Inter-task Communication Research Articles

Computer Control Laboratory

The paper describes a three semester course of the Computer Control Laboratory. Didactic program of the laboratory addresses all aspects related to the design and application of real-time computer control systems: software development technology, system development, and specialized controllers application. The run-time environment used in most laboratory exercises is a multi processor microcomputer controlled by a real-time operating system. The system is able to model different types of intertask communication procedures. Some details on the hardware structures and the operating system are discussed. Process models used as a real-world during the laboratory projects are presented. Some of them are relatively simple, while the others are characterized by a complicated mathematical models.

Ramifications of re-introducing asynchronous exceptions to the Ada language

Re-introduction of asynchronous exceptions into the Ada language has been suggested as a means to solve certain problems in intertask communication. The author argues that such re-introduction would provide inconsistent and nondeterministic semantics, disallow several important classes of optimization, cause considerable distributed overhead, complicate program proof and verification efforts, prove error-prone to use, and offer little new functionality. Further discussions of alternate solutions to the requirements which motivated asynchronous exceptions are encouraged.

Asynchronous communication in Ada

The current definition of Ada manifestly fails to satisfy the requirements for supporting the application domain characteristics of real-time, embedded and distributed systems particularly with respect to the capability for asynchronous message passing. It is shown that, for a number of independent reasons, the potential for allowing Ada tasks to communicate in this manner by extension of the existing purely synchronous rendezvous has significant benefits. Though this is a radical deviation from the existing language definition a firm proposal is made for allowing asynchronous inter-task communication. Some of the resultant semantic and syntactic implications are explored.

Throughput calculation for basic stochastic rendezvous networks

Distributed computer programs using rendezvous for intertask communication have performance effects due to tasks waiting for rendezvous. Tasks have a dual nature as being both servers of requests and customers in making requests of other tasks. In a previous paper, such tasks have been termed ‘active servers’. In this paper, a distributed program is modelled as a stochastic network of tasks related by their rendezvous requests. For purposes of defining the throughput, the tasks are assumed to have maximum concurrency as if each task were executed on its own processor. For small networks, exact throughput values can be found by translating them into equivalent times Petri nets, and the translation procedure is given as an algorithm. An approximation suitable for larger networks, which was found to be accurate to within a few percent in most of the cases tested, is also given.

Architectural support for synchronous task communication

This paper describes the motivation for a set of intertask communication primitives, the hardware support of these primitives, the architecture used in the Sylvan project which studies these issues, and the experience gained from various experiments conducted in this area. We start by describing how these facilities have been implemented in a multiprocessor configuration that utilizes a shared backplane. This configuration represents a single node in the system. The latter part of the paper discusses a distributed multiple node system and the extension of the primitives that are used in this expanded environment. This research is funded by a strategic grant from the Natural Sciences and Engineering Research Council of Canada (Grant No. G1581).

Validating a demonstration tool for graphics-assisted debugging of Ada concurrent programs

A demonstration-quality graphics-assisted debugger is developed for intertask communication in Ada. Based on the static task-specification diagrams of G. Booch (Software Engineering with Ada, Benjamin/Cummings, 1983), the debugger animates the activity of a collection of communicating tasks, and it runs on a DEC GIGI terminal connected to a VAX 11-780 under TeleSoft's partial Ada compiler. The model has been subjected to empirical validation, using undergraduate students as experimental subjects. Subjects were required to debug erroneous tasking programs using both the graphical debugger and a textual one. It is concluded that although the problems to be addressed in the development and evaluation of a graphical debugging tool for Ada tasks are nontrivial, the benefits could be worth the effort. >

Control and communications for multiple, cooperating robots

There is an increasing need for the flexible integration and control of multiple robots of different types and manufacturers in a single workstation. These robots must coordinate their actions with each other in order to improve throughput, to simplify the process of programming them and to accommodate variations in the working environment. We propose a flexible architecture for the control system of multiple, cooperating robots in an integrated, multiple robot system. Our new architecture has major advantages in that it provides for asynchronous operations in and between control modules at the higher levels, retaining synchronous control only at the lowest level where each robot is servoed at its own pose-clock rate. We have outlined a communications and task management architecture which calls for only a simple run-time operating system at the lower levels of the hierarchy. We have shown that the inter-task communication load at the low levels, which is where the worst case occurs, could be handled, at the speeds necessary for robot control, by at least one current message-passing operating system.

