Transactional memory concurrency : new models and systems
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Transactional memory (TM) aims to bring the benefits of ACID transactions to the volatile world of program synchronization. Architectural trends are making software transactions more appealing, as more programmers struggle with the problems of locks as they exploit multi-core processors. This thesis applies TM, which until recently has been restricted to small benchmarks, to a large, real-life system: the Linux operating system kernel. I describe TxLinux, a version of Linux, which is the first OS to use transactional memory for synchronization. TxLinux runs on MetaTM, a simulator co-designed with TxLinux, which models an x86-based Hardware Transactional Memory (HTM) system. The TxLinux/MetaTM effort yields a characterization of real-life OS transactions, exposes previously unconsidered complications (including interaction with interrupts and stack memory) and allows sensitivity studies of various TM microarchitectural parameters. It also provides a flexible platform for future OS, TM and architecture research. Next, I examine ways to increase concurrency by investigating the factors that inhibit concurrency in existing TM models and systems. These include avoidable implementation limitations, overly restrictive serialization models, and inexpressive APIs. After examining the nature of each limitation, I propose a solution for each one. I postulate that the conventional wisdom that every transaction is "for itself" and primarily relates to other transactions by conflicting with them, is a pervasive misperception. This thesis aims to demonstrate that there are other ways of thinking about the relation of one transaction to another. I present three different transaction models to show how (i) co-existence, (ii) cooperation, and (iii) coordination, can each solve important problems facing TM programmers today. Co-existence of multiple transactions on the same processor is enabled using the suspended transactions model. This model, used by TxLinux, can reduce aborts and removes transaction length limitations imposed by interrupts. Cooperation of transactions that access the same data, using the dependence-aware transactions model, can transparently turn transaction aborts into commits. Drawing on serializability theory and notions of spheres of control (which predate ACID transactions), this model is able to accept more execution schedules than any existing TM design. Lastly, the coordination of multiple transactions in the coordinated sibling transactions model, provides programmers a simple and unified way of expressing intratransaction parallelism. This helps move transactions beyond being a drop-in replacement for locks (SLE-style) to instead helping programmers find more parallel work within their programs (both in speculative and non-speculative forms). All three models aim at increasing concurrency, while shifting complexity away from the programmer and into the TM system. I evaluate all three models, using either the MetaTM HTM, or one of the several software (STM) systems this thesis also develops.