A TimeStamp Based Multi-version STM Algorithm

Software Transactional Memory Systems (STM) are a promising alternative for concurrency control in shared memory systems. Multiversion STM systems maintain multiple versions for each t-object. The advantage of storing multiple versions is that it facilitates successful execution of higher number of read operations than otherwise. Multi-Version permissiveness (mv-permissiveness) is a progress condition for multi-version STMs that states that a read-only transaction never aborts. Recently a STM system was proposed that maintains only a single version but is mv-permissive. This raises a natural question: how much concurrency can be achieved by multi-version STM. We show that fewer transactions are aborted in multi-version STMs than single-version systems. We also show that any STM system that is permissive w.r.t opacity must maintain at least as many versions as the number of live transactions. A direct implication of this result is that no single-version STM can be permissive w.r.t opacity. In this paper we present a time-stamp based multiversion STM system that satisfies opacity and is easy to implement. We formally prove the correctness of the proposed STM system. Although many multi-version STM systems have been proposed in literature that satisfy opacity, to the best of our knowledge none of them has been formally proved to be opaque. We also present garbage collection procedure which deletes unwanted versions of the transaction objects. We show that with garbage collection the number of versions maintained is bounded by number of live transactions.