Proactive contention avoidance

In current TM systems, both STM and HTM, if two transactions access the same address, and, one of them writes it, at least one of the two is aborted. However, many times, the aborted transaction was in a valid state, and work was lost for no good reason. In the first part of the chapter we discuss lowering such contention. We focus on methods that never lock-out a transaction, thus, we exclude approaches that serialize writing transaction to allow irrevocable transactions, for example. We are interested only in mechanisms that avoid conflicts and not in contention managers which resolves them. This part is about using the TM that exists, both in hardware and the compiler. The second part of the chapter is about SemanticTM, an algorithm that manages to eliminate the need for aborts, while maintaining parallelism. SemanticTM allows the application to maintain a consistent state without locks and aborts, but is currently restricted to specific scenarios.