Monotonicity properties of cost functions in queueing networks

We establish monotonicity properties of cost functions in queueing networks with blocking, using sample path analysis. For any configuration of queueing stations with general interarrival times and exponentially distributed service times, we show that increasing the initial population and/or the arrival rates increases the expected cost (including blocking penalties for jobs that are rejected) incurred over a finite or an infinite horizon. Routing can be either probabilistic or state dependent. Furthermore, we show that this result also applies to networks with communication-type blocking where no jobs are ever lost. The criticality of the constraint imposed on the service times in demonstrated by a simple counterexample.This work is supported in part by the Office of Naval Research under Contract N00014-87-K-0304 and by the National Science Foundation under Grant ECS-8801912.     

A modular drinking philosophers algorithm

Summary A variant of the drinking philosophers algorithm of Chandy and Misra is described and proved correct in a modular way. The algorithm of Chandy and Misra is based on a particular dining philosophers algorithm and relies on certain properties of its implementation. The drinking philosophers algorithm presented in this paper is able to use an arbitrary dining philosophers algorithm as a subroutine; nothing about the implementation needs to be known, only that it solves the dining philosophers problem. An important advantage of this modularity is that by substituting a more time-efficient dining philosophers algorithm than the one used by Chandy and Misra, a drinking philosophers algorithm withO(1) worst-case waiting time is obtained, whereas the drinking philosophers algorithm of Chandy and Misra hasO(n) worst-case waiting time (forn philosophers). Careful definitions are given to distinguish the drinking and dining philosophers problems and to specify varying degrees of concurrency. Jennifer L. Welch received her B.A. in 1979 from the University of Texas at Austin, and her S.M. and Ph.D. from the Massachusetts Institute of Technology in 1984 and 1988 respectively. She has been a member of technical staff at GTE Laboratories Incorporated in Waltham, Massachusetts and an assistant professor at the University of North Carolina at Chapel Hill. She is currently an assistant professor at Texas A&M University. Her research interests include algorithms and lower bounds for distributed computing.Much of this work was performed while this author was at the Laboratory for Computer Science, Massachusetts Institute of Technology, supported by the Advanced Research Projects Agency of the Department of Defense under contract N00014-83-K-0125, the National Science Foundation under grants DCR-83-02391 and CCR-86-11442, the Office of Army Research under contract DAAG29-84-K-0058, and the Office of Naval Research under contract N00014-85-K-0168. This author was also supported in part by NSF grant CCR-9010730, an IBM Faculty Development Award, and NSF Presidential Young Investigator Award CCR-9158478This author was supported by the Office of Naval Research under contract N00014-91-J-1046, the Advanced Research Projects Agency of the Department of Defense under contract N00014-89-J-1988, and the National Science Foundation under grant CCR-89-15206. The photograph and autobiography of Professor N.A. Lynch were published in Volume 6, No. 2, 1992 on page 121     

Designing networks with compact routing tables

Classes of network topologies are identified in which shortest-path information can be succinctly stored at the nodes, if they are assigned suitable names. The naming allows each edge at a node to be labeled with zero or more intervals of integers, representing all nodes reachable by a shortest path via that edge. Starting with the class of outerplanar networks, a natural hierarchy of networks is established, based on the number of intervals required. The outerplanar networks are shown to be precisely the networks requiring just one interval per edge. An optimal algorithm is given for determining the labels for edges in outerplanar networks.The research of this author was supported in part by the National Science Foundation under Grant DCR-8320124, and by the Office of Naval Research on contract N 00014-86-K-0689.The research of this author was supported in part by the National Science Foundation under Grant DCR-8320124.     

