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 implementations. 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. A power transaction can run concurrently with regular transactions that do not conflict with it. This permits higher levels of concurrency in cases when a (regular) transaction cannot make progress due to conflicts and would require a non-speculative fallback path otherwise. Our idea is backward-compatible with existing HTM systems, imposing no additional cost on transactions that do not use the power mode. Furthermore, using power transactions requires no changes to target applications that employ traditional lock synchronization. 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 the performance of the baseline that does not use power mode, and performs comparably with state-of-the-art related proposals that require more substantial architectural changes.