Practical Experience with Transactional Lock Elision

Transactional Memory (TM) promises both to provide a scalable mechanism for synchronization in concurrent programs, and to offer ease-of-use benefits to programmers. The most straightforward use of TM in real-world programs is in the form of Transactional Lock Elision (TLE). In TLE, critical sections are attempted as transactions, with a fall-back to a lock if conflicts manifest. Thus TLE expects to improve scalability, but not ease of programming. Still, until TLE can deliver performance improvements, transactional styles of programming are unlikely to gain popularity.In this paper, we describe our experiences employing TLE in two real-world programs: the PBZip2 file compression tool, and the x265 video encoder/decoder. We discuss the obstacles we encountered, propose solutions to those obstacles, and introduce open challenges. In experiments using the GCC compiler's hardware and software support for TM, we observe that both are able to outperform the original lock-based code, potentially heralding the readiness of TM to be used more broadly for TLE, if not for truly transactional styles of programming.