Split-ordered lists: Lock-free extensible hash tables

We present the first lock-free implementation of an extensible hash table running on current architectures. Our algorithm provides concurrent insert, delete, and find operations with an expected O(1) cost. It consists of very simple code, easily implementable using only load, store, and compare-and-swap operations. The new mathematical structure at the core of our algorithm is recursive split-ordering, a way of ordering elements in a linked list so that they can be repeatedly “split” using a single compare-and-swap operation. Metaphorically speaking, our algorithm differs from prior known algorithms in that extensibility is derived by “moving the buckets among the items” rather than “the items among the buckets.” Though lock-free algorithms are expected to work best in multiprogrammed environments, empirical tests we conducted on a large shared memory multiprocessor show that even in non-multiprogrammed environments, the new algorithm performs as well as the most efficient known lock-based resizable hash-table algorithm, and in high load cases it significantly outperforms it.