Gregory R. Andrews

Concepts and Notations for Concurrent Programming

Much has been learned in the last decade about concurrent programming. This paper identifies the major concepts and describes some of the more important language notations for writing concurrent programs. The roles of processes, communication and syhchronization are discussed from both an operational and an axiomatic viewpoint. Language notations for expressing concurrent execution and for specifying process interaction are surveyed. Synchronization primitives based on shared variables and on message passing are described. Finally, three general classes of concurrent programming languages are identified and compared.

Paradigms for process interaction in distributed programs

Distributed computations are concurrent programs in which processes communicate by message passing. Such programs typically execute on network architectures such as networks of workstations or distributed memory parallel machines (i.e., multicomputers such as hypercubes). Several paradigms—examples or models—for process interaction in distributed computations are described. These include networks of filters, clients, and servers, heartbeat algorithms, probe/echo algorithms, broadcast algorithms, token-passing algorithms, decentralized servers, and bags of tasks. These paradigms are appliable to numerous practical problems. They are illustrated by solving problems, including parallel sorting, file servers, computing the topology of a network, distributed termination detection, replicated databases, and parallel adaptive quadrature. Solutions to all problems are derived in a step-wise fashion from a general specification of the problem to a concrete solution. The derivations illustrate techniques for developing distributed algorithms.

An overview of the SR language and implementation

SR is a language for programming distributed systems ranging from operating systems to application programs. On the basis of our experience with the initial version, the language has evolved considerably. In this paper we describe the current version of SR and give an overview of its implementation. The main language constructs are still resources and operations. Resources encapsulate processes and variables that they share; operations provide the primary mechanism for process interaction. One way in which SR has changed is that both resources and processes are now created dynamically. Another change is that inheritance is supported. A third change is that the mechanisms for operation invocation—call and send—and operation implementation—proc and in—have been extended and integrated. Consequently, all of local and remote procedure call, rendezvous, dynamic process creation, asynchronous message passing, multicast, and semaphores are supported. We have found this flexibility to be very useful for distributed programming. Moreover, by basing SR on a small number of well-integrated concepts, the language has proved easy to learn and use, and it has a reasonably efficient implementation.

Disassembly of executable code revisited

Machine code disassembly routines form a fundamental component of software systems that statically analyze or modify executable programs, e.g., reverse engineering systems, static binary translators, and link-time optimizers. The task of disassembly is complicated by indirect jumps and the presence of non-executable data - jump tables, alignment bytes, etc. - in the instruction stream. Existing disassembly algorithms are not always able to cope successfully with executable files containing such features, and they fail silently - i.e., produce incorrect disassemblies without any indication that the results they are producing are incorrect. In this paper we examine two commonly-used disassembly algorithms and illustrate their shortcomings. We propose a hybrid approach that performs better than these algorithms in the sense that it is able to detect situations where the disassembly may be incorrect and limit the extent of such disassembly errors. Experimental results indicate that the algorithm is quite effective: the amount of code flagged as incurring disassembly errors is usually quite small.

An Axiomatic Approach to Information Flow in Programs

A new approach to information flow in sequential and parallel programs is presented. Flow proof rules that capture the information flow semantics of a variety of statements are given and used to construct program flow proofs. The method is illustrated by examples. The applications of flow proofs to certifying information flow policies and to solving the confinement problem are considered. It is also shown that flow rules and correctness rules can be combined to form an even more powerful proof system.

Synchronizing Resources

A new proposal for synchronization and communication in parallel programs is presented. The proposal, called synchronization resources, combines and extends aspects of procedures, coroutines, monitors, communicating sequential processes, and distributed processes. It provides a single notation for parallel programming with or without shared variables and is suited for either shared or distributed memory architectures. The essential new concepts are operations, input statements, multiple processes and resources. The proposal is illustrated by solving a variety of parallel programming problems. Key Words and Phrases: parallel programming, processes, synchronization, process communication, monitors, distributed processing, programming languages, operating systems, data bases. CR Categories: 4.20, 4.22, 4.32, 4.35

The distributed programming language SR—Mechanisms, design and implementation

SR is a new language for programming software containing many processes that execute in parallel. The language allows an entire software system that controls a potentially large collection of processors to be programmed as an integrated set of software modules. The key language mechanisms are resources, operations and input statements. The language supports separate compilation, type abstraction, and dynamic communication links; it also contains novel treatments of arrays and procedures. This paper gives an overview of the language mechanisms, discusses some of the major design decisions and describes one implementation.

The evolution of the SR language

As a result of our experience, the SR distributed programming language has evolved. One change is that resources and processes are now dynamic rather than static. Another change is that operations and processes are now integrated in a novel way: all the mechanisms for process interaction — remote and local procedure call, rendezvous, dynamic process creation, and asynchronous message passing — are expressed in similar ways. This paper explains the rationale for these and other changes. We examine the fundamental issues faced by the designers of any distributed programming language and consider the ways in which these issues could be addressed. Special attention is given to the design objectives of expressiveness, simplicity, and efficiency.

Distributed allocation with pools of servers

Distributed systems make possible both a high degree of concurrency and robustness in the face of failure. One approach to achieving these goals is to employ pools of servers implementing major system functions. This paper describes the concept of pools of servers, and presents logically distributed, robust algorithms for one problem arising in this approach: the allocation of servers to clients. Three types of allocation problems are identified: free servers, preferred servers, and retentive servers. Allocation protocols based upon the idea of hash addressing are described and analyzed.

A file replication facility for Berkeley Unix

This paper describes two mechanisms – reproduction sets and metafiles – that provide low‐cost, semi‐automatic file replication and location transparency facilities for an interconnected collection of machines running Berkeley Unix. A reproduction set is a collection of files that the system attempts to keep identical; this is done on a ‘best effort’ basis, with the system relying on the user to handle unusual situations. A metafile is a special file that contains symbolic pathnames of other files, each of which may be on any machine in the network; opening a metafile results in opening an available constituent file. Examples are given to illustrate the use of these mechanisms. Their implementation and performance are also described.