A case for Exiting a Transaction in the Context of Hardware Transactional Memory

Despite the rapid growth in the area of Transactional Memory (TM), there is a lack of standardisation of certain features. The behaviour of a transactional abort is one such feature. All hardware TM and most software TM designs treat abort as a way of restarting the current transaction. However an alternative representation for the same functionality has been expressed in some software transactional memories and programming languages proposals. These allow the termination of a transaction without restarting. In this paper we argue that a similar functionality is required for hardware TM as well. We call this functionality Exit_Transaction, in which a programmer can explicitly ask the underlying TM system to move to the end of the transaction without committing it. We discuss how to extend a hardware TM system to support such a feature and our evaluation with two hardware TM systems shows that by using this functionality a speedup of up to 1.35X can be achieved on the benchmarks tested. This is achieved as a result of lower contention for resources and less false positives.