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Articles 1021 - 1050 of 1102
Full-Text Articles in Computer Sciences
A Visualization System For Correctness Proofs Of Graph Algorithms, Peter A. Gloor, Donald B. Johnson, Fillia Makedon, Panagiotis Metaxas
A Visualization System For Correctness Proofs Of Graph Algorithms, Peter A. Gloor, Donald B. Johnson, Fillia Makedon, Panagiotis Metaxas
Computer Science Technical Reports
In this paper we describe a system for visualizing correctness proofs of graph algorithms. The system has been demonstrated for a greedy algorithm. Prim's algorithm for finding a minimum spanning tree of an undirected, weighted graph. We believe that our system is particularly appropriate for greedy algorithms, though much of what we discuss can guide visualization of proofs in other contexts. While an example is not a proof, our system provides concrete examples to illustrate the operation of the algorithm. These examples can be referred to by the user interactively and alternatively with the visualization of the proof where the …
Multiprocessor File System Interfaces, David Kotz
Multiprocessor File System Interfaces, David Kotz
Computer Science Technical Reports
Increasingly, file systems for multiprocessors are designed with parallel access to multiple disks, to keep I/O from becoming a serious bottleneck for parallel applications. Although file system software can transparently provide high-performance access to parallel disks, a new file system interface is needed to facilitate parallel access to a file from a parallel application. We describe the difficulties faced when using the conventional (Unix-like) interface in parallel applications, and then outline ways to extend the conventional interface to provide convenient access to the file for parallel programs, while retaining the traditional interface for programs that have no need for explicitly …
Multiprocessor File System Interfaces, David Kotz
Multiprocessor File System Interfaces, David Kotz
Dartmouth Scholarship
Increasingly, file systems for multiprocessors are designed with parallel access to multiple disks, to keep I/O from becoming a serious bottleneck for parallel applications. Although file system software can transparently provide high-performance access to parallel disks, a new file system interface is needed to facilitate parallel access to a file from a parallel application. We describe the difficulties faced when using the conventional (Unix-like) interface in parallel applications, and then outline ways to extend the conventional interface to provide convenient access to the file for parallel programs, while retaining the traditional interface for programs that have no need for explicitly …
Spede: A Simple Programming Environment For Distributed Execution (Users' Manual), James Gochee
Spede: A Simple Programming Environment For Distributed Execution (Users' Manual), James Gochee
Dartmouth College Undergraduate Theses
Traditional single processor computers are quickly reaching their full computational potentials. The quest for faster and faster chips have brought technology to the point where the laws of physics are hampering future gains. Significant gains in speed must therefore come from using multiple processors instead of a single processor. This technology usually represents itself in the form of a parallel computer, such as the Connection Machine Model 5. Recently however, much interest has been focused on software that organizes single processor computers to behave like a parallel computer. This is desirable for sites which have large installations of workstations, since …
Spede: Simple Programming Environment For Distributed Execution, James Gochee
Spede: Simple Programming Environment For Distributed Execution, James Gochee
Dartmouth College Undergraduate Theses
One of the main goals for people who use computer systems, particularly computational scientists, is speed. In the quest for ways to make applications run faster, engineers have developed parallel computers, which use more than one CPU to solve a task. However, many institutions already posses significant computational power in networks of workstations. Through software, it is possible to glue together clusters of machines to simulate a parallel environment. SPEDE is one such system, designed to place the potential of local machines at the fingertips of the programmer. Through a simple interface, users design computational objects that can be linked …
How To Encrypt /Usr/Dict/Words In About A Second, Peter Su, Matt Bishop
How To Encrypt /Usr/Dict/Words In About A Second, Peter Su, Matt Bishop
