The main purpose of SLRUs is to track metadata about Postgres transactions.
SLRUs are a general mechanism used by multiple types. Like a lot of things in Postgres, the SLRU system is extensible which means extensions can create new types.
The “least recently used” aspect might be recognizable from cache systems. LRU refers to how the oldest items are evicted from the cache when it’s full, and newer items take their place. This is because the cache has a fixed amount of space (measured in 8KB pages) and thus can only store a fixed amount of items.
Read on to learn more about these two concepts and how things have changed in Postgres 17.