The Impact of Memory Models on Software Reliability in Multiprocessors

TL;DR

This paper investigates how different memory consistency models in multiprocessors affect software reliability, revealing that weaker models increase bug vulnerability with few threads but become less significant as thread count grows.

Contribution

It introduces a probabilistic model to analyze the impact of memory models on concurrency bug vulnerability, providing bounds and insights into their effects based on thread count.

Findings

Weaker memory models increase bug vulnerability with few threads.

As thread count increases, the impact of memory model strength diminishes.

Implications for designing future multi-core systems' memory models.

Abstract

The memory consistency model is a fundamental system property characterizing a multiprocessor. The relative merits of strict versus relaxed memory models have been widely debated in terms of their impact on performance, hardware complexity and programmability. This paper adds a new dimension to this discussion: the impact of memory models on software reliability. By allowing some instructions to reorder, weak memory models may expand the window between critical memory operations. This can increase the chance of an undesirable thread-interleaving, thus allowing an otherwise-unlikely concurrency bug to manifest. To explore this phenomenon, we define and study a probabilistic model of shared-memory parallel programs that takes into account such reordering. We use this model to formally derive bounds on the \emph{vulnerability} to concurrency bugs of different memory models. Our results show that for 2 (or a small constant number of) concurrent threads, weaker memory models do indeed have a higher likelihood of allowing bugs. On the other hand, we show that as the number of parallel threads increases, the gap between the different memory models becomes proportionally insignificant. This suggests the counter-intuitive rule that \emph{as the number of parallel threads in the system increases, the importance of using a strict memory model diminishes}; which potentially has major implications on the choice of memory consistency models in future multi-core systems.