Opacity vs TMS2: Expectations and Reality

Most of the popular Transactional Memory (TM) algorithms are known to be safe because they satisfy opacity, the well-known correctness criterion for TM algorithms. Recently, it has been shown that they are even more conservative, and that they satisfy TMS2, a strictly stronger property than opacity. This paper investigates the theoretical and practical implications of relaxing those algorithms in order to allow histories that are not TMS2. In particular, we present four impossibility results on TM implementations that are not TMS2 and are either opaque or strictly serializable, and one practical TM implementation that extends TL2, a high-performance state-of-the-art TM algorithm, to allow non-TMS2 histories. By matching our theoretical findings with the results of our performance evaluation, we conclude that designing and implementing TM algorithms that are not TMS2, but safe, has inherent costs that limit any possible performance gain.