An occam Style Communications System for UNIX Networks

This document describes the design of a communications system which provides occam style communications primitives under a Unix environment, using TCP/IP protocols, and any number of other protocols deemed suitable as underlying transport layers. The system will integrate with a low overhead scheduler/kernel without incurring significant costs to the execution of processes within the run time environment. A survey of relevant occam and occam3 features and related research is followed by a look at the Unix and TCP/IP facilities which determine our working constraints, and a description of the T9000 transputer's Virtual Channel Processor, which was instrumental in our formulation. Drawing from the information presented here, a design for the communications system is subsequently proposed. Finally, a preliminary investigation of methods for lightweight access control to shared resources in an environment which does not provide support for critical sections, semaphores, or busy waiting, is made. This is presented with relevance to mutual exclusion problems which arise within the proposed design. Future directions for the evolution of this project are discussed in conclusion

Bounded Model Checking for Probabilistic Programs

In this paper we investigate the applicability of standard model checking approaches to verifying properties in probabilistic programming. As the operational model for a standard probabilistic program is a potentially infinite parametric Markov decision process, no direct adaption of existing techniques is possible. Therefore, we propose an on-the-fly approach where the operational model is successively created and verified via a step-wise execution of the program. This approach enables to take key features of many probabilistic programs into account: nondeterminism and conditioning. We discuss the restrictions and demonstrate the scalability on several benchmarks

Fifty years of Hoare's Logic

We present a history of Hoare's logic.Comment: 79 pages. To appear in Formal Aspects of Computin

Programming Languages for Distributed Computing Systems

When distributed systems first appeared, they were programmed in traditional sequential languages, usually with the addition of a few library procedures for sending and receiving messages. As distributed applications became more commonplace and more sophisticated, this ad hoc approach became less satisfactory. Researchers all over the world began designing new programming languages specifically for implementing distributed applications. These languages and their history, their underlying principles, their design, and their use are the subject of this paper. We begin by giving our view of what a distributed system is, illustrating with examples to avoid confusion on this important and controversial point. We then describe the three main characteristics that distinguish distributed programming languages from traditional sequential languages, namely, how they deal with parallelism, communication, and partial failures. Finally, we discuss 15 representative distributed languages to give the flavor of each. These examples include languages based on message passing, rendezvous, remote procedure call, objects, and atomic transactions, as well as functional languages, logic languages, and distributed data structure languages. The paper concludes with a comprehensive bibliography listing over 200 papers on nearly 100 distributed programming languages

Uncovering Bugs in Distributed Storage Systems during Testing (not in Production!)

Testing distributed systems is challenging due to multiple sources of nondeterminism. Conventional testing techniques, such as unit, integration and stress testing, are ineffective in preventing serious but subtle bugs from reaching production. Formal techniques, such as TLA+, can only verify high-level specifications of systems at the level of logic-based models, and fall short of checking the actual executable code. In this paper, we present a new methodology for testing distributed systems. Our approach applies advanced systematic testing techniques to thoroughly check that the executable code adheres to its high-level specifications, which significantly improves coverage of important system behaviors. Our methodology has been applied to three distributed storage systems in the Microsoft Azure cloud computing platform. In the process, numerous bugs were identified, reproduced, confirmed and fixed. These bugs required a subtle combination of concurrency and failures, making them extremely difficult to find with conventional testing techniques. An important advantage of our approach is that a bug is uncovered in a small setting and witnessed by a full system trace, which dramatically increases the productivity of debugging

Observational Determinism for Concurrent Program Security

Noninterference is a property of sequential programs that is useful for expressing security policies for data confidentiality and integrity. However, extending noninterference to concurrent programs has proved problematic. In this paper we present a relatively expressive secure concurrent language. This language, based on existing concurrent calculi, provides first-class channels, higher-order functions, and an unbounded number of threads. Well-typed programs obey a generalization of noninterference that ensures immunity to internal timing attacks and to attacks that exploit information about the thread scheduler. Elimination of these refinement attacks is possible because the enforced security property extends noninterference with observational determinism. Although the security property is strong, it also avoids some of the restrictiveness imposed on previous security-typed concurrent languages

Doctor of Philosophy

dissertationAlmost all high performance computing applications are written in MPI, which will continue to be the case for at least the next several years. Given the huge and growing importance of MPI, and the size and sophistication of MPI codes, scalable and incisive MPI debugging tools are essential. Existing MPI debugging tools have, despite their strengths, many glaring de ficiencies, especially when it comes to debugging under the presence of nondeterminism related bugs, which are bugs that do not always show up during testing. These bugs usually become manifest when the systems are ported to di fferent platforms for production runs. This dissertation focuses on the problem of developing scalable dynamic verifi cation tools for MPI programs that can provide a coverage guarantee over the space of MPI nondeterminism. That is, the tools should be able to detect diff erent outcomes of nondeterministic events in an MPI program and enforce all those di fferent outcomes through repeated executions of the program with the same test harness. We propose to achieve the coverage guarantee by introducing efficient distributed causality tracking protocols that are based on the matches-before order. The matches-before order is introduced to address the shortcomings of the Lamport happens-before order [40], which is not sufficient to capture causality for MPI program executions due to the complexity of the MPI semantics. The two protocols we propose are the Lazy Lamport Clocks Protocol (LLCP) and the Lazy Vector Clocks Protocol (LVCP). LLCP provides good scalability with a small possibility of missing potential outcomes of nondeterministic events while LVCP provides full coverage guarantee with a scalability tradeoff . In practice, we show through our experiments that LLCP provides the same coverage as LVCP. This thesis makes the following contributions: •The MPI matches-before order that captures the causality between MPI events in an MPI execution. • Two distributed causality tracking protocols for MPI programs that rely on the matches-before order. • A Distributed Analyzer for MPI programs (DAMPI), which implements the two aforementioned protocols to provide scalable and modular dynamic verifi cation for MPI programs. • Scalability enhancement through algorithmic improvements for ISP, a dynamic verifi er for MPI programs