Self-stabilization of dynamic systems assuming only read/write atomicity

Summary Three self-stabilizing protocols for distributed systems in the shared memory model are presented. The first protocol is a mutual-exclusion prootocol for tree structured systems. The second protocol is a spanning tree protocol for systems with any connected communication graph. The thrid protocol is obtianed by use offair protoco combination, a simple technique which enables the combination of two self-stabilizing dynamic protocols. The result protocol is a self-stabilizing, mutualexclusion protocol for dynamic systems with a general (connected) communication graph. The presented protocols improve upon previous protocols in two ways: First, it is assumed that the only atomic operations are either read or write to the shared memory. Second, our protocols work for any connected network and even for dynamic network, in which the topology of the network may change during the excution. Shlomi Dolev received his B.Sc. in Civil Engineering and B.A. in Computer Science in 1984 and 1985, and his M.Sc. and Ph.D. in computer Sciene in 1989 and 1992 from the Technion Israel Institute of Technology. He is currently a post-dotoral fellow in the Department of Computer Science at Texas A & M Univeristy. His current research interests include the theoretical aspects of distributed computing and communcation networks. Amos Israeli received his B.Sc. in Mathematics and Physics from Hebrew University in 1976, and his M.Sc. and D.Sc. in Computer Science from the Weizmann Institute in 1980 and the Technion in 1985, respectively. Currently he is a sensior lecturer at the Electrical Engineering Department at the Technion. Prior tot his he was a postdoctoral fellow at the Aiken Computation Laboratory at harvard. His research interests are in Parellel and Distributed Computing and in Robotics. In particular he has worked on the design and analysis of Wait-Free and Self-Stabilizing distributed protocols. Shlomo Moran received his B.Sc. and D.Sc. degrees in matheamtics from Technion, Israel Institute of Technology, Haifa, in 1975 and 1979, respectively. From 1979 to 1981 he was assistant professors and a visiting research specialist at the University of Minnesota, Minneapolis. From 1981 to 1985 he was a senior lecturer at the Department of Computer Science. Technion, and from 1985 to 1986 he visted at IBM Thoas J. Watson Research Center, Yorktown Heights. From 1986 to 1993 he was an associated professor at the Department of Computer Science, Technin. in 1992–3 he visited at AT & T Bell Labs at Murray Hill and at Centrum voor Wiskunde en Informatica, Amsterdam. From 1993 he is a full professor at the Department of Computer Science, Technion. His researchinterests include distributed algorithm, computational complexity, combinatorics and grapth theory.Part of this research was supported in part by Technion V.P.R. Funds — Wellner Research Fund, and by the Foundation for Research in Electronics, Computers and Communictions, administrated by the Israel Academy of Sciences and Humanities.     

One-bit algorithms

Many algorithms in distributed systems assume that the size of a single message depends on the number of processors. In this paper, we assume in contrast that messages consist of a single bit. Our main goal is to explore how the one-bit translation of unbounded message algorithms can be sped up by pipelining. We consider two problems. The first is routing between two processors in an arbitrary network and in some special networks (ring, grid, hypercube). The second problem is coloring a synchronous ring with three colors. The routing problem is a very basic subroutine in many distributed algorithms; the three coloring problem demonstrates that pipelining is not always useful. Amotz Bar-Noy received his B.Sc. degree in Mathematics and Computer Science in 1981, and his Ph.D. degree in Computer Science in 1987, both from the Hebrew University of Jerusalem, Israel. Between 1987 and 1989 he was a post-doctoral fellow in the Department of Computer Science at Stanford University. He is currently a visiting scientist at the IBM Thomas J. Watson Research Center. His current research interests include the theoretical aspects of distributed and parallel computing, computational complexity and combinatorial optimization. Joseph (Seffi) Naor received his B.A. degree in Computer Science in 1981 from the Technion, Israel Institute of Technology. He received his M.Sc. in 1983 and Ph.D. in 1987 in Computer Science, both from the Hebrew University of Jerusalem, Israel. Between 1987 and 1988 he was a post-doctoral fellow at the University of Southern California, Los Angeles, CA. Since 1988 he has been a post-doctoral fellow in the Department of Computer Science at Stanford University. His research interests include combinatorial optimization, randomized algorithms, computational complexity and the theoretical aspects of parallel and distributed computing. Moni Naor received his B.A. in Computer Science from the Technion, Israel Institute of Technology, in 1985, and his Ph.D. in Computer Science from the University of California at Berkeley in 1989. He is currently a visiting scientist at the IBM Almaden Research Center. His research interests include computational complexity, data structures, cryptography, and parallel and distributed computation.Supported in part by a Weizmann fellowship and by contract ONR N00014-85-C-0731Supported by contract ONR N00014-88-K-0166 and by a grant from Stanford's Center for Integrated Systems. This work was done while the author was a post-doctoral fellow at the University of Southern California, Los Angeles, CAThis work was done while the author was with the Computer Science Division, University of California at Berkeley, and Supported by NSF grant DCR 85-13926     

