Progressive Transactional Memory in Time and Space

TL;DR

This paper investigates the fundamental time and space complexity limits of progressive lock-based transactional memory systems, revealing inherent costs in read validation and establishing a new lower bound on remote memory references.

Contribution

It provides the first lower bound on remote memory references in progressive TMs and characterizes the inherent costs of read validation in lock-based transactional memory.

Findings

Read validation can require quadratic steps in the number of data items.

A progressive TM can have raca0n d7 a0loga0na0 RMRs in execution.

Established the first RMR complexity lower bound for transactional memory.

Abstract

Transactional memory (TM) allows concurrent processes to organize sequences of operations on shared \emph{data items} into atomic transactions. A transaction may commit, in which case it appears to have executed sequentially or it may \emph{abort}, in which case no data item is updated. The TM programming paradigm emerged as an alternative to conventional fine-grained locking techniques, offering ease of programming and compositionality. Though typically themselves implemented using locks, TMs hide the inherent issues of lock-based synchronization behind a nice transactional programming interface. In this paper, we explore inherent time and space complexity of lock-based TMs, with a focus of the most popular class of \emph{progressive} lock-based TMs. We derive that a progressive TM might enforce a read-only transaction to perform a quadratic (in the number of the data items it reads) number of steps and access a linear number of distinct memory locations, closing the question of inherent cost of \emph{read validation} in TMs. We then show that the total number of \emph{remote memory references} (RMRs) that take place in an execution of a progressive TM in which $n$ concurrent processes perform transactions on a single data item might reach $\Omega(n \log n)$, which appears to be the first RMR complexity lower bound for transactional memory.