Distributed Real-time Systems Research Articles

Comments, with reply, on 'A noninvasive architecture to monitor real-time distributed systems', by J. Tsai et al

The commenter maintains that the above-titled article (see ibid., vol.23, no.3, p.22-3, 1990) on unobtrusive real-time system monitoring fails to demonstrate much appreciation for the software side and as a result provides a somewhat misleading view of the state of the art in real-time system monitoring. He states that the article fails to mention that modern real-time application software written in a high-level programming language rarely conforms to the static structure assumed by the monitor. The authors reply that their article clearly states that the principal objective is to develop a hardware-based noninvasive real-time monitoring system to ensure minimal interference in the execution of a target real-time distributed computing system. They point out that the monitor does not assume that the application software has to be a static structure and present an argument in support of this claim.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Two algorithms for mutual exclusion in real-time distributed computer systems

Two algorithms developed utilizing a priority-based event-ordering which manage mutual exclusion in distributed systems—computer networks—are proposed. In these systems, processes communicate only by messages and do not share memory. The computer network functions either (a) in an environment requiring priorities or (b) in a real-time environment. The algorithms are based on broadcast requests and token passing service approach, but the token need not be passed if no process wishes to enter the critical section. These algorithms are fully distributed and are insensitive to the relative speeds of node computers and communication links. They use only N messages per critical section, where N is the number of nodes (processes). The algorithms are optimal in the sense that a symmetrical, distributed algorithm cannot use fewer messages if requests are processed by each node computer concurrently. Both algorithms ensure freedom from starvation. There are mechanisms to handle node insertion and removal, node failure, the loss of the token, the existence of more than one token, and delivery of messages out of order.

Deciding properties of timed transition models

Real-time distributed systems are modeled by a times transition model (TTM). For any finite-state TTM, decision procedures are provided for checking a small but important class of properties (specified in real-time temporal logic). The procedures are linear in the size of the system reachability graph. The class of properties includes invariance, precedence, eventuality and real-time response specifications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Real-time Data Processing and Distribution System of the Hakuho-Maru

Real-time Data Processing and Distribution System of the Hakuho-Maru

Real-Time Expert Systems and Their Applications

Real-time expert systems differ from simple consultation type expert systems in their relation with the external world. While the latter prompt the user for input data and control the rate at which input will be absorbed and processed, the former are driven by external events at a rate which cannot be influenced by the expert system. Because sequences of data relating to different lines of reasoning interleave each other, multitasking operation is a key feature of such systemsRTEX is an expert system building tool for creating embedded real-time applications in process control, network management, manufacturing control, industrial robots and simulation. It has been written in Ada and integrates the advanced concepts used in real-time distributed software systems with many features of the well established A.I. toolsAs a solid test application a procedure tracking system far monitoring nuclear power plants has been built. Monitoring the behaviour of a nuclear power plant after an incident has occurred involves a high degree of parallel reasoning. While diagnosing the incident and forwarding corrective actions, constant monitoring of critical functions is of primary importance. In the RTEX application the behaviour of the nuclear power plant is modelized by a flow of messages carrying measurement values of important plant parameters. A hierarchy of micro expert systems cooperate at run-time to assist the operator

Operating system support for adaptive distributed real-time systems in DRAGON SLAYER

We analyze current and future needs in building adaptive real-time systems, and then formulate a series of design requirements for their hardware/interconnection, operating system, and application levels from which we derive a plea for a layered and integrated system design. We report on the novel distributed operating system DRAGON SLAYER which provides highly reliable and adaptive kernel services for real-time applications. It supports the novel file system MELODY which, in order to both meet real-time constraints and provide for high availability in a hazardous environment, allows for replicating, relocating, or deleting of file copies. Such copies may also assume a different status regarding the recency, reliability, and consistency of their information: They may be public or private. Status changes as well as file replication, deletion, or relocation are analyzed and managed by local file assigners , based on cost and time delay criteria and - for public copies - on cooperation with all file assigners responsible for the other public copies of a given file. A distributed implementation of our integrated system design model is currently being completed in our labs in order to serve as a distributed real-time testbed. For systems modeled according to our design requirements all relevant parameters could be checked and fine-tuned according to the environmental situation for the tested system.- We also report on a sensitivity study that shows how promptly the MELODY file system reacts to changing environmental situations (changing request patterns). Our findings are discussed, future work is outlined.

