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.

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Transactional Memory (TM) is a compelling alternative to simplify multi-threaded programming that traditionally relies on error-prone lockbased synchronization for implementing cooperative tasks. Lazy-Lazy hardware TM is one of the most efficient schemes in today's Hardware TM systems (HTMs). Nonetheless, the commit protocol in these systems has severe impact on performance and energy. The SEQ in Scalable TCC implementation (STCC-SEQ) is the most popular and efficient commit protocol to date. In this paper, we propose GCommit, a cost-effective hardware-based STCC-SEQ protocol. GCommit employs a G-Arbiter microarchitecture for achieving minimal-latency and high-efficient commits. We implement G-Arbiter with a standard 45 nm cell library. For a target 16-core CMP, a G-Arbiter just represents 0.07% of the whole on-chip area, requiring marginal energy consumption. Full-system simulations of the target system with the STAMP benchmarks show that GCommit achieves average reductions of 15.7% and 13.7% in execution time and energy, respectively, when compared with STCC-SEQ.