Leaplist: lessons learned in designing tm-supported range queries

Hillel Avni, Nir Shavit, Adi Suissa

Abstract: We introduce Leaplist, a concurrent data-structure that is tailored to provide linearizable range queries. A lookup in Leaplist takes O (log n) and is comparable to a balanced binary search tree or to a Skiplist. However, in Leaplist, each node holds up-to K immutable key-value pairs, so collecting a linearizable range is K times faster than the same operation performed non-linearizably on a Skiplist.We show how software transactional memory support in a commercial compiler helped us create an efficient lock-based implementation of Leaplist. We used this STM to implement short transactions which we call Locking Transactions (LT), to acquire locks, while verifying that the state of the data-structure is legal, and combine them with a transactional Consistency Oblivious Programming (COP) [2] mechanism to enhance data structure traversals.We compare Leaplist to prior implementations of Skiplists, and show that while updates in the Leaplist are slower, lookups are somewhat faster, and for range-queries the Leaplist outperforms the Skiplist's non-linearizable range query operations by an order of magnitude. We believe that this data structure and its performance would have been impossible to obtain without the STM support.