Efficient emulation of single-hop radio network with collision detection on multi-hop radio network with no collision detection

Summary This paper presents an efficient randomized emulation ofsingle-hop radio networkwith collision detection onmulti-hop radio networkwithout collision detection. Each step of the single-hop network is emulated by rounds of the multi-hop network and succeeds with probability 1–. (n is the number of processors,D the diameter and the maximum degree). It is shown how to emulate any polynomial algorithm such that the probability of failure remains . A consequence of the emulation is an efficient randomized algorithm for choosing a leader in a multi-hop network. Reuven Bar-Yehuda was born in Iran, on July 17th 1951. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion — Israel Institute of Technology, Haifa, Israel, in 1978, 1980, and 1983, respectively. He is currently a Senior Lecturer of Computer Science at the Technion. From 1984 to 1986, he was a visiting assistant professor in the Computer Science Dept. at the Duke Univesity His research interests include computational geometry, VLSI, graph algorithms and distributed algorithms. Oded Goldreich was born in Tel-Aviv, Israel, on February 4th 1957. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion — Israel Institute of Technology, Haifa, Israel, in 1980, 1982, and 1983, respectively. He is currently an Associate Professor of Computer Science at the Technion. From 1983 to 1986, he was a postdoctoral fellow at MIT's Laboratory for Computer Science. His research interests include cryptography and related areas, relation between randomness and algorithms, and distributed computation. Alon Itai was born in Scotland, on December 12th 1946. Received B.Sc. in Mathematics from the Hebrew University in Jerusalem in 1969. M.Sc., and Ph.D. in Computer Science from the Weizmann Institute of Science, Rehovot, Israel in 1971 and 1976. He is currently an Associate Professor of Computer Science at the Technion. His research interests include randomized and distributed algorithms, computational learning theory and performance evaluation.The second author was partially supported by grant No. 86-00301 from the United States—Israel Bi-national Science Foundation BSF), Jerusalem, Israel.     

Strong stable properties in distributed systems

Summary A stable property in a distributed system is a global property which once true, remains true forever. This paper refines this notion by formally introducing the concept ofstrong stable properties. A strong stable property has the nice property that it can be correctly evaluated on the consistent part of uncoordinated snapshots. Termination and deadlock are shown to be strong stable properties, whereas distributed garbage is not. We also show how to derive a simple generic algorithm for the detection of a strong stable property. The generic algorithm is illustrated by two examples: termination detection and deadlock detection. Incidentally the paper presents a very simple algorithm for termination detection. Andre Schiper has been a professor of Computer Science at EPFL (Federal Institute of Technology in Lausanne, Switzerland) since 1985, leading the Operating Systems laboratory. He graduated in Physics from the Federal Institute of technology in Zürich and received his Ph.D. in Computer Science from EPFL in 1980. In 1981–82 he spent one year at the University of Rennes, France. From 1983 to 1985, he was professor at the Engineering School in Yverdon, Switzerland. Between 1989 and 1991 André Schiper was head of the Department of Computer Science of EPFL, and during the academic year 1992–93 he was on sabbatical leave at Cornell University, Ithaca (NY). His research interests are in the areas of operating systems, distributed and fault-tolerant distributed systems, and parallelism. He is currently involved in the European Esprit project BROADCAST whose objective is the design and implementation of large scale distributed computing systems. Alain Sandoz graduated in Mathematics from the University of Neuchâtel, Switzerland, in 1984 and in Computer Science from the Federal Institute of Technology in Lausanne, Switzerland, in 1988. He received his Ph.D. in Computer Science from the Federal Institute of Technology in Lausanne in 1992. His dissertation was concerned with modelling causal relationships between transactions in distributed and replicated database systems. From 1992 to 1994 he was involved in research on fault-tolerant and large scale distributed computing systems. He is currently working on the development of information systems for the Swiss government.     

On the complexity of global computation in the presence of link failures: the general case

Summary This paper presents (m logn) and (mn) messages lower bounds on the problem of computing a gobal sensitive function in biderectional networks with link failures (i.e., dynamically changing topology), wheren andm are the total number of nodes and links in the network. The (m logn) lower bound is under the assumption thatn is a-priori known to the nodes, while the second bound is for the case in which such knowledge is not available. A global sensitive function ofn variables is a function that may not be computed without the knowledge of the values of all then variables (e.g. maximum, sum, etc). Thus, computing such a function at one node of a distributed network requires this node to communicate with every other node in the network. Though lower bounds higher than (m) messages are known for this problem in the context of link failures, none holds for dense bidirectional networks. Moreover, we are not aware of any other nontrivial lower bound higher than (m) for dense bidirectional networks. Yehuda Afek received a B.Sc. in Electrical Engineering from the Technion and an M.Sc. and Ph.D. in Computer Science from the University of California, Los-Angeles. In 1985 he joined the Distributed Systems Research Department in AT&T Bell Laboratories as a Member of Technical Staff. In 1988 he joined the Computer Science Department in Tel-Aviv University, where he now holds a permanent position. From 1989 to 1994 he was also a consultant for AT&T Bell Laboratories. His interests include communication protocols, distributed computing and asynchronous shared memory systems. Danny Hendler was born in Kiryat-Haim near Haifa, Israel, on April 17th 1961. He received his B.Sc. and M.Sc. in Computer Science from Tel-Aviv University, Israel, in 1986 and 1993, respectively. In the past 8 years he has worked as a free lance software-consultant, specializing mainly in communication, telephony and voice-mail applications.     