Computer Science Technical Reports
We present an implementation of the Data Encryption Standard on the Connection Machine architecture. The DES encryption algorithm is ideally suited to the Connection Machine because it consists of bit serial operations, and thousands of encryptions can be done in parallel, independently of one another. Thus, our code encrypts passwords about ten times faster than the fastest competition that we know about. In addition, the nature of the Connection Machine's architecture is such that some of the optimizations that make DES run much faster on conventional architectures have no effect on the performance of the Connection Machine. Our comparison of …
Concurrent Local Search For Fast Proximity Algorithms On Parallel And Vector Architectures, Peter Su
Concurrent Local Search For Fast Proximity Algorithms On Parallel And Vector Architectures, Peter Su
Computer Science Technical Reports
This paper presents a fast algorithm for solving the all-nearest-neighbors problem. The algorithm uses a data parallel style of programming which can be efficiently utilized on a variety of parallel and vector architectures [4,21,26]. I have implemented the algorithm in C on one such architecture, the Cray Y-MP. On one Cray CPU, the implementation is about 19 times faster than a fast sequential algorithm running on a Sparc workstation. The main idea in the algorithm is to divide the plane up into a fixed grid of cells, or buckets. When the points are well distributed, the algorithm processes each query …
On The De Bruijn Torus Problem, Glenn Hurlbert, Garth Isaak
On The De Bruijn Torus Problem, Glenn Hurlbert, Garth Isaak
Computer Science Technical Reports
A (kn;n)k-de Bruijn Cycle is a cyclic k-ary sequence with the property that every k-ary n-tuple appears exactly once contiguously on the cycle. A (kr, ks; m, n)k-de Bruijn Torus is a k-ary krXks toroidal array with the property that every k-ary m x n matrix appears exactly once contiguously on the torus. As is the case with de Bruijn cycles, the 2-dimensional version has many interesting applications, from coding and communications to pseudo-random arrays, spectral imaging, and robot self-location. J.C. Cock proved the existence of such tori for all m, n, and k, and Chung, Diaconis, and Graham asked …
Optimal Algorithms For Multipacket Routing Problems On Rings, Fillia Makedon, Antonios Symvonis
Optimal Algorithms For Multipacket Routing Problems On Rings, Fillia Makedon, Antonios Symvonis
Computer Science Technical Reports
We study multipacket routing problems. We divide the multipacket routing problem into two classes, namely, distance limited and bisection limited routing problems. Then, we concentrate on rings of processors. We prove a new lower bound of 2n/ 3 routing steps for the case of distance limited routing problems. We also give an algorithm that tightens this lower bound. For bisection limited problems the lower bound is kn/ 4,k >2, where k is the number of packets per processor. The trivial algorithm needs in the worst case k | n /2| steps to terminate. An algorithm that completes the routing in …
Practical Prefetching Techniques For Parallel File Systems, David Kotz, Carla Schlatter Ellis
Practical Prefetching Techniques For Parallel File Systems, David Kotz, Carla Schlatter Ellis
Dartmouth Scholarship
Parallel disk subsystems have been proposed as one way to close the gap between processor and disk speeds. In a previous paper we showed that prefetching and caching have the potential to deliver the performance benefits of parallel file systems to parallel applications. In this paper we describe experiments with practical prefetching policies, and show that prefetching can be implemented efficiently even for the more complex parallel file access patterns. We test these policies across a range of architectural parameters.
Caching And Writeback Policies In Parallel File Systems, David Kotz, Carla Schlatter Ellis
Caching And Writeback Policies In Parallel File Systems, David Kotz, Carla Schlatter Ellis
Dartmouth Scholarship
Improvements in the processing speed of multiprocessors are outpacing improvements in the speed of disk hardware. Parallel disk I/O subsystems have been proposed as one way to close the gap between processor and disk speeds. Such parallel disk systems require parallel file system software to avoid performance-limiting bottlenecks. We discuss cache management techniques that can be used in a parallel file system implementation. We examine several writeback policies, and give results of experiments that test their performance.
