Why a Pool? (Approximated LRU/LFU)
Scanning all keys to find the *perfect* one to evict is slow. Instead, Redis performs a high-speed approximation: it takes a small, random sample of keys and places the best candidates into this 16-slot pool. It then evicts the single best key *from the pool after a final check*. This is much faster and a key reason Redis remains responsive under memory pressure.