In-network traffic regulation for Transactional Memory

Hardware Transactional Memory (HTM) promises to simplify parallel programming on shared-memory chip multiprocessors by providing atomic execution of code blocks. Concurrently, Networks-On-Chip (NOCs) have emerged as an efficient on-chip communication infrastructure but have been largely neglected in HTM designs. In this work, we explore the interaction between the HTM paradigm and NOCs. In the process, we find a huge source of unnecessary network traffic incurred by transactional requests that are unsuccessful. This problem is identified as false forwarding that adversely affects network performance and energy efficiency. Surprisingly, 39% (up to 79% for a specific workload) of the transactional requests have incurred false forwarding over a wide spectrum of workloads. To combat this problem, we propose TMNOC, a novel approach that exploits the co-design of HTM and NOCs to mitigate false forwarding. Transactional requests that have a high probability to fail are filtered out in-network as early as possible to save energy and improve concurrency in the memory system. Experimental results show that our design reduces total network traffic by 20% on average (up to 40%) for a set of high-contention benchmarks representative of future TM workloads, thereby reducing energy consumption by an average of 24% (up to 39%). Meanwhile, the contention in the coherence directory is reduced by 66% on average. These improvements are achieved with only 5% area overhead added to a conventional on-chip router design.