Effects Of Replication On The Duration Of Failure In Distributed Databases, Donald B. Johnson, Larry Raab
Effects Of Replication On The Duration Of Failure In Distributed Databases, Donald B. Johnson, Larry Raab
Computer Science Technical Reports
Replicating data objects has been suggested as a means of increasing the performance of a distributed database system in a network subject to link and site failures. Since a network may partition as a consequence of such failures, a data object may become unavailable from a given site for some period of time. In this paper we study duration failure, which we define as the length of time, once the object becomes unavailable from a particular site, that the object remains unavailable. We show that, for networks composed of highly-reliable components, replication does not substantially reduce the duration of failure. …
Availability Issues In Data Replication In Distributed Database, Donald B. Johnson, Larry Raab
Availability Issues In Data Replication In Distributed Database, Donald B. Johnson, Larry Raab
Computer Science Technical Reports
Replication of data at more than one site in a distributed database has been reported to increase the availability in data in systems where sites and links are subject to failure. We have shown in results summarized in this paper that in many interesting cases the advantage is slight. A well-placed single copy is available to transactions almost as much of the time as is correct replicated data no matter how ingeniously it is managed. We explain these findings in terms of the behavior of the partitions that form in networks where components fail. We also show that known and …
Complexity Of Network Reliability And Optimal Database Placement Problems, Donald B. Johnson, Larry Raab
Complexity Of Network Reliability And Optimal Database Placement Problems, Donald B. Johnson, Larry Raab
Computer Science Technical Reports
A fundamental problem of distributed database design in an existing network where components can fail is finding an optimal location at which to place the database in a centralized system or copies of each data item in a decentralized or replicated system. In this paper it is proved for the first time exactly how hard this placement problem is under the measure of data availability. Specifically, we show that the optimal placement problem for availability is #P- complete, a measure of intractability at least as severe as NP-completeness. Given the anticipated computational difficulty of finding an exact solution, we go …
Ilona: An Advanced Cai Tutorial System For The Fundamentals Of Logic, Otto Mayer, Graham E. Oberem, Fillia Makedon
Ilona: An Advanced Cai Tutorial System For The Fundamentals Of Logic, Otto Mayer, Graham E. Oberem, Fillia Makedon
Computer Science Technical Reports
An advanced tutorial system for teaching the fundamentals of logic has been developed to run on UNIX work stations and commonly available micro-computers. An important part of this tutorial is the intelligent problem solving environment which allows students to practise wiriting logical sentences in mathematical notation. A natural language system for intelligent logic narrative analysis (ILONA) allows students to type in their own logical sentences in plain English and then have the computer check their working when they write these in mathematical form. ILONA is an intelligent tutoring system which allows students a great deal of initiative in problem solving …
Prefetching And Caching Techniques In File Systems For Mimd Multiprocessors, David F. Kotz
Prefetching And Caching Techniques In File Systems For Mimd Multiprocessors, David F. Kotz
Dartmouth Scholarship
The increasing speed of the most powerful computers, especially multiprocessors, makes it difficult to provide sufficient I/O bandwidth to keep them running at full speed for the largest problems. Trends show that the difference in the speed of disk hardware and the speed of processors is increasing, with I/O severely limiting the performance of otherwise fast machines. This widening access-time gap is known as the “I/O bottleneck crisis.” One solution to the crisis, suggested by many researchers, is to use many disks in parallel to increase the overall bandwidth. \par This dissertation studies some of the file system issues needed …
An Object-Oriented Learning/Design Support Environment, Fillia Makedon, Julie C. Jumes, Jill P. David
An Object-Oriented Learning/Design Support Environment, Fillia Makedon, Julie C. Jumes, Jill P. David
Computer Science Technical Reports
We present an object-oriented experimental learning and design support environment, call AVT, for an Algorithm Visualization Tool, implemented in Digitalk's Smalltalk/V1 on a Macintosh II2, AVT provides a domain- independent visualization tool, an exploratory learning environment, and an experimental heuristic design environment. Algorithm visualization is the exploration of ways to visualize intuitively the computational behavior of an algorithm using multiple views, some of which are visual in the graphical sense [2,4]. AVT employs other views (combining text and graphics) to explain the problem, the strategy, the heuristics, and the reasoning process behind the solutions. User interaction in AVT includes not …