Performance properties of vertically partitioned object-oriented systems

A vertically partitioned structure for the design and implementation of object-oriented systems is proposed, and their performance is demonstrated. It is shown that the application-independent portion of the execution overheads in object-oriented systems can be less than the application-independent overheads in conventionally organized systems built on layered structures. Vertical partitioning implements objects through extended type managers. Two key design concepts result in performance improvement: object semantics can be used in the state management functions of an object type and atomicity is maintained at the type manager boundaries providing efficient recovery points. The performance evaluation is based on a case study of a simple but nontrivial distributed real-time system application. >

Semantic information and consistency in distributed realtime systems

Considerable research effort has been devoted to the problem of developing techniques for achieving high availability of critical data in distributed realtime systems. One approach is to use replication. Replicated data is stored redundantly at multiple sites so that it can be used even if some of the copies are not available due to failures. This paper presents an algorithm for maintaining consistency and improving the performance of databases with replicated data in distributed realtime systems. The semantic information of read-only transactions is exploited for improved efficiency, and a multiversion technique is used to increase the degree of concurrency. Related issues including version management and consistency of the states seen by transactions are discussed.

Network access protocols for real-time distributed control systems

The concepts for pertinent network parameters are introduced, and an analytical base for system design is established. A simulation methodology has been developed for performance evaluation of real-time control system networks for distributed dynamical processes (e.g. autonomous manufacturing), electric power and chemical plants, and aircraft and spacecraft. The network-induced delays, resulting from data latencies, data loss, and missynchronism between system components, degrade the dynamic performance of the controlled process and are a source of potential instability. The impact of delays induced by different network access protocols on the control system performance is investigated. The results of combined discrete-event and continuous-time simulation are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Computer as a Camera-Operation and Image-Quality Manager

Specific hardware and software is needed in a television camera to cope with function monitoring and control. The real-time distributed system concept allows for complete function control together with an automatic setup system. Intelligent, knowledge-based algorithms are built into this setup system, which can take care of corrections for all aging effects. After tube replacement all tube parameters, including back-focus adjustment, are optimized for high-quality performance. In the authors' opinion, the computer in the camera of the future will become invisible to the human operator. It will control the technical operation and image quality of the camera, with additional operational features, but its presence will no longer be evident.

Processor Tradeoffs in Distributed Real-Time Systems

Optimizing the design of real-time distributed systems is important since the systems are frequently critical to life. This optimization is a difficult problem, and heuristics and designer judgment are called for in the process. The chief cause of the difficulty is the large number of parameters under the designer's control which impact performance and life-cycle cost. We study the interplay between the more important parameters in this paper using two objective measures, i. e., the mean cost and the probability of dynamic failure in [6], [10]. Among these are the processor burn-in time and processor replacement policy. A central feature of this work is a look at how the application requirements affect the optimality of the distributed systems; indeed, the application requirements are an integral part of the analysis.

Clock Synchronization in Distributed Real-Time Systems

The generation of a fault-tolerant global time base with known accuracy of synchronization is one of the important operating system functions in a distributed real-time system. Depending on the types and number of tolerated faults, this paper presents upper bounds on the achievable synchronization accuracy for external and internal synchronization in a distributed real-time system. The concept of continuous versus instantaneous synchronization is introduced in order to generate a uniform common time base for local, global, and external time measurements. In the last section, the functions of a VLSI clock synchronization unit, which improves the synchronization accuracy and reduces the CPU load, are described. With this unit, the CPU overhead and the network traffic for clock synchronization in state-of-the-art distributed real-time systems can be reduced to less than 1 percent.

