Primus Inter Pares: Improving Parallelism in Hardware Transactional Memory

Hardware transactional memory (HTM) is supported by recent processors from Intel and IBM. HTM is attractive because it can enhance concurrency while simplifying programming. Today's HTM systems rely on existing coherence protocols, which implement a requester-wins strategy. This, in turn, leads to very poor performance when transactions frequently conflict, causing them to resort to a non-speculative fallback path. Often, such a path severely limits concurrency. In this paper, we propose very simple architectural changes to the existing requester-wins HTM architectures. These changes permit higher levels of concurrency when transactions cannot make progress and require a fallback path. The idea is to support a special mode of execution in HTM, called power mode, which can be used to enhance conflict resolution between regular and so-called power transactions. Our idea is backward-compatible with existing HTM code, imposing no additional cost on transactions that do not use the power mode. In addition, it supports dynamic undesired data sharing detection, indicating when transactions whose data sets should be disjoint, are not. Using extensive evaluation of micro- and STAMP benchmarks in a transactional memory simulator and real hardware-based emulation, we show that our technique significantly improves performance of the baseline that does not use power mode, and performs similarly or better than state-of-the-art related proposals that require mode substantial architectural changes.