A Metric Towards Efficient Exhaustive Test Pattern Generation, Dimitrios Kagaris, Fillia Makedon, Spyros Tragoudas
A Metric Towards Efficient Exhaustive Test Pattern Generation, Dimitrios Kagaris, Fillia Makedon, Spyros Tragoudas
Computer Science Technical Reports
A viable technique [7] in built-in self-test (BIST)[2] is to generate test patterns pseudo-exhaustively by using linear feedback shift registers (LFSR's). The goal is to find an appropriate primitive polynomial of degree d that will generat 2d test patterns in order to exercise all circuit outputs simultaneously. In an attempt to reduce the degree d of the polynomial the following strategy was proposed in [6,5]. In the first phase, partition the circuit into segments by inserting a small number of register cells, so that the input dependency of any circuit element in the segments is no more than d. Then, …
On Minimizing Hardware Overhead For Exhaustive Circuit Testability, Dimitrios Kagaris, Fillia Makedon, Spyros Tragoudas
On Minimizing Hardware Overhead For Exhaustive Circuit Testability, Dimitrios Kagaris, Fillia Makedon, Spyros Tragoudas
Computer Science Technical Reports
Exhaustive built-in self testing is given much attention as a viable technique in the context of VLSI technology. In this paper, we present heuristic in order to make exhaustive testing of combinational circuits practical. The goal is to place a small number of register cells on the nets of the input circuit so that the input dependency of combinational elements in the circuit is less than a small given integer k. Our heuristic guarantees that each output can be individually tested with 2k test patterns and can be used as a subroutine to generat efficient test patterns to test all …
Privacy-Enhanced Electronic Mail, Matt Bishop
Privacy-Enhanced Electronic Mail, Matt Bishop
Computer Science Technical Reports
The security of electronic mail sent through the Internet may be described in exactly three words: there is none. The Privacy and Security Research Group has recommended implementing mechanisms designed to provide security enhancements. The first set of mechanisms provides a protocol to provide privacy, integrity, and authentication for electronic mail; the second provides a certificate-based key management infrastructure to support key distribution throughout the internet, to support the first set of mechanisms. This paper describes these mechanisms, as well as the reasons behind their selection and how these mechanisms can be used to provide some measure of securtiy in …
Implementation Notes On Bdes(1), Matt Bishop
Implementation Notes On Bdes(1), Matt Bishop
Computer Science Technical Reports
This note describes the implementation of bdes, the file encryption program being distributed in the 4.4 release of the Berkeley Software Distribution. It implements all modes of the Data Encryption Standard program.
Connected Components In O(Lg3/2|V|) Parallel Time For The Crew Pram, Donald B. Johnson, Panagiotis Metaxas
Connected Components In O(Lg3/2|V|) Parallel Time For The Crew Pram, Donald B. Johnson, Panagiotis Metaxas
Computer Science Technical Reports
Computing the connected components of an undirected graph G = (V,E) on |V| = n vertices and |E| = m edges is a fundamental computational problem. The best known parallel algorithm for the CREW PRAM model runs on O(lg2n) time using n2/lg2n processors [CLC82,HCS79]. For the CRCW PRAM model in which concurrent writing is permitted, the best known algorithm runs in O(lg n) time using almost (n+m)/lg n processors [SV82,CV86,AS87]. Unfortunately, simulating this algorithm on the weaker CREW model increases its running time to O(lg2n) [CDR86, KR90,Vis83]. We present here an efficient and simple algorithm that runs in O(lg 3/2n) …
Optimal Parallel And Sequential Algorithms For The Vertex Updating Problem Of A Minimum Spanning Tree, Donald B. Johnson, Panagiotis Metaxas
Optimal Parallel And Sequential Algorithms For The Vertex Updating Problem Of A Minimum Spanning Tree, Donald B. Johnson, Panagiotis Metaxas
Computer Science Technical Reports
We present a set of rules that can be used to give optimal solutions to the vertex updating problem for a minimum spanning tree: Update a given MST when a new vertex z is introducted, along with weighted edges that connect z with the vertices of the graph. These rules lead to simple parallel algorithms that run in O(lg n) parallel time using n/lg n EREW PRAMs. They can also be used to derive simple linear-time sequential algorithms for the same problem. Furthermore, we show how our solution can be used to solve the multiple vertex updating problem.