High-performance operating system primitives for robotics and real-time control systems

To increase speed and reliability of operation, multiple computers are replacing uniprocessors and wired-logic controllers in modern robots and industrial control systems. However, performance increases are not attained by such hardware alone. The operating software controlling the robots or control systems must exploit the possible parallelism of various control tasks in order to perform the necessary computations within given real-time and reliability constraints. Such software consists of both control programs written by application programmers and operating system software offering means of task scheduling, intertask communication, and device control. The Generalized Executive for real-time Multiprocessor applications (GEM) is an operating system that addresses several requirements of operating software. First, when using GEM, programmers can select one of two different types of tasks differing in size, called processes and microprocesses. Second, the scheduling calls offered by GEM permit the implementation of several models of task interaction. Third, GEM supports multiple models of communication with a parameterized communication mechanism. Fourth, GEM is closely coupled to prototype real-time programming environments that provide programming support for the models of computation offered by the operating system. GEM is being used on a multiprocessor with robotics application software of substantial size and complexity.

V60 real-time operating system

This paper describes the requirements for 32-bit microprocessor real-time operating systems, design objectives and the implementation of the V60(∗)/V70(∗∗) Real-Time Operating System (RTOS) and its programming supports. (∗), (∗∗) V60 and V70 are trademarks of NEC Corporation. The V60/V70 RTOS is designed for NEC's 32-bit microprocessor series, V60 and V70. It provides real-time operation with a reliable task environment, a high-level intertask communication mechanism based on Ada(∗)'s rendezvous and fault tolerant capability. These functions are implemented efficiently using V60/V70's architectural feature, e.g. multiple virtual address space, ring level protection and context switching instructions. The RTOS is constructed from three separable modules, real-time kernel, file system and UNIX (∗∗) supervisor. This structure can cover a wide range of application from small embedded systems to large fault tolerant systems. A high-level language and system generation support tools are also provided to increase application program productivity. (∗) Ada is a registered trademark of U.S. Government, Ada Joint Program Office. (∗∗) UNIX is a trademark of AT&T Bell Laboratories.

A Characterization and Analysis of Parallel Processor Interconnection Networks

The permuting properties of various interconnection networks have been extensively studied. However, not too much attention has been focused on how the permuting properties interact with the mapping of tasks to processors in realizing the communication requirements between tasks. In this paper we focus on characterizing the abilities of some interconnection networks in realizing intertask communication that can be specified as permutations of the task names. From the point of view of the intertask communications requirements, the perceived permuting capabilities may depend upon the specific assignment of tasks to processors. Distinct network permutations may actually result in equivalent intertask communication patterns depending upon the mapping of tasks to processors. Characterizations of networks are developed based upon the theory of permutation groups. A number of properties as well as limitations of these networks become evident from this characterization. Finally, a class of switching networks is identified, that possess many useful properties that make them preferable to multistage interconnection networks in specific applications.

A modular operating system for the cray‐1

AbstractA new operating system for the CRAY‐1 supercomputer implements four types of objects that can be manipulated by user tasks. A directory object is used to implement a hierarchical directory structure, a file object stores random access data, a pipe object permits intertask communication and a process object is used to execute user programs. The advantages of implementing a restricted number of object types in the kernel and the lessons learned from developing a supercomputer operating system from scratch are discussed.

Fault-Tolerance Considerations in Large, Multiple-Processor Systems

Researcher have long conjectured upon the possibility of constructing large, massively-parallel computing engines by interconnecting many conventional processing elements to form an integrated supersystem. The rapid expansion in very large scale integration, or VLSI, circuit technology during the past decade has accelerated research in this direction. As advances in VLSI push basic component or chip functionalities to the processor level and beyond, it becomes natural to view complex processing elements as the basic components of much larger systems. Several names for such systems have been proposed, including network computers, multicomputers, and distributed multiprocessors. Despite the naming differences, these systems have the following salient features: (1) A large number of basically autonomous processing elements interconnected by a structure that allows high-bandwidth communication between them. At the system level, these processing elements and interconnection facilities are viewed as the basic components of the system. Each processing node has its own local memory and there is no sharing of memory between nodes. (2) A high degree of distribution of control or operating system functions among the processing elements. (3) Highly parallel computation performed by constructing applications as collections of several or many distinct tasks. These tasks may execute concurrently on different processors, withmore » necessary intertask communication carried out over the communication facilities linking the nodes. The collection of cooperating tasks comprising an application is sometimes referred to as a task force.« less

Symmetric intertask communication

We argue for the need of supporting a symmetric select construct, in which entry calls as well as accepts can be alternatives. We present several situations in which a symmetric select leads to a more natural programming style. We show that several semantic principles are violated by a nonsymmetric select, while being satisfied by a symmetric one. In particular, the suggested symmetric intertask communication mechanism is fully abstract and composable, and has a distributed termination rule which reduces the risk of deadlock. Our discussion is in terms of Ada™.