Appraising fairness in languages for distributed programming

The relations among various languages and models for distributed computation and various possible definitions of fairness are considered. Natural semantic criteria are presented which an acceptable notion of fairness should satisfy. These are then used to demonstrate differences among the basic models, the added power of the fairness notion, and the sensitivity of the fairness notion to irrelevant semantic interleavings of independent operations. These results are used to show that from the considerable variety of commonly used possibilities, only strong process fairness is appropriate forCSP if these criteria are adopted. We also show that under these criteria, none of the commonly used notions of fairness are fully aceptable for a model with an n-way synchronization mechanism. The notion of fairness most often mentioned for Ada is shown to be fully acceptable. For a model with nonblockingsend operations, some variants of common fairness definitions are appraised, and two are shown to satisfy the suggested criteria. Krzysztof R. Apt was born in 1949 in Poland. Received his Ph.D. in 1974 from Polish Academy of Sciences in Warsaw in mathematical logic. From 1974 until 1981 worked at various scientific institutions in the Netherlands and from 1981 until 1987 at C.N.R.S. in Paris, France. Spent 1985 as a visiting scientist at IBM Research Centre in Yorktown Heights, U.S.A. Currently holding an Endowed Professorship at the Department of Computer Sciences at the University of Texas at Austin; also a senior research scientist at the Centre for Mathematics and Computer Science in Amsterdam, the Netherlands. His research interests include program correctness and semantics, methodology of distributed computing, use of logic as a programming language and non-standard forms of reasoning. He has served on editorial boards of a number of journals and program committees of numerous conferences in computer science. Lectured in a dozen countries on four continents. Also, he has run two marathons and crossed Sumatra on a bicycle. Shmuel Katz received his B.A. in Mathematics and English Literature from U.C.L.A., and his M.Sc. and Ph.D. in Computer Science (1976) from the Weizmann Institute in Rehovot, Israel. From 1976 to 1981 he was a researcher at the IBM Israel Scientific Center. Presently, he is a Senior Lecturer in the Computer Science Department at the Technion in Haifa, Israel. In 1977–78, he visited for a year at the University of California, Berkeley, and in 1984–85 was at the University of Texas at Austin. He has also been a consultant for the MCC Software Technology Program. His research interests include the methodology of programming, specification methods, program verification and semantics, distributed programming, data structures, and programming languages. Nissim Francez received his B.A. in Mathematics and Philosophy from the Hebrew University in Jerusalem, and his M.Sc. and Ph.D. in computer science (1976) from the Weizmann Institute of Science, Rehovot, Israel. In 1976–77 he spent a postdoctoral year at Queen's university, Belfast, where he was introduced by C.A.R. Hoare to CSP. In 1977–78 he was an assistant professor at USC, Los Angeles. From 1978 he is with the Computer Science Department at the Technion. In 1982–83 he was on a sabbatical leave at IBM T.J. Watson Research Center. He has been a consultant for MCC's software technology program, working on multiparty activities in distributed systems. He had summer appointments in Harvard University, IBM T.J. Watson Research Center, Utrecht University, CWI (Amsterdam) and at MCC. He also served in several program committees. His research interests include program verification and the semantics of programming languages, mainly for concurrent and distributed programming. Is also interested in logic programming and recursive query evaluation and in compiler constration. He is the author of the first book onFairness. Unfortunately, he is incapable of Marathon running...     

Verification of distributed programs using representative interleaving sequences

Summary We present a formal proof method for distributed programs. The semantics used to justify the proof method explicitly identifies equivalence classes of execution sequences which are equivalent up to permuting commutative operations. Each equivalence class is called an interleaving set or a run. The proof rules allow concluding the correctness of certain classes of properties for all execution sequences, even though such properties are demonstrated directly only for a subset of the sequences. The subset used must include a representative sequence from each interleaving set, and the proof rules, when applicable, guarantee that this is the case. By choosing a subset with appropriate sequences, simpler intermediate assertions can be used than in previous formal approaches. The method employs proof lattices, and is expressed using the temporal logic ISTL. Shmuel Katz received his B.A. in Mathematics and English Literature from U.C.L.A., and his M.Sc. and Ph.D. in Computer Science (1976) from the Weizmann Institute in Rechovot, Israel. From 1976 to 1981 he was at the IBM Israel Scientific Center. Presently, he is on the faculty of the Computer Science Department at the Technion in Haifa, Israel. In 1977–1978 he visited for a year at the University of California, Berkeley, and in 1984–1985 was at the University of Texas at Austin. He has been a consultant and visitor at the MCC Software Technology Program, and in 1988–1989 was a visiting scientist at the I.B.M. Watson Research Center. His research interests include the methodology of programming, specification methods, program verification and semantics, distributed programming, data structures, and programming languages. Doron Peled was born in 1962 in Haifa. He received his B.Sc. and M.Sc. in Computer Science from the Technion, Israel in 1984 and 1987, respectively. Between 1987 and 1991 he did his military service. He also completed his D.Sc. degree in the Technion during these years. Dr. Peled was with the Computer Science department at Warwick University in 1991–1992. He is currently a member of the technical staff with AT & T Bell Laboratories. His main research interests are specification and verification of programs, especially as related to partial order models, fault-tolerance and real-time. He is also interested in semantics and topology.This research was carried out while the second author was at the Department of Computer Science, The Technion, Haifa 32000, Israel     

