Constant-Time Snapshots with Applications to Concurrent Data Structures

Abstract

We present an approach for efficiently taking snapshots of the state of a collection of CAS objects. Taking a snapshot allows later operations to read the value that each CAS object had at the time the snapshot was taken. Taking a snapshot requires a constant number of steps and returns a handle to the snapshot. Reading a snapshotted value of an individual CAS object using this handle is wait-free, taking time proportional to the number of successful CASes on the object since the snapshot was taken. Our fast, flexible snapshots yield simple, efficient implementations of atomic multi-point queries on concurrent data structures built from CAS objects. For example, in a search tree where child pointers are updated using CAS, once a snapshot is taken, one can atomically search for ranges of keys, find the first key that matches some criteria, or check if a collection of keys are all present, simply by running a standard sequential algorithm on a snapshot of the tree. To evaluate the performance of our approach, we apply it to two search trees, one balanced and one not. Experiments show that the overhead of supporting snapshots is low across a variety of workloads. Moreover, in almost all cases, range queries on the trees built from our snapshots perform as well as or better than state-of-the-art concurrent data structures that support atomic range queries.