Multi-versioning in transactional memory

Reducing the number of aborts is one of the biggest challenges of most transactional systems: existing TMs may abort many transactions that could, in fact, commit without violating correctness. Historically, the commonly used method for reducing the abort rate was maintaining multiple object versions. Multiversion concurrency control is a classical approach for providing concurrent access to the database in database management systems. Its idea is to let a reading transaction obtain a consistent snapshot corresponding to an arbitrary point in time (e.g., defined at the beginning of a transaction) – concurrent updates are isolated through maintaining old versions rather than via scheduling decisions. Multi-versioning was adopted by transactional memory algorithms as well. In this chapter we overview the multi-versioning approach by studying the inherent properties of STMs that use multiple versions to guarantee successful commits of all read-only transactions. We first consider the challenges of garbage collecting of old object versions, and show that no STM can be optimal in the number of previous versions kept, while following the na¨ıve approach of keeping a constant number of last versions per object might lead to an exponential memory growth. We then show the potential performance challenges of multi-versioned STMs, including disjoint-access parallelism and visibility of read-only transactions. We demonstrate the advantages of implementing multi-versioned STMs in managed memory environments by presenting SelectiveMulti-Versioning (SMV) algorithm. SMV relies on automatic garbage collection, and thus efficiently deals with old versions while still allowing invisible read-only transactions.