Distributed algorithms for constructing a minimum-weight spanning tree in a broadcast network

Two distributed algorithms are presented for a network using a common communication channel (e.g. radio) in which all nodes are within signal range and in line of sight of each other: (a) an algorithm to compute all internode distances (in terms of propagation delays) in the network. the algorithm requires only 2 messages per node, and provides each node with the distances to all other nodes. (b) An algorithm for constructing a minimum-weight spanning tree (MST) in such a network. This algorithm starts out with the information provided by (a) and ends with each node possessing the explicit knowledge of the full MST. The algorithm requires at most log₂ N messages per node. The internal processing in each node needsO(N logN) time andO(N) space. All messages required by (a) and (b) contain at most one edge weight plus 2 log₂ N bits. Some possible applications of the algorithms are: position-location, tuning acknowledgement time-out mechanisms, tuning the scheduling functions of access protocols that are sensitive to individual internode propagation delays, and selecting performance effective transmission sequences for round robin access protocols.Yaron I. Gold received the B.Sc. (Cum Laude, 1970) and M.Sc. (1975), both in Electrical Engineering, from the Technion, Israel institute of Technology, and the Ph.D. (1981) in Computer Science, from the University of Minnesota, Minneapolis.From 1970 to 1975 Yaron Gold served as Research and Development Officer in the Israeli Defense Forces, leading a group of several scientists, engineers and technicians. From 1982 to 1984 he was on the faculty of the Department of Electrical Engineering the Department of Electrical Engineering and Computer Science at the University of Connecticut. During that period he also served as consultant for United Technologies Corporation and for Battelle Laboratories. Presently, Dr. Gold is on the faculty of the Computer Science Department at the Technion.His research interests include Computer Networks and Communications, Simulation and Intelligent Systems.Shlomo Moran received the B.Sc. and D.Sc. degrees in mathematics from Technion, Israel Institute of Technology, Haifa, in 1975 and 1979, respectively.From 1979 to 1981 he was assistant professor and a visiting research specialist at the University of Minnesota, Minneapolis. From 1981 to 1985 he was a senior lecturer at the Department of Computer Science, Technion, and from 1985 to 1986 he was a World Trade visitor at IBM Thomas J. Watson Research Center, Yorktown Heights. Currently he is associate professor at the Department of Computer Science, Technion.His research interests include distributed algorithms, computational complexity, combinatorics, graph theory and geometric aspects of communication networks.This work was supported in part by NSF grant ECS-8307186Part of this research was done while this author was with the University of ConnecticutPart of this research was done while this author was with IBM, Thomas J. Watson Research Center     

Self-stabilizing extensions for meassage-passing systems

Summary A self-stabilizing program eventually resumes normal behavior even if excution begins in, an abnormal initial state. In this paper, we explore the possibility of extending an arbitrary program into a self-stabilizing one. Our contributions are: (1) a formal definition of the concept of one program being aself-stabilizing extension of another; (2) a characterization of what properties may hold in such extensions; (3) a demonstration of the possibility of mechanically creating such extensions. The computtional model used is that of an asynchronous distributed message-passing system whose communication topology is an arbitrary graph. We contrast the difficulties of self-stabilization in thismodel with those of themore common shared-memory models. Shmuel Katz received his B.A. in Mathematics and Englisch Literature from U.C.L.A., and his M.Sc. and Ph.D. in Computer Science (1976) from the Weizmann Institute in Rechovot, Israel. From 1976 to 1981 he was a research at the IBM Israel Scientific Center. Presently, he is an Associate Professor in the Computer Science Department at the Technion in Haifa, Israel. In 1977–78 he visited for a year at the University of California, Berkeley, and in 1984–85 was at the University of Texas at Austin. He has been a consultant and vistor at the MCC Software Technology Program, and in 1988–89 was a visiting scientist at the IBM Watson Research Center. His research interests include the methodology of programming, specification methods, program verification and semantics, distributed programming, data structure, and programming languages. Kenneth J. Pery has performed research in the area of distributed computing since obtaining Masters and Doctorate degrees in Computer Science from Cornell Univesity. His current interest is in studying problems of a partical nature in a formal context. He was graduated from Princeton University in 1979 with a B.S.E. degree in Electrical Engineering and Computer Science.The Research of this author was partially supported by Research Grant 120-749 and the Argentinian Research Fund at the Technion     