A study of affine matching with bounded sensor error

Affine transformations of the plane have been used in a number of model-based recognition systems. Because the underlying mathematics are based on exact data, in practice various heuristics are used to adapt the methods to real data where there is positional uncertainty. This paper provides a precise analysis of affine point matching under uncertainty. We obtain an expression for the range of affine-invariant values that are consistent with a given set of four points, where each image point lies in an -disc of uncertainty. This range is shown to depend on the actualx-y-positions of the data points. In other words, given uncertainty in the data there are no representations that are invariant with respect to the Cartesian coordinate system of the data. This is problematic for methods, such as geometric hashing, that are based on affine-invariant representations. We also analyze the effect that uncertainty has on the probability that recognition methods using affine transformations will find false positive matches. We find that there is a significant probability of false positives with even moderate levels of sensor error, suggesting the importance of good verification techniques and good grouping techniques.This report describes research done in part at the Artificial Intelligence Laboratory of the Massachusetts Institute of Technology. Support for the laboratory's research is provided in part by an ONR URI grant under contract N00014-86-K-0685, and in part by DARPA under Army contract number DACA76-85-C-0010 and under ONR contracts N00014-85-K-0124 and N00014-91-J-4038. WELG is supported in part by NSF contract number IRI-8900267. DPH is supported at Cornell University in part by NSF grant IRI-9057928 and matching funds from General Electric and Xerox, and in part by AFOSR under contract AFOSR-91-0328.     

On the recognition of parameterized 2D objects

Determining the identity and pose of oceluded objects from noisy data is a critical step in interacting intelligently with an unstructured environment. Previous work has shown that local measurements of position and surface orientation may be used in a constrained search process to solve this problem, for the case of rigid objects, either two-dimensional or three-dimensional. This paper considers the more general problem of recognizing and locating objects that can vary in parameterized ways. We consider two-dimensional objects with rotational, translational, or scaling degrees of freedom, and two-dimensional objects that undergo stretching transformations. We show that the constrained search method can be extended to handle the recognition and localization of such generalized classes of object families.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 an Office of Naval Research University Research Initiative grant under contract N00014-86-K-0180, in part by the Advanced Research Projects Agency of the Department of Defense under Army contract number DACA76-85-C-0010, and in part by DARPA under. Office of Naval Research contract N00014-85-K-0124. A preliminary version of this work appeared in the proceedings of the First International Conference on Computer Vision, London, England, 1987.     

On the analysis of cooperation and antagonism in networks of communicating processes

We propose a new method for the analysis of cooperative and antagonistic properties of communicating finite state processes (FSPs). This algebraic technique is based on a composition operator and on the notion of possibility equivalence among FSPs. We demonstrate its utility by showing that potential blocking, termination, and lockout can be decided in polynomial time for loosely connected networks of tree FSPs. Potential blocking and termination are examples of cooperative properties, while lockout is an antagonistic one. For loosely connected networks of (the more general) acyclic FSPs, the cooperative properties become NP-complete and the antagonistic ones PSPACE-complete. For tightly coupled networks of tree FSPs, we also have NP-completeness for the cooperative properties. For the harder case of FSPs with cycles, we provide a natural extension of the method.A preliminary version of this paper appeared as an extended abstract in theProceedings of the Fourth Annual ACM Symposium on Principles of Distributed Computing, August, 1985, pp. 23–38. P. C. Kanellakis was supported by ONR-DARPA Grant N00014-83-K-0146, NSF Grant DCR-8302391, and by the Office of Army Research under contract DAAG29-84-K-0058. S. A. Smolka was supported by National Science Foundation Grant DCR-8505873.     

The ALFALFA Entomology Pest Identification System

Rule groups with preconditions,posteonditions,and termination conditions were added tothe ADVISE Meta-Expert System.Multiple,varying goals are also an attribute of the new rulegroups.By treating the data collection process as separating from the rule inference engine,techniques for enhanced data acquisition were developed using semantic networks to describerelations among variables and to restructure value sets for variables dynamically.Having thusextended the ADVISE tools,an automated key to alfalfa field pest identification was selected as atest application and found to be particularly well suited by the new features.A need fordisjunctive(“OR”)constructs in the right hand side of rules is discussed,and directions forfuture applications are given.     

