Scheduling and serialization techniques for transactional memories

Abstract

In the last few years, Transactional Memories (TMs) have been shown to be a parallel programming model that can effectively combine performance improvement with ease of programming. Moreover, the recent introduction of (H)TM-based ISA extensions, by major microprocessor manufacturers, also seems to endorse TM as a programming model for today's parallel applications. One of the central issues in designing Software TM (STM) systems is to identify mechanisms or heuristics that can minimize contention arising from conflicting transactions. Although a number of mechanisms have been proposed to tackle contention, such techniques have a limited scope, because conflict is avoided by either interrupting or serializing transaction execution, thus considerably impacting performance. This work explores a complementary approach to boost the performance of STM through the use of schedulers. A TM scheduler is a software component that decides when a particular transaction should be executed. Their effectiveness is very sensitive to the accuracy of the metrics used to predict transaction behaviour, particularly in high-contention scenarios. This work proposes a new Dynamic Transaction Scheduler -DTS to select a transaction to execute next, based on a new policy that rewards success and an improved metric that measures the amount of effective work performed by a transaction.Hardware TMs (HTM) are an interesting mechanism to implement TM as they integrate the support for transactions at the lowest, most efficient, architectural level. On the other hand, for some applications, HTMs can have their performance hindered by the lack of scalability and by limitations in cache store capacity. This work presents an extensive performance study of the implementation of HTM in the Haswell generation of Intel x86 core processors. It evaluates the strengths and weaknesses of this new architecture by exploring several dimensions in the space of TM application characteristics. This detailed performance study provides insights on the constraints imposed by the Intel's Transaction Synchronization Extension (Intel's TSX) and introduces a simple, but efficient, serialization policy for guaranteeing forward progress on top of the best-effort Intel's HTM which was critical to achieving performance. ix x I thank all the faculty members, staff and students of the Computer Science Institute at UNICAMP as well as of the Department of Computing Science at the University of Alberta, whose presence and spirit have made both fantastic places to learn, to think, and to have fun.Finally, and most importantly, I would like to thank my family. They were always supporting me and encouraging me with their best wishes. Most especially, I would like to thank my wife and best friend, Lia, for her love and tremendous support in each step of the way.