Laws for Communicating Parallel Processes

Key Words and Phrases: parallel processes, parallel or asynchronous computations, partial orders of events, Actor theory. CR Categories: 5.21, 5.24, 5.26. This report describes research done at the Artificial Intelligence Laboratory of the Massachusetts Institute of Technology. Support for the laboratory's artificial intelligence research is provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014-75-C-0522.This paper presents some laws that must be satisfied by computations involving communicating parallel processes. The laws are stated in the context of the actor theory, a model for distributed parallel computation, and take the form of stating plausible restrictions on the histories of parallel computations to make them physically realizable. The laws are justified by appeal to physical intuition and are to be regarded as falsifiable assertions about the kinds of computations that occur in nature rather than as proven theorems in mathematics. The laws are used to analyze the mechanisms by which multiple processes can communicate to work effectively together to solve difficult problems. Since the causal relations among the events in a parallel computation do not specify a total order on events, the actor model generalizes the notion of computation from a sequence of states to a partial order of events. The interpretation of unordered events in this partial order is that they proceed concurrently. The utility of partial orders is demonstrated by using them to express our laws for distributed computation.MIT Artificial Intelligence Laboratory Department of Defense Advanced Research Projects Agenc

Process algebra with strategic interleaving

In process algebras such as ACP (Algebra of Communicating Processes), parallel processes are considered to be interleaved in an arbitrary way. In the case of multi-threading as found in contemporary programming languages, parallel processes are actually interleaved according to some interleaving strategy. An interleaving strategy is what is called a process-scheduling policy in the field of operating systems. In many systems, for instance hardware/software systems, we have to do with both parallel processes that may best be considered to be interleaved in an arbitrary way and parallel processes that may best be considered to be interleaved according to some interleaving strategy. Therefore, we extend ACP in this paper with the latter form of interleaving. The established properties of the extension concerned include an elimination property, a conservative extension property, and a unique expansion property.Comment: 19 pages, this version is a revision of the published versio

A model of time dependent behavior in concurrent software systems

A great difficulty in building distributed systems lies in being able to predict what the systems behavior will be. A distributed or communicating system is defined here to be one in in which the hardware consists of a set of processors each with their own memory, connected by some communication medium (there is no shared memory), and the software is assumed to be of the CSP (Hoare's Communicating Sequential Processes) type.In the past few years some theories have been proposed to model features of communicating systems. Milner's Calculus of communicating Systems (CCS), Winskel's Synchronization Trees (ST), Hennessy's Acceptance Trees (AT), and Hoare and Brookes's theory of communicating processes are examples of formal models of such systems. All of these models concentrate on modelling observable properties of a system.Event Dependency Trees (EDT) is a new representation of communicating systems that models the time dependent nature of such systems. None of the representations mentioned above explicitly represent time but time is precisely the factor that introduces so much variability and complexity into such software and systems. EDT provides a representation based on trees and a set of operations over the EDT trees that can be used to produce deadlock-free software. The model supplies potentially important information for the design and construction of distributed, parallel software systems

Run-time Mapping of Applications to a Heterogeneous SoC

This paper presents an iterative hierarchical approach to map an application to a parallel heterogeneous SoC architecture at run-time. The application is modeled as a set of communicating processes. The optimization objective is to minimize the energy consumption of the SoC, while still providing the required Quality of Service. This approach is flexible, scalable and the performance looks promisin

Semantics of communicating parallel processes.

Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science.Bibliography: leaves 187-189.Ph.D

On the Control of Asynchronous Automata

The decidability of the distributed version of the Ramadge and Wonham controller synthesis problem,where both the plant and the controllers are modeled as asynchronous automataand the controllers have causal memoryis a challenging open problem.There exist three classes of plants for which the existence of a correct controller with causal memory has been shown decidable: when the dependency graph of actions is series-parallel, when the processes are connectedly communicating and when the dependency graph of processes is a tree. We design a class of plants, called decomposable games, with a decidable controller synthesis problem.This provides a unified proof of the three existing decidability results as well as new examples of decidable plants

An Evolutionary Approach for Learning Attack Specifications in Network Graphs

This paper presents an evolutionary algorithm that learns attack scenarios, called attack specifications, from a network graph. This learning process aims to find attack specifications that minimise cost and maximise the value that an attacker gets from a successful attack. The attack specifications that the algorithm learns are represented using an approach based on Hoare's CSP (Communicating Sequential Processes). This new approach is able to represent several elements found in attacks, for example synchronisation. These attack specifications can be used by network administrators to find vulnerable scenarios, composed from the basic constructs Sequence, Parallel and Choice, that lead to valuable assets in the network

Methods to Model-Check Parallel Systems Software

We report on an effort to develop methodologies for formal verification of parts of the Multi-Purpose Daemon (MPD) parallel process management system. MPD is a distributed collection of communicating processes. While the individual components of the collection execute simple algorithms, their interaction leads to unexpected errors that are difficult to uncover by conventional means. Two verification approaches are discussed here: the standard model checking approach using the software model checker SPIN and the nonstandard use of a general-purpose first-order resolution-style theorem prover OTTER to conduct the traditional state space exploration. We compare modeling methodology and analyze performance and scalability of the two methods with respect to verification of MPD.Comment: 12 pages, 3 figures, 1 tabl