Programming simultaneous actions using common knowledge

This work applies the theory of knowledge in distributed systems to the design of efficient fault-tolerant protocols. We define a large class of problems requiring coordinated, simultaneous action in synchronous systems, and give a method of transforming specifications of such problems into protocols that areoptimal in all runs: these protocols are guaranteed to perform the simultaneous actions as soon as any other protocol could possibly perform them, given the input to the system and faulty processor behavior. This transformation is performed in two steps. In the first step we extract, directly from the problem specification, a high-level protocol programmed using explicit tests for common knowledge. In the second step we carefully analyze when facts become common knowledge, thereby providing a method of efficiently implementing these protocols in many variants of the omissions failure model. In the generalized omissions model, however, our analysis shows that testing for common knowledge is NP-hard. Given the close correspondence between common knowledge and simultaneous actions, we are able to show that no optimal protocol for any such problem can be computationally efficient in this model. The analysis in this paper exposes many subtle differences between the failure models, including the precise point at which this gap in complexity occurs.This research was supported by the Office of Naval Research under contract N00014-85-K-0168, by the Office of Army Research under contract DAAG29-84-K-0058, by the National Science Foundation under Grant DCR-8302391, and by the Defense Advanced Research Projects agency (DARPA) under contract N00014-83-K-0125, and was performed while both authors were at MIT. A preliminary version of this work appeared in theProceedings of the 27th Annual IEEE Symposium on Foundations of Computer Science, Toronto, 1986.This author was primarily supported by an IBM postdoctoral fellowship.     

The complexity of hashing with lazy deletion

We examine a version of the dynamic dictionary problem in which stored items have expiration times and can be removed from the dictionary once they have expired. We show that under several reasonable assumptions about the distribution of the items, hashing with lazy deletion uses little more space than methods that use eager deletion. The simple algorithm suggested by this observation was used in a program for analyzing integrated circuit artwork.Support for this author was provided in part by NSF research grants MCS-81-05324 and DCR-84-03613, by an IBM research contract, by an IBM Faculty Development Award, and by ONR and DARPA under Contract N00014-83-K-0146 and ARPA Order No. 4786.     

Implementing a Scheme-based parallel processing system

The Scheme language can be converted into a parallel processing language by adding two new data types (placeholders andweak pairs), two processor synchronization primitives, and a task distribution mechanism. The mechanisms that support task creation, scheduling, and task synchronization are built using these extensions and features already present in the sequential language. Implementing the core of the parallel processing component in Scheme itself provides testbed for a variety of experiments and extensions.MultiScheme, the system resulting from these extensions, supports Halstead's future construct as the simple model for parallelism. By revealing the underlying placeholders on top of which this construct is built, Multischeme supports a variety of additional parallel programming techniques. It supports speculative computation through a simple procedural interface and the automatic garbage collection of tasks. The qlet and qlambda constructs of the QLisp language are also easily implemented in MultiScheme, as are the more familiar fork and join constructs of imperative programming.This research was supported in part by the Defense Advanced Research Projects Agency and was monitored by the Office of Naval Research under contract numbers N00014-83-K-0125, N00014-84-K0099, N00014-86-K-0180, and MDA903-84-C-0033. Additional funds and resources were provided by BBN Advanced Computers Inc., and the Hewlett-Packard Corporation. The work was performed as part of the author's dissertation research at the Massachusetts Institute of Technology.     

Emulating the human interpretation of line-drawings as three-dimensional objects

The human vision system has the ability to interpret two-dimensional images as three-dimensional objects. In this article, we present a program that emulates this ability for the case of images consisting of line-drawings. As a by-product of the approach, we provide an explanation of the Necker cube illusion.Support for the Artificial Intelligence Laboratory is provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014-85-K-0124.     

