XPS: FULL: Collaborative Research: Rethinking Architecture Support for Memory Consistency

XPS：完整：协作研究：重新思考对内存一致性的架构支持

• 批准号: 1629126
• 负责人: Michael Bond
• 金额: $ 34.39万
• 依托单位: Ohio State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-01 至 2021-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1629126&HistoricalAwards=false
• 关键词: XPS; FULL; Collaborative; Research; Rethinking

Despite decades of progress, writing correct parallel software to realize the value of modern parallel computer hardware remains extremely difficult. A key problem is that today's computer systems do not give all programs clear behavioral guarantees; "ill-synchronized" code, in which parallel computations are incompletely or incorrectly coordinated, has ill-defined, often destructive behavior. This problem is a key theoretical and practical flaw in nearly all parallel computer systems. This proposal addresses this challenge, by proposing a new class of parallel computer architectures with strong behavioral guarantees, even for ill-synchronized code. The key idea is to make systems safely terminate ill-synchronized program executions before they can cause problems. To avoid degrading availability, the project includes mechanisms to avoid terminating program executions when possible, by falling back to more permissive, yet safe and predictable behavioral guarantees, and by resolving potential errors caused by ill-synchronized code. The intellectual merits of the project are that it provides crucial behavioral guarantees even to ill-synchronized parallel code. The project eliminates outdated hardware models that not only provide inadequate behavioral guarantees, but are also complex, and power-hungry. The project is the first in this domain to directly address availability and correctness together. The project's broader significance and importance are that it will improve the reliability of all parallel systems, which affects all aspects of life: medicine, energy, transportation, health, defense, and business. The stronger guarantees provided by this project avoid costly, dangerous failures and decrease the cost of application development, even in mature languages. The project will generate results relevant to industry and will influence academia through publication. The project will directly influence secondary and higher education in computing, fostering a diverse, future STEM workforce.To provide strong behavioral guarantees to all code -- even if incorrectly synchronized -- the proposed architectures provide region-atomic memory consistency guarantees for coarse-grained code regions. In these architectures, a program's execution is either a serialization of code regions, or it terminates with an exception that indicates an error could have left memory inconsistent. The architectures provide this strong memory consistency model to all program executions, departing from mainstream approaches to coherence and consistency that favor weaker guarantees without a clear benefit in complexity or performance. In systems executing ill-synchronized code, frequent exceptions may too often terminate program executions, degrading availability. The proposed architectures avoid degrading availability by tolerating consistency violations with a well-defined snapshot isolation semantics that avoids exceptions, but does not guarantee serializability of code regions. The architectures further address availability by resolving exceptions, leveraging commutativity of code to avoid unnecessary exceptions for commutative operations, as well as using dynamic symbolic analysis to resolve exceptions by combining symbolic memory updates.

尽管取得了数十年的进步，但编写正确的并行软件来实现现代并行计算机硬件的价值仍然极其困难。一个关键问题是，当今的计算机系统并没有为所有程序提供明确的行为保证； “不同步”代码，其中并行计算不完全或不正确地协调，具有不明确的、通常具有破坏性的行为。这个问题是几乎所有并行计算机系统的一个关键理论和实践缺陷。该提案通过提出一种新的并行计算机架构来解决这一挑战，该架构具有强大的行为保证，即使对于不同步的代码也是如此。关键思想是让系统在不同步的程序产生问题之前安全地终止它们。为了避免降低可用性，该项目包含了一些机制，可以通过回退到更宽松、但安全且可预测的行为保证，以及解决由不同步的代码引起的潜在错误来避免在可能的情况下终止程序执行。该项目的智力优点在于，它甚至为不同步的并行代码提供了关键的行为保证。该项目消除了过时的硬件模型，这些模型不仅提供了不足的行为保证，而且复杂且耗电。该项目是该领域第一个直接同时解决可用性和正确性问题的项目。该项目更广泛的意义和重要性在于，它将提高所有并行系统的可靠性，影响生活的各个方面：医学、能源、交通、健康、国防和商业。 该项目提供的更强有力的保证可以避免代价高昂的危险故障，并降低应用程序开发的成本，即使是在成熟的语言中也是如此。 该项目将产生与工业相关的成果，并将通过出版影响学术界。该项目将直接影响计算领域的中等和高等教育，培养多元化的未来 STEM 劳动力。为了向所有代码提供强有力的行为保证（即使同步不正确），所提出的架构为粗粒度提供了区域原子内存一致性保证代码区域。 在这些体系结构中，程序的执行要么是代码区域的序列化，要么因异常而终止，该异常表明错误可能导致内存不一致。 这些架构为所有程序执行提供了这种强大的内存一致性模型，偏离了支持较弱保证的主流连贯性和一致性方法，而在复杂性或性能方面没有明显的优势。 在执行不同步代码的系统中，频繁的异常可能会经常终止程序执行，从而降低可用性。所提出的架构通过使用定义良好的快照隔离语义来容忍一致性违规，从而避免了可用性下降，该语义可以避免异常，但不能保证代码区域的可串行化。这些架构通过解决异常、利用代码的交换性来避免交换操作中不必要的异常以及使用动态符号分析通过组合符号内存更新来解决异常来进一步解决可用性问题。