Tel-aviv University Raymond and Beverly Sackler Faculty of Exact Sciences School of Computer Science

A crucial property required from software transactional memory systems (STMs) is that transactions, even ones that will eventually abort, will operate on consistent states. A known technique for providing this property is through the introduction of a globally shared version clock whose values are used to tag memory locations. Realizing global clock is the root of significant overhead, this thesis presents TLC 1, the first thread-local clock mechanism for allowing transactions to operate on consistent states. TLC is the proof that one can devise coherent-state STM systems without a global clock. Existing optimized global clock implementations: GV4, GV5 and GV6 are examined and a new one, GV7 is proposed. Another contribution of this thesis is the definition of the abstract data type which has the minimal characteristics that are required to maintain STM consistent states. We name the object transactional consistency verifier (TCV) and prove all the clocking schemes demonstrated are TCV implementations. A set of benchmarks is presented within the context of the TL2 STM algorithm, using TLC, and is compared to known global clock schemes and our new GV7. Our benchmarks show TLC perform as well as a global clock on small scale machines. The big promise of the TLC approach is in providing a decentralized solution for future large scale machines. On such machines, a globally coherent clock based solution is most likely infeasible, and TLC promises a way for transactions to operate consistently in a distributed fashion. Hillel Avni, Nir Shavit: Maintaining Consistent Transactional States without a Global Clock. SIROCCO 2008 ii