A system organization for parallel image processing

One of the problems in the development of multiprocessor systems for image analysis is the selection and efficient utilization of an interconnection network between the multiple processing units. This paper proposes a system organization centered around a class of interconnection networks and a global bus. Control schemes are developed for realizing the intertask communication requirements typically encountered in the parallel formulation of problems for image analysis. These schemes are simple, distributed and efficient. The utility of this organization is demonstrated by evaluating the performance of two applications.

Modular, object-oriented microcomputer architecture

The article proposes a multi-resource, distributed, matrix-structured, reconfigurable, fault-tolerant microcomputer system, particularly suited for efficient multiprocessing and multiprogramming operation. The system implements some ideas of the dynamic and the flexible architectures. The proposed system possess a high degree of regularity by implementing large amounts of identical units. The system implements a new Tree-Structured Architecture (TSA), proposed in this article. The TSA is object-oriented, implements the notions of capability-based-addressing and the single-level-store and it is particularly designed to narrow the semantic gap between HLL's and the machine. It encourages modular programming and directly supports the concept of tasks and intertask communication. The system hardware is modular and modules can be added or taken out, without change in system software.

A Tree Structured Architecture for semantic gap reduction

The article proposes a new Tree-Structured-Architecture (TSA). The TSA is object-oriented, implements the notions of capability-based-addressing and the single-level-store, and it is particularly designed to narrow the semantic gap. It encourages modular programming and directly supports the concepts of tasks and inter-task communication, making it particularly suitable for multiprocessing and multiprogramming implementation.The TSA is implemented on a multi-resource, distributed, matrix-structured, reconfigurable, fault-tolerant computing system, proposed in this article. The system implements some ideas of the dynamic and the flexible architecture. The system possesses a high degree of regularity by implementing large amounts of identical units. The hardware is modular, permitting the addition or removal of groups of units, without a change in the system's software.

A control kernel to support ada intertask communication on a distributed multiprocessor computer system

The control kernel software necessary to support the execution of the Ada intertask communication primitives on a distributed multiprocessor computer system is described. The control kernel software has been written in a high-level procedural language. It is therefore portable, and represents part of a run-time support environment for a concurrent programming language known as Martlet. Martlet combines the tasking features of Ada with the sequential features of the language Pascal. The choice between master-slave and separate autonomous kernels in a multiprocessor system is analysed, and the required data structures and associated routines used in an actual implemented system are described. Finally, the mechanism of mapping interrupts into external entry calls is presented

The application of the RMX/80 in the multimicroprocessor system for digital signal processing

A multimicroprocessor system, based on Intel 8085 processors, has been designed and applied in digital signal processing. Multiple master/dual bus multiprocessor architecture has been applied, which enables up to eight masters to run concurrently. Several inter-master communications, as communication through common memory, synchronization and mutual exclusion, have been used. One master of the system, dedicated to digital signal analysis, has been realized as real-time multitasking system, using the RMX/80 operating system. The major steps in designing this system are described in the paper. They include designing the hardware and the software components of the system, designing and coding user tasks and generating the software configuration. Intertask communication, based on the Command task, has been proposed. It provides flexible software communication between this master, realized as the multitasking system, and the other part of the multimicroprocessor system. The specific functions of digital signal analysis have also been described.

A control kernel to support ada intertask communication on a distributed multiprocessor computer system

The control kernel software necessary to support the execution of the Ada intertask communication primitives on a distributed multiprocessor computer system is described. The control kernel software has been written in a high-level procedural language. It is therefore portable, and represents part of a run-time support environment for a concurrent programming language known as Martlet. Martlet combines the tasking features of Ada with the sequential features of the language Pascal. The choice between master-slave and separate autonomous kernels in a multiprocessor system is analysed, and the required data structures and associated routines used in an actual implemented system are described. Finally, the mechanism of mapping interrupts into external entry calls is presented

Two Years of Experience with the PETRA Control System

PETRA control system experience after March '79 is reported. We discuss system structure influence on reliability , flexibility , maintainability , and the development of correlated functions , task allocation , and intertask communication. Operational aspects of man-machine communication , man-machine interface as well as interactive language approaches are presented.