MB server, CHECKPOINT, source navigation @REL_17_10
Last reviewed2026-06-19

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