Software transactional memory (STM) allows programmers to easily implement concurrent data structures. STMs simplify atomicity. Recent STMs can achieve good performance for some workloads but they have some limitations. In particular, STMs typically cannot support long-running reads which access a large number of addresses that are frequently updated. Multiversioning is a common approach used to support this type of workload. However, multiversioning is often expensive and can reduce the performance of transactions where versioning is not necessary. In this work we present Multiverse, a new STM that combines the best of both unversioned TM and multiversioning. Multiverse features versioned and unversioned transactions which can execute concurrently. A main goal of Multiverse is to ensure that unversioned transactions achieve performance comparable to the state of the art unversioned STM while still supporting fast versioned transactions needed to enable long running reads. We implement Multiverse and compare it against several STMs. Our experiments demonstrate that Multiverse achieves comparable or better performance for common case workloads where there are no long running reads. For workloads with long running reads and frequent updates Multiverse significantly outperforms existing STMS. In several cases for these workloads the throughput of Multiverse is several orders of magnitude faster than other STMs.

翻译：软件事务内存（STM）使程序员能够轻松实现并发数据结构。STM简化了原子性操作。近期的STM在某些工作负载下能够取得良好性能，但仍存在一些局限性。具体而言，STM通常无法支持访问大量频繁更新地址的长时读操作。多版本化是支持此类工作负载的常用方法。然而，多版本化通常代价高昂，且可能降低无需版本化的事务性能。本文提出Multiverse——一种融合无版本化事务内存与多版本化优势的新型STM。Multiverse支持版本化与无版本化事务的并发执行。其主要设计目标是确保无版本化事务达到与先进无版本化STM相当的性能，同时支持实现长时读操作所需的快速版本化事务。我们实现了Multiverse并与多种STM进行对比实验。实验结果表明，在无非版本化长时读的常规工作负载下，Multiverse取得了相当或更优的性能；对于存在长时读且频繁更新的工作负载，Multiverse显著优于现有STM系统。在此类工作负载的多个场景中，Multiverse的吞吐量比其他STM快数个数量级。