Conflict management is a key design dimension of hardware transactional memory (HTM) systems, and the implementation of efficient mechanisms for detection and resolution becomes critical when conflicts are not a rare event. Current designs address this problem from two opposite perspectives, namely, lazy and eager schemes. While the former approach is based on an purely optimistic view that is not well-suited when conflicts become frequent, the latter results too pessimistic because resolves conflicts too conservatively, often limiting concurrency unnecessarily. In this paper, we present a hybrid, pseudo-optimistic scheme of conflict resolution for HTM systems that recaptures the concept of speculation to allow transactions to continue their execution past conflicting accesses. Simulation results show that our proposal is capable of combining the advantages of both classical approaches. For the STAMP transactional benchmarks, our hybrid scheme outperforms both eager and lazy systems with average reductions in execution time of 8 and 17%, respectively, and it decreases network traffic by another 17% compared to the eager policy.

SUMMARYTransactional memory is currently being advocated as a promising alternative to lock‐based synchronization because it simplifies multithreaded programming. In this way, future many‐core chip multiprocessor architectures may need to provide hardware support for transactional memory. On the other hand, energy consumption constitutes nowadays a first class consideration in multicore processor designs. In this work, we characterize the performance and energy consumption of two well‐known hardware transactional memory systems that employ opposite policies for data versioning and conflict management. More specifically, we compare a LogTM‐SE eager‐eager system and a version of the Scalable Transactional Coherence and Consistency lazy‐lazy system that enable parallel commits. To do so, we extended the Multifacet GEMS simulator to estimate the energy consumed in the on‐chip caches according to CACTI and used the interconnection network energy model given by Orion 2. Results show that the energy consumption of the eager‐eager system is 38% higher in average than in the lazy‐lazy case, whereas performance differences between the two systems are 26% in average. We found that even though lazy‐lazy beats eager‐eager on average, there are considerable deviations in performance depending on the particular characteristics of each application and the settings of both systems. Finally, from this characterization, we observe that a significant part of the energy consumed in some applications in eager‐eager is spent on the back‐off delay phase and explore more energy‐efficient hardware back‐off mechanisms. For lazy‐lazy systems, the way in which memory lines are assigned to the L2 cache banks affects the number of parallel commits in some applications, and we study an alternative fine‐grained assignment. Copyright © 2012 John Wiley & Sons, Ltd.

Abstract. One of the key design points of any hardware transactional memory (HTM) system is the conflict detection mechanism, and its efficient implementation becomes critical when conflicts are not a rare event. While many contemporary proposals rely on the coherence protocol to carry out conflict detection at the private cache levels, this approach is not optimal for systems that use a directory to maintain coherence over an unordered, scalable network, such as tiled CMPs. In this paper, we present a new scheme of conflict detection for HTM systems, which moves this key mechanism from the private caches to the directory level. We propose a novel transactional book-keeping method and describe how this detection can be carried out more efficiently at the directory. Simulation results show that our approach obtains reductions in execution time between 25 and 55% for transactional benchmarks with a high number of conflicts, with an average improvement over LogTM-SE of 15%.