Zombie Hashing: Reanimating Tombstones in Graveyard

Linear probing-based hash tables offer high data locality and are considered among the fastest in real-world applications. However, they come with an inherent tradeoff between space efficiency and speed, i.e. when the hash table approaches full capacity, its performance tends to decline considerably due to an effect known as primary clustering. As a result they are only used at low load factors. Tombstones (markers for deleted elements) can help mitigate the effect of primary clustering in linear probing hash tables. However, tombstones require periodic redistribution, which, in turn, requires a complete halt of regular operations. This makes linear probing not suitable in practical applications where periodic halts are unacceptable. In this paper, we present a solution to forestall primary clustering in linear probing hash tables, ensuring high data locality and consistent performance even at high load factors. Our approach redistributes tombstones within small windows, deamortizing the cost of mitigating primary clustering and eliminating the need for periodic halts. We provide theoretical guarantees that our deamortization method is asymptotically optimal in efficiency and cost. We also design an efficient implementation within dominant linear-probing hash tables and show performance improvements. We introduce Zombie hashing in two variants: ordered (compact) and unordered (vectorized) linear probing hash tables. Both variants achieve consistent, high throughput and lowest variance in operation latency compared to other state-of-the-art hash tables across numerous churn cycles, while maintaining 95% space efficiency without downtime. Our results show that Zombie hashing overcomes the limitations of linear probing while preserving high data locality.