Time, Fences and the Ordering of Events in TSO

2025-08-15

AI Summary

The TSO memory model improves performance via store buffering but weakens sequential consistency, complicating correctness verification—particularly in determining the necessity of synchronization primitives (e.g., memory fences, atomic RMW operations). Method: We adapt Lamport’s happens-before relation to TSO semantics, introducing a TSO-specific happens-before definition that rigorously captures inter-core temporal dependencies. We prove that constructing a complete happens-before chain is a generalized lower bound for linearizability. Leveraging asynchronous message-passing semantics, we formally analyze how fences and RMWs uniquely enable causal chain formation. Contribution/Results: We derive precise, necessary conditions for synchronization primitives under TSO. Our framework establishes a theoretical foundation and practical criteria for balancing correctness and performance in weak memory models, enabling principled reasoning about synchronization necessity in concurrent systems.

Technology Category

• Constraint Satisfaction and Optimization: Satisfiability Modulo Theories
• Multiagent Systems: Other Foundations of Multi Agent Systems
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Web performance, measurement, and characterization
• Security and Privacy: Data transparency and provenance
• Graph Algorithms and Modeling for the Web: Algorithms and analysis for incomplete, noisy, or partially observed Web-related graphs

Abstract

The Total Store Order (TSO) is arguably the most widely used relaxed memory model in multiprocessor architectures, widely implemented, for example in Intel's x86 and x64 platforms. It allows processes to delay the visibility of writes through store buffering. While this supports hardware-level optimizations and makes a significant contribution to multiprocessor efficiency, it complicates reasoning about correctness, as executions may violate sequential consistency. Ensuring correct behavior often requires inserting synchronization primitives such as memory fences ($F$) or atomic read-modify-write ($RMW$) operations, but this approach can incur significant performance costs. In this work, we develop a semantic framework that precisely characterizes when such synchronization is necessary under TSO. We introduce a novel TSO-specific occurs-before relation, which adapts Lamport's celebrated happens-before relation from asynchronous message-passing systems to the TSO setting. Our main result is a theorem that proves that the only way to ensure that two events that take place at different sites are temporally ordered is by having the execution create an occurs-before chain between the events. By studying the role of fences and $RMW$s in creating occurs-before chains, we are then able to capture cases in which these costly synchronization operations are unavoidable. Since proper real-time ordering of events is a fundamental aspect of consistency conditions such as Linearizability, our analysis provides a sound theoretical understanding of essential aspects of the TSO model. In particular, we are able to generalize prior lower bounds for linearizable implementations of shared memory objects. Our results capture the structure of information flow and causality in the TSO model by extending the standard communication-based reasoning from asynchronous systems to the TSO memory model.

Problem

• Characterizing necessary synchronization under TSO memory model
• Analyzing role of fences and RMWs in event ordering
• Extending causality reasoning from async systems to TSO

Innovation

• Develops a semantic framework for TSO synchronization
• Introduces novel TSO-specific occurs-before relation
• Analyzes fence and RMW roles in event ordering