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

本项目的目标是研究一种新的并发编程一致性模型--版本一致性，以此作为提高并发系统性能和可扩展性的手段。在版本一致性下，这可以被视为释放一致性的放松，如果进程正在访问相同版本的内存，则保证它们(仅)看到相同的内存内容。这样的版本是通过类似于源代码的版本控制系统的调用来创建、检索和/或合并的。实质上受益于版本一致性的一类重要应用是确定性并发运行时，其中的目标是确保程序在给定相同输入的情况下产生相同的输出，而不受任何非确定性定时影响的影响。在以前的工作中，PI在使用版本一致性的情况下，实现了确定性运行时DThread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“p线程对等”，在这种情况下，实施确定性只会导致可以忽略不计的性能影响。一旦实现了这一点，“默认决定论”将成为主流计算机系统的一种可行选择。随着计算机处理器不断从集中式的单一“核心”体系结构发展到高度分布式和并行的“多核心系统”，编写正确的程序来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，从资源利用率低到程序有严重缺陷，可能导致数据丢失，甚至生命损失。这个项目研究(A)一种降低高度并行系统编程的复杂性以解决这些问题的方法，以及(B)一种保证即使是不正确的并行程序每次都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，并修复软件缺陷，否则可能是间歇性的和难以识别的。

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.