Efficient algorithms for optimistic crash recovery

Summary Recovery from transient processor failures can be achieved by using optimistic message logging and checkpointing. The faulty processorsroll back, and some/all of the non-faulty processors also may have to roll back. This paper formulates the rollback problem as a closure problem. A centralized closure algorithm is presented together with two efficient distributed implementations. Several related problems are also considered and distributed algorithms are presented for solving them. S. Venkatesan received the B. Tech. and M. Tech degrees from the Indian Institute of Technology, Madras in 1981 and 1983, respectively and the M.S. and Ph.D. degrees in Computer Science from the University of Pittsburgh in 1985 and 1988. He joined the University of Texas at Dallas in January 1989, where he is currently an Assistant Professor of Computer Science. His research interests are in fault-tolerant distributed systems, distributed algorithms, testing and debugging distributed programs, fault-tolerant telecommunication networks, and mobile computing. Tony Tony-Ying Juang is an Associate Professor of Computer Science at the Chung-Hwa Polytechnic Institute. He received the B.S. degree in Naval Architecture from the National Taiwan University in 1983 and his M.S. and Ph.D. degrees in Computer Science from the University of Texas at Dallas in 1989 and 1992, respectively. His research interests include distributed algorithms, fault-tolerant distributed computing, distributed operating systems and computer communications.This research was supported in part by NSF under Grant No. CCR-9110177 and by the Texas Advanced Technology Program under Grant No. 9741-036     

Modelling knowledge and action in distributed systems

We present a formal model that captures the subtle interaction between knowledge and action in distributed systems. We view a distributed system as a set ofruns, where a run is a function from time toglobal states and a global state is a tuple consisting of anenvironment state and alocal state for earch process in the system. This model is a generalization of those used in many previous papers.Actions in this model are associated with functions from global states to global states. Aprotocol is a function from local states to actions. We extend the standard notion of a protocol by definingknowledge-based protocols, ones in which a process' actions may depend explicitly on its knowledge. Knowledge-based protocols provide a natural way of describing how actions should take place in a distributed system. Finally, we show how the notion of one protocolimplementing another can be captured in our model. Joseph Y. Halpern received a B.Sc. in mathematics from the University of Toronto in 1975, and a Ph.D. in mathematics from Harvard University in 1981. In between, he spent two years as the head of the Mathematics Department at Bawku Secondary School, in Ghana. After a year as a visiting scientist at MIT, he joined IBM in 1982. He is currently the manager of the Mathematics and Related Computer Science Department at the IBM Almaden Research Center, and a consulting professor in the Computer Science Department at Stanford. His major research interests are reasoning about knowledge, distributed computation, and logics of programs. He was program chairman and organizer of the first conference of Theoretical Aspects of Reasoning About Knowledge, program chairman of the Fifth ACM Symposium on Principles of Distributed Computing, and was the co-recipient (with Ronald Fagin) of the MIT Publisher's Prize for the Best Paper Paper at the 1985 International Joint Conference on Artificial Intelligence. Ronald Fagin is manager of the Foundations of Computer Science group at the IBM Almaden Research Center. He received his B.A. degree in mathematics from Dartmouth College in 1967 and his Ph.D. in mathematics, specializing in mathematical logic, from the University of California at Berkeley in 1973. He joined IBM in 1973 at the Thomas J. Watson Research Center. In 1975, he transferred to the San Jose Research Laboratory (now the IBM Almaden Research Center) where most of his research has centered on applications of logic to computer science. In particular, he has done research on the theory of relational databases and, more recently, on theories of knowledge and belief. He has received three IBM Outstanding Innovation Awards for his contributions to relational database theory, extendible hashing, and reasoning about knowledge. He was co-recipient (with Joe Halpern) of the MIT Press Publisher's Prize for the Best Paper at the 1985 International Joint Conference on Artificial Interlligence.Some material in this paper appeared in preliminary form in Halpern and Fagin (1985). An abridged version of the paper appeared in Vogt F (ed) Proceeding of Concurrency 88 (Lecture Notes in Computer Science Vol. 335) Springer-Verlag, 1988, pp 18–32     

