ClickHouse Recycles Small Objects Through Per-Size Free Lists
ClickHouse’s ArenaWithFreeLists keeps one free list per power-of-two size class and threads the free-list link through the freed block itself.
Data structures
ClickHouse's LRU Cache Keeps Its Order in a Linked List
ClickHouse’s LRU cache is a hash map of cells pointing into a linked list, so a lookup is an O(1) splice and eviction is a pop from the front.