Wait-Free Multi-Word Compare-and-Swap Using Greedy Helping and Grabbing

Abstract

We present a new algorithm for implementing a multi-word compare-and-swap functionality supporting the Read and CASN operations. The algorithm is wait-free under reasonable assumptions on execution history and benefits from new techniques to resolve conflicts between operations by using greedy helping and grabbing. Although the deterministic scheme for enabling grabbing somewhat sacrifices fairness, the effects are insignificant in practice. Moreover, unlike most of the previous results, the CASN operation does not require the list of addresses to be sorted before the operation is invoked, and the Read operation can read the current value without applying helping when the word to be read is within an ongoing transaction. Experiments using micro-benchmarks varying important parameters in three dimensions have been performed on two multiprocessor platforms. The results show similar performance as the lock-free algorithm by Harris et al. for most scenarios, and significantly better performance on scenarios with very high contention. This is altogether extraordinary good performance considering that the new algorithm is wait-free.