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: 1629196
• 负责人: Brandon Lucia
• 金额: $ 48.11万
• 依托单位: Carnegie-Mellon 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=1629196&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 The project eliminates outdated hardware models that not only provide inadequate behavioral guarantees, but are also complex, and power-hungry.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.

尽管进步了数十年，但编写正确的并行软件来实现现代平行计算机硬件的价值仍然非常困难。一个关键问题是，当今的计算机系统并未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，其定义不明，通常是破坏性的行为。在几乎所有平行的计算机系统中，此问题是一个关键的理论和实用缺陷。该提案通过提出具有强大行为保证的新的平行计算机体系结构，即使对于不同步的代码，也可以解决这一挑战。关键的想法是使系统安全终止不同步的程序执行，然后才会引起问题。为了避免降级可用性，该项目包括在可能的情况下避免终止程序执行的机制，通过回到更宽松，安全和可预测的行为保证，并解决由不同步的代码引起的潜在错误。该项目的智力优点在于，即使对不同步的并行代码，它也提供了至关重要的行为保证。该项目消除了过时的硬件模型，这些模型不仅提供了行为不足的保证，而且还很复杂且渴望。该项目是该域中第一个共同解决可用性和正确性的项目。该项目的重要性和重要性是，它将提高所有平行系统的可靠性，从而影响生活的各个方面：医学，能源，运输，健康，国防和业务。 该项目提供的更强的保证避免了昂贵，危险的失败，并降低了应用程序开发的成本，即使在成熟的语言中也是如此。 该项目将产生与行业相关的结果，并将通过出版物影响学术界。该项目将直接影响计算中的二级和高等教育，促进多样化的未来STEM劳动力。为所有代码提供强大的行为保证 - 即使不正确同步 - 拟议的体系结构为粗粒度代码区域提供了区域 - 原子的记忆一致性保证。 在这些体系结构中，程序的执行要么是代码区域的序列化，要么终止了例外，表明错误可能使内存不一致。 这些体系结构为所有程序执行提供了这种强大的内存一致性模型，从主流方法偏离了连贯性和一致性，从而在复杂性或绩效方面没有明显的好处，从而有利于弱保证。 在执行不同步代码的系统中，频繁的例外可能经常终止程序执行，从而降低可用性。提出的体系结构通过使用明确定义的快照隔离语义来避免违反一致性，避免了可用性，从而避免了例外，但不能保证代码区域的序列化性。这些体系结构通过解决异常来进一步解决可用性，利用代码的交换性来避免不必要的通勤操作例外，并使用动态符号分析来通过结合符号内存更新来解决异常。