Fractional cascading: II. Applications

This paper presents several applications offractional cascading, a new searching technique which has been described in a companion paper. The applications center around a variety of geometric query problems. Examples include intersecting a polygonal path with a line, slanted range search, orthogonal range search, computing locus functions, and others. Some results on the optimality of fractional cascading, and certain extensions of the technique for retrieving additional information are also included.The first author was supported in part by NSF grants MCS 83-03925 and the Office of Naval Research and the Defense Advanced Research Projects Agency under contract N00014-83-K-0146 and ARPA Order No. 4786. Part of this work was done while the second author was employed by the Xerox Palo Alto Research Center.     

An assessment of multilisp: Lessons from experience

Multilisp is a parallel programming language derived from the Scheme dialect of Lisp by addition of thefuture construct. It has been implemented on Concert, a 32-processor shared-memory multiprocessor. A statistics-gathering feature of Concert Multilisp producesparallelism profiles showing the number of processors busy with computing or overhead, as a function of time. Experience gained using parallelism profiles and other measurement tools on several application programs has revealed three basic ways in whichfuture generates concurrency. These ways are illustrated on two example programs: the Lisp mapping functionmapcar and the partitioning routine from Quicksort. Experience with Multilisp programming exposes issues relating to side effects, error and exception handling, low-level operations for explicit manipulation of futures and tasks, and speculative computing, which are also discussed. The basic outlines of Multilisp are now fairly clear and have stood the test of being used for several applications, but further language design work is especially needed in the areas of speculative computing and exception handling.This research was supported in part by the Defense Advanced Research Projects Agency and was monitored by the Office of Naval Research under contract numbers N00014-83-K-0125 and N00014-84-K-0099.     

Topological numbering of features on a mesh

Assume we are given ann ×n binary image containing horizontally convex features; i.e., for each feature, each of its row's pixels form an interval on that row. In this paper we consider the problem of assigning topological numbers to such features, i.e., assign a number to every featuref so that all features to the left off in the image have a smaller number assigned to them. This problem arises in solutions to the stereo matching problem. We present a parallel algorithm to solve the topological numbering problem inO(n) time on ann ×n mesh of processors. The key idea of our solution is to create a tree from which the topological numbers can be obtained even though the tree does not uniquely represent the to the left of relationship of the features.The work of M. J. Atallah was supported by the Office of Naval Research under Grants N00014-84-K-0502 and N00014-86-K-0689, and the National Science Foundation under Grant DCR-8451393, with matching funds from AT&T. Part of this work was done while he was a Visiting Scientist at the Center for Advanced Architectures project of the Research Institute for Advanced Computer Science, NASA Ames Research Center, Moffett Field, CA 94035, USA. S. E. Hambrusch's work was supported by the Office of Naval Research under Contracts N00014-84-K-0502 and N00014-86K-0689, and by the National Science Foundation under Grant MIP-87-15652. Part of this work was done while she was visiting the International Computer Science Institute, Berkeley, CA 94704, USA. The work of L. E. TeWinkel was supported by the Office of Naval Research under Contract N00014-86K-0689.     

The complexity of planar compliant motion planning under uncertainty

We consider the computational complexity of planning compliant motions in the plane, given geometric bounds on the uncertainty in sensing and control. We can give efficient algorithms for generating and verifying compliant motion strategies that are guaranteed to succeed as long as the sensing and control uncertainties lie within the specified bounds. We also consider the case where a compliant motion plan is required to succeed over some parametric family of geometries. While these problems are known to be intractable in three dimensions, we identify tractable subclasses in the plane.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 Office of Naval Research under Office of Naval Research Contract N00014-81-K-0494 and in part by the Advanced Research Projects Agency under Office of Naval Research Contracts N00014-85-K-0124 and N00014-82-K.-0334. The author was funded in part by a NASA fellowship administered by the Jet Propulsion Laboratory, and in part by the Mathematical Sciences Institute and the National Science Foundation.     

