Software-improved hardware lock elision

With hardware transactional memory (HTM) becoming available in mainstream processors, lock-based critical sections may now initiate a hardware transaction instead of taking the lock, enabling their concurrent execution unless a real data conflict occurs. However, just a few transactional aborts can cause the lock to be acquired non-transactionally resulting in the serialization of all the threads, severely degrading the amount of speedup obtained. In this paper we provide two software extension mechanisms that considerably improve the concurrency and speedup levels attained by lock based programs using HTM-based lock elision. The first sacrifices opacity to achieve higher levels of concurrency, and the second retains opacity while reaching slightly lower levels of concurrency. Evaluation on STAMP and on data structure benchmarks on an Intel Haswell processor shows that these techniques improve the speedup by up to 3.5 times and 10 times respectively, compared to using Haswell's hardware lock elision as is.