The Ordering Requirements of Relativistic and Reader-Writer Locking Approaches to Shared Data Access

The semantics of reader-writer locks allow read-side concurrency. Unfortunately, the locking primitives serialize access to the lock variable to an extent that little or no concurrency is realized in practice for small critical sections. Relativistic programming is a methodology that also allows readside concurrency. Relativistic programming uses different ordering constraints than reader-writer locking. The different ordering constraints allow relativistic readers to proceed without synchronization so relativistic readers scale even for very short critical sections. In this paper we explore the differences between the ordering constraints for reader-writer locking and relativistic programs. We show how and why the different ordering constraints allow relativistic programs to have both better performance and better scalability than their reader-writer locking counterparts.