Showing papers in "Distributed Computing in 1989"

Probabilistic clock synchronization

Abstract: A probabilistic method is proposed for reading remote clocks in distributed systems subject to unbounded random communication delays. The method can achieve clock synchronization precisions superior to those attainable by previously published clock synchronization algorithms. Its use is illustrated by presenting a time service which maintains externally (and hence, internally) synchronized clocks in the presence of process, communication and clock failures.

Decentralization of process nets with centralized control

Abstract: The behavior of a net of interconnected, communicating processes is described in terms of the joint actions in which the processes can participate. A distinction is made between centralized and decentralized action systems. In the former, a central agent with complete information about the state of the system controls the execution of the actions; in the latter no such agent is needed. Properties of joint action systems are expressed in temporal logic. Centralized action systems allow for simple description of system behavior. Decentralized (two-process) action systems again can be mechanically compiled into a collection of CSP processes. A method for transforming centralized action systems into decentralized ones is described. The correctness of this method is proved, and its use is illustrated by deriving a process net that distributedly sorts successive lists of integers.

Synthesizing systolic arrays with control signals from recurrence equations

Abstract: We present a technique for synthesizing systolic arrays which have non-uniform data flow governed by control signals. The starting point for the synthesis is anAffine Recurrence Equation—a generalization of the simple recurrences encountered in mathematics. A large class of programs, including most (single and multiple) nested-loop programs can be described by such recurrences. In this paper we extend our earlier work (Rajopadhye and Fujimoto 1986) in two principal directions. Firstly, we characterize a class of transformations calleddata pipelining and show that they yield recurrences that havelinear conditional expressions governing the computation. Secondly, we discuss the synthesis of systolic arrays that have non-uniform data flow governed by control signals. We show how to derive the control signals in such arrays by applying similar pipelining transformations to theselinear conditional expressions. The approach is illustrated by deriving the Guibas-Kung-Thompson architecture for computing the cost of optimal string parenthesization.

Fault tolerant processes

Abstract: A process is said to be fault tolerant if the system provides proper service despite the failure of the process. For supporting fault-tolerant processes, measures have to be provided to recover messages lost due to the failure. One approach for recovering messages is to use message-logging techniques. In this paper, we present a model for message-logging based schemes to support fault-tolerant processes and develop conditions for proper message recovery in asynchronous systems. We show that requiring messages to be recovered in the same order as they were received before failure is a stricter requirement than necessary. We then propose a distributed scheme to support fault-tolerant processes that can also handle multiple process failures.

Case Studies of the Bending Vibration and Whirling Motion of Drill Collars

Abstract: Description of the principal sources of bottomhole assembly (BHA) bending vibration: drill-collar whirling and linear coupling between weight on bit and bending vibration of an initially curved BHA. Evolution of the consequences of bending vibrations in terms of drill-collar wear and connection fatigue. Downhole measurement of bending moment are used to detect and identify bending-vibration events.

Understanding naming in distributed systems

Abstract: Naming in distributed systems is modelled as a string translation problem. Viewing names as strings and name resolution mechanisms as syntax directed translators provides a formal handle on the loosely understood concepts associated with naming: we give precise definitions for such informal terminology as name spaces, addresses, routes, source-routing, and implicit-routing; we identify the properties of naming systems, including under what conditions they support unique names, relative names, absolute names, and synonyms; and we discuss how the basic elements of the model can be implemented by name servers.

Deadlock-free message routing in multicomputer networks

Abstract: The execution of a concurrent computation by a network of processors requires a routing algorithm that is deadlock free. Many routing algorithms proposed for processor networks have the potential of deadlock due to the cyclic topology of the network. In this paper we first formalize the concept of message routing. Next, we show a method by which a deadlock-free routing algorithm can be constructed out of a given routing algorithm. Finally the method is illustrated by constructing deadlock-free routing algorithms for cartesian product processor networks.

Randomized function evaluation on a ring

Abstract: LetR be a unidirectional asynchronous ring ofn identical processors each with a single input bit. Letf be any cyclic nonconstant function ofn boolean variables. Moran and Warmuth (1986) prove that anydeterministic algorithm that evaluatesf onR has communication complexity Ω (n logn) bits. They also construct a family of cyclic nonconstant boolean functions that can be evaluated inO(n logn) bits by a deterministic algorithm. This contrasts with the following new results:

Step semantics for "true'' concurrency with recursion

Abstract: We present a variety of denotational linear time semantics for a language with recursion and “true” concurrency in a form of synchronous co-operation, which in the literature is known as step semantics. We show that this can be done by a generalization of known results for interleaving semantics. A general method is presented to define semantical operators and denotational semantics in the Smyth powerdomain of streams. With this method, first a naive and then more sophisticated semantics for synchronous co-operation are developed, which include such features as interleaving and synchronization. Then we refine the semantics to deal with a bounded number of processors, subatomic actions, maximal parallelism and a real-time operator. Finally, it is indicated how to apply these ideas to branching-time models, where it becomes possible to analyze deadlock behaviour as well as a form of “true” concurrency.