CSR: Small: Multi-Version Concurrency Control (MVCC) for Main Memory and its Implications for Deterministic Concurrency

这个项目的目标是研究版本一致性，一个新的并发编程一致性模型，作为提高并发系统性能和可伸缩性的一种手段。在版本一致性下，可以看作是释放一致性的放松，保证进程（只有）在访问相同版本的内存时才能看到相同的内存内容。这些版本是通过调用创建、检索和/或合并的，类似于源代码的版本控制系统。从版本一致性中获益的一类重要应用程序是确定性并发运行时，其目标是确保给定相同输入的程序产生相同的输出，而不受任何非确定性计时效应的影响。在之前的工作中，PI使用版本一致性为确定性运行时dthread实现了高达50%的性能提升。在这个项目中，目标之一是实现确定性运行时的“pthreads parity”，在这种情况下，强制执行确定性只会对性能产生微不足道的影响。如果实现了这一点，“默认决定论”将成为主流计算机系统的可行选择。随着计算机处理器继续从集中的单“核”架构发展到高度分布式和并行的“多核系统”，编写正确的程序以有效地利用这种极其强大的硬件变得越来越困难。这导致了许多有害的影响，从资源利用率低下到严重缺陷的程序，其中可能导致数据丢失，甚至可能导致生命损失。本项目研究(a)降低编程高度并行系统以解决这些问题的复杂性的方法，以及(b)保证即使是不正确的并行程序每次也能产生相同结果的方法。后一部分将帮助程序员编写正确的程序，并修复可能断断续续且难以识别的软件缺陷。

The objective of this project is to investigate version consistency, a new concurrent programming consistency model, as a means of enhancing the performance and scalability of concurrent systems. Under version consistency, which can be seen as a relaxation of release consistency, processes are guaranteed to see the same memory contents (only) if they are accessing the same version of the memory. Such versions are created, retrieved and/or merged through calls analogous with version control systems for source code.An important class of applications that benefits substantially from version consistency is deterministic concurrency runtimes, where the goal is to ensure that a program produces the same output given the same input, independent of any non-deterministic timing effects. In prior work, the PI achieved up to 50% performance gain for the deterministic runtime DThreads, using version consistency. In this project, one of the goals is to achieve ``pthreads parity" for a deterministic runtime, where enforcing determinism incurs only negligible performance impact. When and if this is achieved, ``determinism by default'' becomes a feasible option for mainstream computer systems.As computer processors continue to evolve from a centralized single ``core'' architecture to highly distributed and parallel ``multi-core systems'', writing correct programs that make efficient use of this extremely powerful hardware is becoming increasingly difficult. This results in a number of detrimental effects ranging from poor resource utilization, to seriously flawed programs where loss of data, or even loss of life may result. This project investigates (a) a means of reducing the complexity of programming highly parallel systems to combat these problems, and (b) a means of guaranteeing that even an incorrect parallel program produces the same result every time. This latter part will help programmers write correct programs, and to fix software flaws that may otherwise be intermittent and difficult to identify.