AE1: An extension matrix approximate method for the general covering problem

A new approach, the extension matrix approach, is introduced and used to show that some optimization problems in general covering problem areNP-hard. Approximate solutions for these problems are given. Combining these approximate solutions, this paper presents an approximately optimal covering algorithm,AE1. Implementation shows thatAE1 is efficient and gives optimal or near optimal results.This research was supported in part by the National Science Foundation under Grant DCR 84-06801, Office of Naval Research under Grant N00014-82-K-0186, Defense Advanced Research Project Agency under Grant N00014-K-85-0878, and Education Ministry of the People's Republic of China.On leave from Harbin Institute of Technology, Harbin, China.     

On multiple moving objects

This paper explores the motion-planning problem for multiple moving objects. The approach taken consists of assigning priorities to the objects, then planning motions one object at a time. For each moving object, the planner constructs a configuration space-time that represents the time-varying constraints imposed on the moving object by the other moving and stationary objects. The planner represents this space-time approximately, using two-dimensional slices. The space-time is then searched for a collision-free path. The paper demonstrates this approach in two domains. One domain consists of translating planar objects; the other domain consists of two-link planar articulated arms.This report describes research performed at the Artificial Intelligence Laboratory of the Massachusetts Institute of Technology. Michael Erdmann is supported in part by a fellowship from General Motors Research Laboratories. Tomás Lozano-Pérez is supported by an NSF Presidential Young Investigator grant. Support for the Laboratory's Artificial Intelligence research is provided in part by the System Development Foundation, in part by the Office of Naval Research under Office of Naval Research Contract N00014-81-K-0494, and in part by the Advanced Research Projects Agency under Office of Naval Research Contracts N00014-80-C-0505 and N00014-82-K-0344.     

Addressing real-time constraints in the design of autonomous agents

The Phoenix project is an experiment in the design of autonomous agents for a complex environment. The project consists of a simulator of the environment, a basic agent architecture, and specific implementation of agents based on real-time techniques; the first two parts have been constructed, the third is on-going. The facets of Phoenix that facilitate real-time research are: a simulator parameterized for varying environmental conditions and instrumented to record behaviors, an agent architecture designed to support adaptable planning and scheduling, and methods for reasoning about real-time constraints.This research has been supported by DARPA, # F30602-85-C-0014; the Office of Naval Research, under a University Research Initiative grant, N00014-86-K-0764; the Office of Naval Research, # N00014-88-K0009, and a grant from the Digital Equipment Corporation. We wish to thank Mike Greenberg for his keen understanding of design issues and mastery of programming that made Phoenix what it is today. We also wish to thank Paul Silvey and David Westbrook for their help.     

Scheduling multihop CDMA networks in the presence of secondary conflicts

This paper presents an algorithm for producing near-optimal conflict-free schedules for networks operating under code division multiple access (CDMA). A procedure for finding a lower bound on the length of such schedules is also presented. The presence of both primary and secondary conflicts (due to imperfectly orthogonal CDMA codes) are accounted for by these algorithms. The complexity of both algorithms is analyzed and computational experience with both procedures is presented. Using the lower bound, it is shown that the heuristic is effective. The complexity analysis demonstrates that it is efficient enough to use in networks of realistic size, even when the schedules must be produced in real time.This work was sponsored in part by the Naval Research Laboratory under Contract N00014-84-K-2011 and by the New York State Foundation for Science and Technology as part of its Centers for Advanced Technology Program.     

Scheduling customers in a non-removal real-time system with an application to disk scheduling

In this article, we consider the problem of scheduling customers in a real-time system in which all customers are required to be serviced. Such anon-removal system can be distinguished from aremoval real-time system in which customers can be removed prior to completing service. We describe and evaluate a simple paradigm for mapping policies for removal systems to policies for non-removal systems. We show that several policies known to be optimal for removal systems map into policies which are also optimal for non-removal systems. The article concludes with an application of this paradigm to scheduling requests with real-time constraints on disk subsystems.This work is supported, in part, by the Office of Naval Research under contract N00014-87-K-0796 and the National Science Foundation under grant IRI-9114197.