Concurrent Programming Model Research Articles

A CONCURRENT OOP MODEL FOR COMPUTER-AIDED ENGINEERING USING BLACKBOARD ARCHITECTURE

A concurrent object-oriented programming (COOP) model is presented for computer-aided engineering using a blackboard architecture. The basic components of the model are a hierarchy of tasks, global and local controllers, knowledge sources, global and local blackboards, controlling data slots, and numerical data slots. For interprocess communications, named pipes, message queues, and shared memory are used. The model is implemented in C + + on a simulated concurrent processing platform using the system functions provided by UNIX operating system. The COOP model is applied to a complex engineering problem, analysis of high-performance composite laminates and performance results are reported.

Animating an actor programming model

Interest in object-oriented programming systems for parallel computing has been growing for many years. This paper explores a concurrent object-oriented programming model known as actors, and describes an animation (or simulation) of this model implemented in another object-oriented programming system, namely Smalltalk-80 . This animation has been used for educational purposes, particularly at final-year undergraduate level.

The implementation of a distribution model for logic programming based on multiple-headed clauses

We present the the distributed implementation of a new communication model for concurrent logic programming. Tha main novelty with respect to STREAM-parallel logic languages is that inter-process communication is performed via multiple-headed clauses, rather that by variables shared among AND processes. We describe an abstract machine for the model which extend the Warren Abstract Machine.

The conversation deadlock problem in client-server model

Different problems have to be solved for an effective use of conversations as a mean to improve the overall reliability of a concurrent program. In this paper we address the so called Deserter Process Problem. We investigate the causes which generate deserter processes, distinguishing between the case in which a process does not enter a conversation and the case in which a process does not reach the acceptance test line. The first case is related with a particular type of deadlocks. A model of concurrent programs with conversations, and conditions, which are useful to detect deadlocks in these programs, are presented.

The ensemble system

ENSEMBLE is a programmming system for writing concurrent programs on a personal computer (PC).ENSEMBLE consists of a library of Turbo Pascal 1 units to be used with applications written in the same language and executable on IBM PCs or compatibles under the MS-DOS 2 operating system. ENSEMBLE provides the programmer with a set of facilities and predefined abstractions that simplify the construction of concurrent programs. The main abstractions are coroutines and tasks.ENSEMBLE's main goal is to provide an environment that can be easily used by students for gaining experience in a wide variety of concurrent programming models and techniques. However, ENSEMBLE was designed to be also a satisfactory production tool for writing concurrent and real-time software on a PC.

Petri net models of concurrent Ada programs

In this paper we give a translation of a subset of the programming language Ada to Petri nets. The subset contains Ada rendezvous and dynamic task creation, but does not contain loops. The design of the translation algorithm was influenced by methods of denotational semantics which made the implementation of the algorithm particularly simple in the programming language ML. This approach makes possible proofs about the nature of the Petri net models. Properties of a concurrent Ada program can be studied by analyzing the Petri net model. It is possible to determine the possible interleavings of statements and do static deadlock detection. We are interested in applying the models of Ada programs to the problem of determining which program segments form the critical path in the time it takes an Ada program to execute.

Synchronizing events in replicated systems

The application of N-Modular Redundancy (NMR) in the construction of software permits redundant systems to be robust with respect to failures in replicated processors. In order to exhibit consistent behavior, all processors in an NMR node must process input requests in the same order. This paper investigates the introduction of synchronization points among replicated computations in a nondeterministic model for concurrent programming. A formal specification is given of a distributed synchronization mechanism that does not introduce any additional deadlock in the replication of a basic single system. We propose this synchronization mechanism as a means of preventing the occurrence of the so-called sequencing failures. The properties of the proposed solution are specified and proved using the COSY path-expression formalism. Finally, some implementation schemes are proposed which satisfy the given specification.

The geometry of semaphore programs

Synchronization errors in concurrent programs are notoriously difficult to find and correct. Deadlock, partial deadlock, and unsafeness are conditions that constitute such errors. A model of concurrent semaphore programs based on multidimensional, solid geometry is presented. While previously reported geometric models are restricted to two-process mutual exclusion problems, the model described here applies to a broader class of synchronization problems. The model is shown to be exact for systems composed of an arbitrary, yet fixed number of concurrent processes, each consisting of a straight line sequence of arbitrarily ordered semaphore operations.

Modular redundancy in a message passing system

Modular redundancy in the form of replicated computations in a concurrent programming model consisting of communicating sequential processes is investigated. Some conditions are given which must always be verified to ensure correctness in the presence of nondeterminism. Then some implementations which satisfy the given conditions are proposed. This approach permits redundant systems to be robust with respect to failures in redundant processors, and also permits the use of software fault tolerance techniques such as <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> -version programming. The concurrent programming model which has been chosen is based on a set of active entities, i.e. processes, each running in a local protected environment. The processes interact using message passing only.

The temporal semantics of concurrent programs

The formalism of Temporal Logic is suggested as an appropriate tool for formalizing the semantics of concurrent programs. A simple model of concurrent program is presented in which n processors are executing concurrent n disjoint programs under a shared memory environment. The semantics of such a program specifies the class of state sequences which are admissible as proper execution sequences under the program. The two main criteria which are required are 1. (a) Each state is obtained from its predecessor in the sequence by exactly one processor performing an atomic instruction in its process. 2. (b) Fair Scheduling: no processor which is infinitely often enabled will be indefinitely delayed. The basic elements of Temporal Logic are introduced in a particular logic framework DX. The usefulness of Temporal Logic notation in describing properties of concurrent programs is demonstrated. A construction is then given for assigning to a program P a temporal formula W( P) which is true on all proper execution sequences of P. In order to prove that a program P possesses a property R 1, one has only to prove the implications W( P)⊃ R. An example of such proof is given. It is then demonstrated that specification of the Temporal character of the program's behavior is absolutely essential for the unambiguous understanding of the meaning of programming constructs.