Distributed optimistic concurrency control with reduced rollback

Concurrency control algorithms have traditionally been based on locking and timestamp ordering mechanisms. Recently optimistic schemes have been proposed. In this paper a distributed, multi-version, optimistic concurrency control scheme is described which is particularly advantageous in a query-dominant environment. The drawbacks of the original optimistic concurrency control scheme, namely that inconsistent views may be seen by transactions (potentially causing unpredictable behavior) and that read-only transactions must be validated and may be rolled back, have been eliminated in the proposed algorithm. Read-only transactions execute in a completely asynchronous fashion and are therefore processed with very little overhead. Furthermore, the probability that read-write transactions are rolled back has been reduced by generalizing the validation algorithm. The effects of global transactions on local transaction processing are minimized. The algorithm is also free from dedlock and cascading rollback problems. Divyakant Agrawal is currently a graduate student in the Department of Computer Science at the State University of New York at Stony Brook. He received his B.E. degree from Birla Institute of Technology and Science, Pilani, India in 1980. He worked with Tata Burroughs Limited, from 1980 to 1982. He completed his M.S. degree in Computer Science from SUNY at Stony Brook in 1984. His research interests include design of algorithms for concurrent systems, optimistic protocols and distributed systems. Arthur Bernstein is a Professor of Computer Science at the State University of New York at Stony Brook. His research is concerned with the design and verification of algorithms involving asynchronous activity and with languages for expressing such algorithms. Pankaj Gupta is currently a graduate student in the Department of Computer Science at the State University of New York at Stony Brook. He received M.S. degree in Electrical Engineering from SUNY at Stony Brook in 1982 and M.S. degree in Computer Science from SUNY at Stony Brook in 1985. His research interests include distributed systems, concurrency control, and databases. Soumitra Sengupta is currently a graduate student in the Department of Computer Science at the State University of New York at Stony Brook. He received his B.E. degree from Birla Institute of Technology and Science, Pilani, India in 1980. He worked with Tata Consultancy Services, from 1980 to 1982. He completed his M.S. degree in Computer Science from SUNY at Stony Brook in 1984. His research interests include distributed algorithms, logic databases and concurrency control.This work was supported by the National Science Foundation under grant, DCR-8502161 and the Air Force Office of Scientific Research under grant AFOSR 810197     

Adaptive solutions to the mutual exclusion problem

Summary Algorithms for mutual exclusion that adapt to the current degree of contention are developed. Afilter and a leader election algorithm form the basic building blocks. The algorithms achieve system response times that are independent of the total number of processes and governed instead by the current degree of contention. The final algorithm achieves a constant amortized system response time. Manhoi Choy was born in 1967 in Hong Kong. He received his B.Sc. in Electrical and Electronic Engineerings from the University of Hong Kong in 1989, and his M.Sc. in Computer Science from the University of California at Santa Barbara in 1991. Currently, he is working on his Ph.D. in Computer Science at the University of California at Santa Barbara. His research interests are in the areas of parallel and distributed systems, and distributed algorithms. Ambuj K. Singh is an Assistant Professor in the Department of Computer Science at the University of California, Santa Barbara. He received a Ph.D. in Computer Science from the University of Texas at Austin in 1989, an M.S. in Computer Science from Iowa State University in 1984, and a B.Tech. from the Indian Institute of Technology at Kharagpur in 1982. His research interests are in the areas of adaptive resource allocation, concurrent program development, and distributed shared memory.A preliminary version of the paper appeared in the 12th Annual ACM Symposium on Principles of Distributed ComputingWork supported in part by NSF grants CCR-9008628 and CCR-9223094     

Self-stabilization over unreliable communication media

Summary A self-stabilizing system has the property that it will converge to a desirable state when started from any state. Most previous researchers assumed that processes in self-stabilizing systems may communicate through shared variables while those that studied meassage passing systems allowed messages with unbounded size. This paper discusses the development of self-stabilizing systems which communicate through message passing, and in which messages may be lost in transit. The systems presented all use fixed size message headers. First, a selfstabilizing version of theAlternating Bit Protocol, a fundamental communication protocol for transmitting data across an unreliable communication medium, is presented. Secondly, the alternating-bit protocol is used to construct a self-stabilizing token ring. Yehuda Afek received a B.Sc. in Electrical Engineering from the Technion and an M.S. and Ph.D. in Computer Science from the University of California, Los Angeles. In 1985 he joined the Distributed Systems research Department in AT&T Bell Laboratories and in 1988 he joined the Department of Computer Science in Tel-Aviv University. His interests include communication protocols, distributed systems, and asynchronous shared memories. Geoffrey M. Brown received the BS degree in Engineering from Swarthmore College in 1982, the MS degree in Electrical Engineering from Stanford University in 1983, and the Ph.D. degree in Electrical Engineering from the University of Texas at Austin in 1987. From 1983 to 1984 he worked for Motorola in Austin, TX. Currently he is an Assistant Professor in the School of Electrical Engineering at Cornell University. In 1990, Brown was named a Presidential Young Investigator by the National Science Foundation.This work supported in part by NSF grant CCR-9058180     

On optimizing the satisfiability (SAT) problem

1IntroductionThesatisfiability(SAT)problemistodeterminewhetherthereexistsanassignmentofvaluesin{0,1}toasetofBooleanvariables{x1,xm}thatmakesaconjunctivenormalform(CNF)formulatrue.ThesatisfiabilityproblemofaCNFformulawithatmostlliteralsineachclauseiscalledthel-SATproblem.Theoretically,for>3,theSATproblemisawell-knownNP-completeproblem.Andthus,thereexistsnopolynomialtimealgorithmfortheSATproblemontheassumptionthatPNP.Ontheotherhand,theSATproblemisfundamentalinsolvingmanypracticalprob…     

