Achieving Forward Progress Guarantee in Small Hardware Transactions

Hardware-transactional-memory (HTM) manages to pique interest from academia and industry alike because of its potential to ease concurrent-programming without compromising on performance. It offers a simple "all-or-nothing" idea to the programmer, making a piece of code appear atomic in hardware. Despite this and many elegant HTM implementations in research, only best-effort HTM is available commercially. Best-effort HTM lacks forward progress guarantee making it harder for the programmer to create a concurrent scalable fallback path. This has made HTM's adaptability limited. With a scope to support a myriad of applications, HTMs do a trade off between design and verification complexity vs forward progress guarantee. In this letter, we argue that limiting the scope of applications helps HTM attain guaranteed forward progress. We support lock-free programs by using HTM as multi-word-atomics and demonstrate strategic design choices to achieve lock-freedom completely in hardware. We use lfbench, a lock-free micro-benchmark-suite, and Arm's best-effort HTM (ARM_TME) on the gem5 simulator, as our base. We demonstrate the performance tradeoffs between design choices of a deferral-based, NACK-based, and NACK-with-backoff approaches. We show that NACK-with-backoff performs better than the others without compromising scalability for both read- and write-intensive applications.