On Partial Wait-Freedom in Transactional Memory

TL;DR

This paper investigates the feasibility and complexity of achieving partial wait-freedom in transactional memory systems, focusing on read-only transactions, and reveals significant complexity costs associated with such guarantees.

Contribution

It introduces the concept of partial wait-freedom in transactional memory and analyzes its complexity implications under various desirable properties.

Findings

Read-only transactions can be made wait-free, but at a high complexity cost.

Guaranteeing partial wait-freedom with properties like invisibility increases complexity.

Full wait-freedom remains impossible for dynamic transactional memory.

Abstract

Transactional memory (TM) is a convenient synchronization tool that allows concurrent threads to declare sequences of instructions on shared data as speculative \emph{transactions} with "all-or-nothing" semantics. It is known that dynamic transactional memory cannot provide \emph{wait-free} progress in the sense that every transaction commits in a finite number of its own steps. In this paper, we explore the costs of providing wait-freedom to only a \emph{subset} of transactions. Since most transactional workloads are believed to be read-dominated, we require that read-only transactions commit in the wait-free manner, while updating transactions are guaranteed to commit only if they run in the absence of concurrency. We show that this kind of partial wait-freedom, combined with attractive requirements like read invisibility or disjoint-access parallelism, incurs considerable complexity costs.