Atomic snapshots using lattice agreement

Summary Thesnapshot object is an important tool for constructing wait-free asynchronous algorithms. We relate the snapshot object to thelattice agreement decision problem. It is shown that any algorithm for solving lattice agreement can be transformed into an implementation of a snapshot object. The overhead cost of this transformation is only a linear number of read and write operations on atomic single-writer multi-reader registers. The transformation uses an unbounded amount of shared memory. We present a deterministic algorithm for lattice agreement that usedO (log² n) operations on 2-processorTest & Set registers, plusO (n) operations on atomic single-writer multi-reader registers. The shared objects are used by the algorithm in adynamic mode, that is, the identity of the processors that access each of the shared objects is determined dynamically during the execution of the algorithm. By a randomized implementation of 2-processorsTest & Set registers from atomic registers, this algorithm implies a randomized algorthm for lattice agreement that uses an expected number ofO (n) operations on (dynamic) atomic single-writer multi-reader registers. Combined with our transformation this yields implementations of atomic snapshots with the same complexity.Cambridge Research Laboratory, Digital Equipment Corporation Hagit Attiya received the B.Sc. degreeiin Mathematics and Computer Science from the Hebrew University of Jerusalem, in 1981, the M.Sc. and Ph.D. degrees in Computer Science from the Hebrew University of Jerusalem, in 1983 and 1987, respectively. She is presently a senior lecturer at the departtment of Computer Science at the Technion, Israel Institute of Technology. Prior to this, she has been a post-doctoral research associate at the Laboratory for Computer Science at M.I.T. Her general research interests are distributed computation and theoretical computer science. More specific interests include fault-tolerance, timing-based and asynchronous algorithms. Maurice Herlihy received the A.B. degree in Mathematics from Harvard University, and the M.S. and the Ph.D. degrees in Computer Science from M.I.T. From 1984 to 1989 he was a faculty member in the Computer Science Department at Carnegie Mellon University in Pittsburgh, PA. In 1989 he joined the research staff at the Digital Equipment Corporation's Cambridge Research Laboratory in Cambridge MA. Since 1994, he has been on the faculty at the Computer Science Department at Brown University. Dr. Herlihy's research interests encompass practical and theoretical aspects of distributed and concurrent computation. Ophir achman received a B.A. in computer science from the Technion, Haifa, Israel in 1989 and M.Sc. in computer science from the Technion, Haifa, Israel, in 1992. He is now studying for a D.Sc. in computer science at the Technion. His currentarea of research is distributed computing, and in particular, asynchronous shared memory systems.This work appeared in preliminary form in proceedings ofthe 6th International Workshop on Distributed Algorithms [12]. This research was partially supported by grant No. 92-0233 from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Technion V.P.R. funds — B. and G. Greenberg Research Fund (Ottawa), and the fund for the promotion of research in the TechnionPart of the work of this author was performed while visiting DEC Cambridge Research Laboratory     

Bounded time-stamps

Summary Time-stamps are labels which a system adds to its data items. These labels enable the system to keep track of the temporal precedence relations among its data elements. Many distributed protocols and some applications use the natural numbers as time-stamps. The natural numbers however are not useful for bounded protocols. In this paper we develop a theory ofbounded time-stamps. Time-stamp schemes are defined and the complexity of their implementation is analyzed. This indicates a direction for developing a general tool for converting time-stamp based protocols to bounded protocols. Amos Israeli received his B.Sc. in Mathematics and Physics from Hebrew University in 1976, and his M.Sc. and D.Sc. in Computer Science from the Weizmann Institute in 1980 and the Technion in 1985, respectively. Currently he is a senior lecturer at the Tlectrical Engineering Department at the Technion. Prior to this he was a postdoctoral fellow at the Aiken Computation Laboratory at Harvard. His research interests are in Parallel and Distributed Computing and in Robotics. In particular he has worked on the design and analysis of Wait-Free and Self-Stabilizing distributed protocols. Ming Li received his M.S. and Ph.D. in Computer Science from Wayne State University in 1980 and Cornell University 1985, respectively. Currently he is an associate professor at the Computer Science Department at the University of Waterloo. His research interests are in Theory of Computing, Kolmogorov Complexity, and Machine Learning.Supported in part by the Weizmann fellowship and NSF Grant DCR-86-00379Supported in part by ONR Grant N00014-85-k-0445 and Army Research Office Grant DAAL03-86-K-0171 at Harvard University, by NSF Grant kDCR-86-06366 at Ohio State University, and by NSERC Operating Grant OGP0036747. Most of this work was done when the authors were at Aiken Computation Laboratory at Harvard University. The authors also acknowledge the hospitality of the computer science department at York University, Canada     