Adding Time to Synchronous Process Communications

In distributed real-time systems, communicating processes cannot be delayed for arbitrary amounts of time while waiting for messages. Thus, communication primitives used for real-time programming usually allow the inclusion of a deadline or timeout to limit potential delays due to synchronization. This paper interprets timed synchronous communication as having absolute deadlines. Various ways of implementing deadlines are discussed, and two useful timed synchronous communication problems are identified which differ in the number of participating senders and receivers and type of synchronous communication. For each problem, a simple algorithm is presented and shown to be correct. The algorithms are shown to guarantee maximal success and to require the smallest delay intervals during which processes wait for synchronous communication. We also evaluate the number of messages used to reach agreement.

Module replication and assignment for real-time distributed processing systems

Response time is an important design criterion for real-time systems. A new analytic model is developed to estimate task response time. It considers such factors as interprocessor communication, module precedence relationship, module scheduling, interconnection network delay, and assignment of modules and files to computers. Since module assignment as well as its replication have great impact on task response time, a new algorithm is developed to iteratively search for module assignments and replications that reduce task response time. An objective function is introduced that is based on the sum of task response time and delay penalty for the violations of thread response time requirements. With this objective function, good module allocations and replications, which minimize task response time and yet satisfy the thread response time requirements, can be determined by the proposed algorithm. To validate the algorithm, we compare the assignments generated by the algorithm for some sample distributed systems to the optimal module assignments obtained from exhaustive search. It shows that with a very small number of initial module assignments, our algorithm is able to generate the optimal or close-to-optimal assignments. The algorithm is also applied to a real-time distributed system for space defense applications where exhaustive search for the optimal assignment is not feasible. The generated module assignments (with replications) satisfy the specified thread response times, and compare closely with the simulation results. A series of experiments is also performed to characterize the behavior of the algorithm. In conclusion, the algorithm can serve as a valuable tool for assigning modules with replications for distributed systems.

Optimal precision in the presence of uncertainty

We consider the problem of achieving coordinated actions in a real-time distributed system. In particular, we consider how closely (in terms of real time) processors can be guaranteed to perform a particular action, in a system where message transmission is guaranteed, but there is some uncertainty in message transmission time. We present an algorithm to achieve optimal precision in arbitrary networks. In networks where clocks run at the rate of real time, the optimal precision achievable in a network is exactly how tightly clocks can be guaranteed to be synchronized.

Availability analysis for the design of distributed processing networks

This paper presents an analytical model for evaluating the availability of distributed processing systems. The model, which is based on a well-known probability formula, has been implemented in FORTRAN and used for the design of a real-time distributed processing system that requires high availability. It is applied to a sample distributed processing system to illustrate its usefulness and to provide good insight into the design of highly available distributed processing systems. Effects of homogeneous/heterogeneous clustering and standby spare are illustrated. The concept of availability threshold where the distributed system may gain or lose availability over the individual processor availability is introduced. It has proven to be a valuable tool for the tradeoffs in the allocation of system requirements. The concluding remarks summarize the lessons learned and suggest cost-effective ways to achieve higher availability for the design of distributed processing systems.

Using Real-Time to Achieve Co-Ordination in Distributed Computer Control Systems

Although a high-speed Local Area Network environment offers great potential for building reliable and flexible real-time distributed computer control systems (DCCS), such communication systems possess undesirable delay and message loss characteristics. In order to overcome these problems, the unique requirements implied by real-time in DCCS are examined and a new protocol based on the inclusion of a validity time as an atomic attribute of every message is proposed as a solution.

Language constructs for real-time distributed systems

The paper observes syntactic and semantic requirements for a language for programming real-time distributed systems. A proposal for language features that meet these requirements is offered, and the features are applied to an example.

ACRE: a real-time distributed system

Abstract This paper describes the development and design objectives that were applied to the Post Office's in-house ACRE (automatic call recording equipment). ACRE is connected into the PO operator manual board system and allows the operator to handle calls more efficiently. The call details are automatically recorded on magnetic cartridge for billing purposes at the end of the call. High security, reliability (MTBF > 50 years) and maximum system flexibility are essential system requirements which have significantly influenced the structure and design of programs and equipment. A measure of the success of this project is the export of OPAS (developed from ACRE and manufactured under license by Standard Telephones and Cables Ltd).

A real-time distributed processing system using GEC 4000 series computers

A real-time distributed processing system using GEC 4000 series computers