Model-Based Proactive Read-Validation in Transaction Processing Systems

Concurrency control protocols based on read-validation schemes allow transactions which are doomed to abort to still run until a subsequent validation check reveals them as invalid. These late aborts do not favor the reduction of wasted computation and can penalize performance. To counteract this problem, we present an analytical model that predicts the abort probability of transactions handled via read-validation schemes. Our goal is to determine what are the suited points-along a transaction lifetime-to carry out a validation check. This may lead to early aborting doomed transactions, thus saving CPU time. We show how to exploit the abort probability predictions returned by the model in combination with a threshold-based scheme to trigger read-validations. We also show how this approach can definitely improve performance-leading up to 14 % better turnaround-as demonstrated by some experiments carried out with a port of the TPC-C benchmark to Software Transactional Memory

Prompt Application-Transparent Transaction Revalidation in Software Transactional Memory

Software Transactional Memory (STM) allows encapsulating shared-data accesses within transactions, executed with atomicity and isolation guarantees. The assessment of the consistency of a running transaction is performed by the STM layer at specific points of its execution, such as when a read or write access to a shared object occurs, or upon a commit attempt. However, performance and energy efficiency issues may arise when no shared-data read/write operation occurs for a while along a thread running a transaction. In this scenario, the STM layer may not regain control for a considerable amount of time, thus not being able to early detect if such transaction has become inconsistent in the meantime. To tackle this problem we present an STM architecture that, thanks to a lightweight operating system support, is able to perform a fine-grain periodic (hence prompt) revalidation of running transactions. Our proposal targets Linux and x86 systems and has been integrated with the open source TinySTM package. Experimental results with a port of the TPC-C benchmark to STM environments show the effectiveness of our solution

Opacity: A Correctness Condition for Transactional Memory

Transactional memory is perceived as an appealing alternative to critical sections for general purpose concurrent programming. Despite the large amount of recent work on transactional memory implementations, however, its actual specification has never been precisely defined. This paper presents \emph{opacity}, a new correctness criterion for transactional memory systems. Opacity extends the notion of strict serializability, itself a strong form of the classical serializability property, with the requirement that even \emph{non-committed} transactions are prevented from accessing inconsistent state. Yet opacity does not preclude versioning, invisible reads and lazy updates, often used by modern TM implementations. In fact, most transactional memory systems we know of ensure opacity. We prove a tight bound on the inherent cost of implementing opacity. The bound highlights a trade-off that explains some of the differences between current transactional memory systems, and also draws a sharp complexity line between opacity on one hand, and the combination of strict serializability and strict recoverability on the other hand