A fast,scalable mutual exclusion algorithm

Summary This paper is concerned with synchornization under read/write atomicity in shared memory multi-processors. We present a new algorithm forN-process mutual exclusion that requires only read and write operations and that hasO(logN) time complexity, where time is measured by counting remote memory references. The time complexity of this algorithm is better than that of all prior solutions to the mutual exclusion problem that are based upon atomic read and write instructions; in fact, the time complexity of most prior solutions is unbounded. Performance studies are presented that show that our mutual exclusion algorithm exhibits scalable performance under heavy contention. In fact, its performance rivals that of the fastest queue-based spin locks based on strong primitives such as compare-and-swap and fetch-and-add. We also present a modified version of our algorithm that generates onlyO(1) memory references in the absence of contention. Jae-Heon Yang received the B.S. and M. S. degrees in Computer Engineering from Seoul National University in 1985 and 1987, respectively, and the Ph.D. degree in Computer Science from the University of Maryland at College Park in 1994. Since June 1994, he has been an Assistant Professor of Computer Science at Mills College in Oakland, California. From 1987 to 1989, he was a junior researcher at the Korea Telecommunication Authority Research Center. His research interests include distributed computing and operating systems. James H. Anderson received the M. S. degree in Computer Science from Michigan State University in 1982, the M.S. degree in Computer Science from Purdue University in 1983, and the Ph.D. degree in Computer Sciences from the University of Texas at Austin in 1990. Since August 1993, he has been an Assistant Professor of Computer Science at the University of North Carolina at Chapel Hill. Prior to joining the University of North Carolina, he was an Assistant Professor of Computer Science for three years at the University of Maryland at College Park Professor Anderson's main research interests are within the area of coneurrent and distributed computing. His current interests include wait-free algorithms, scalabde synchronization mechanisms for shared-memory systems, and object-sharing strategies for hard real-time applications.Preliminary version was presented at the Twelfth Annual ACM Symposium on Principles of Distributed Computing Ithaca, New York, August 1993 [15]. Work supported, in part, by NSF Contracts CCR-9109497 and CCR-9216421 and by the Center for Excellence in Space Data and Information Sciences (CESDIS)     

Naming symmetric processes using shared variables

Summary Implementations of inter-process communication and synchronization in distributed systems usually rely on the existence of unique ids for the processes. We consider the problem of generating such ids for identical processes in a shared-variable system. A randomized protocol that assigns distinct ids to the processes within an expected polynomial number of rounds using a polynomial number of boolean atomic variables is presented. Ömer Eeciolu obtained his Ph.D. degree in mathematics from the University of California, San Diego in 1984. At present he is an Associate Professor in the Computer Science department of the University of California, Santa Barbara, where he has been on the faculty since 1985. His principal areas of research are parallel algorithms, bijective and enumerative combinatorics, and combinatorial algorithms. His current interest in parallel algorithms involve approximation and numerical techniques on distributed memory systems while his combinatorial interests center around computational geometry, bijective methods, and ranking algorithms for combinatorial structures. Ambuj K. Singh is an Assistant Professor in the Department of Computer Science at the University of California, Santa Barbara. He received a Ph.D. in Computer Science from the University of Texas at Austin in 1989, an M.S. in Computer Science from Iowa State University in 1984, and a B. Tech. from the Indian Institute of Technology at Kharagpur in 1982. His research interests are in the areas of adaptive resource allocation, concurrent program development, and distributed shared memory.Work supported in part by NSF grants CCR-9008628 and CCR-9223094     

Curve Matching,Time Warping,and Light Fields: New Algorithms for Computing Similarity between Curves

The problem of curve matching appears in many application domains, like time series analysis, shape matching, speech recognition, and signature verification, among others. Curve matching has been studied extensively by computational geometers, and many measures of similarity have been examined, among them being the Fréchet distance (sometimes referred in folklore as the “dog-man” distance). A measure that is very closely related to the Fréchet distance but has never been studied in a geometric context is the Dynamic Time Warping measure (DTW), first used in the context of speech recognition. This measure is ubiquitous across different domains, a surprising fact because notions of similarity usually vary significantly depending on the application. However, this measure suffers from some drawbacks, most importantly the fact that it is defined between sequences of points rather than curves. Thus, the way in which a curve is sampled to yield such a sequence can dramatically affect the quality of the result. Some attempts have been made to generalize the DTW to continuous domains, but the resulting algorithms have exponential complexity. In this paper we propose similarity measures that attempt to capture the “spirit” of dynamic time warping while being defined over continuous domains, and present efficient algorithms for computing them. Our formulation leads to a very interesting connection with finding short paths in a combinatorial manifold defined on the input chains, and in a deeper sense relates to the way light travels in a medium of variable refractivity. Alon Efrat earned his Bachelor in Applied Mathematics from the Technion, Israel’s Institute of Technology in 1991, earned his Master in Computer Science from the Technion in 1993, and his Ph.D. in Computer Science from Tel-Aviv University in 1998. During the years 1998–2000 he was a Post Doctorate Research Assistant at the Computer Science Department at Stanford University, and at IBM Almaden Research Center. Since 2000, he is an assistant Professor at the Computer Science Department at the University of Arizona. His main research areas are Computational Geometry, and its applications. Quanfu Fan is a Ph.D. student in the department of Computer Science at the University of Arizona. His research interests include object recognition and image understanding, information retrieval, and geometric algorithms.