Multipacket Routing On Rings, Fillia Makedon, Adonios Simvonis
Multipacket Routing On Rings, Fillia Makedon, Adonios Simvonis
Computer Science Technical Reports
We study multipacket routing problems. We divide the multipacket routing problem into two classes, namely, distance limited and bisection limited routing problems. Then, we concentrate on rings of processors. Having a full understanding of the multipacket routing problem on rings is essential before trying to attack the problem for the more general case of r-dimensional meshes and tori. We prove a new lower bound of 2n/3 routing steps for the case of distance limited routing problems. We also give an algorithm that tightens this lower bound. For bisection limited problems, we present an algorithm that completes the routing in near …
A Parallel Algorithm For The Minimum Spanning Tree, Donald B. Johnson, Panagiotis Metaxas
A Parallel Algorithm For The Minimum Spanning Tree, Donald B. Johnson, Panagiotis Metaxas
Computer Science Technical Reports
No abstract provided.
A Security Analysis Of Version 2 Of The Network Time Protocol Ntp: A Report To The Privacy And Security Research Group, Matt Bishop
Computer Science Technical Reports
The Network Time Protocol is being used throughout the Internet to provide an accurate time service. This paper examines the security requirements of such a service, analyzes version 2 of the NTP protocol to determine how well it meets these requirements, and suggests improvements where appropriate.
An Overview Of Computer Viruses In A Research Environment, Matt Bishop
An Overview Of Computer Viruses In A Research Environment, Matt Bishop
Computer Science Technical Reports
The threat of attack by computer viruses is in reality a very small part of a much more general threat, specifically attacks aimed at subverting computer security. This paper examines computer viruses as malicious logic in a research and development environment, relates them to various models of security and integrity, and examines current research techniques aimed at controlling the threats viruses in particular, and malicious logic in gerneral, pose to computer systems. Finally, a brief examination of the vulnerabilities of research and development systems that malicious logic and computer viruses may exploit is undertaken.
A Tight Upper Bound On The Benefits Of Replication And Consistency Control Protocols, Donald B. Johnson, Larry Raab
A Tight Upper Bound On The Benefits Of Replication And Consistency Control Protocols, Donald B. Johnson, Larry Raab
Computer Science Technical Reports
We present an upper bound on the performance provided by a protocol guaranteeing mutually exclusive access to a replicated resource in a network subject to component failure and subsequent partitioning. The bound is presented in terms of the performance of a single resource in the same network. The bound is tight and is the first such bound known to us. Since mutual exclusion is one of the requirements for maintaining the consistency of a database object, this bound provides an upper limit on the availability provided by any database consistency control protocol, including those employing dynamic data relocation and replication. …
The Duke Internet Programming Contest, Owen Astrachan, Vivek Khera, David Kotz
The Duke Internet Programming Contest, Owen Astrachan, Vivek Khera, David Kotz
Dartmouth Scholarship
On the evening of October 23, 1990, electronic mail messages started to pour into the computers at the Duke University Computer Science Department. Teams of programmers from all over the world were registering to compete in the first global (as far as the authors are aware) programming contest to be held on the Internet. During the three hour competition, modeled after the annual ACM scholastic programming contest, 60 teams from 37 institutions in 5 countries attempted to solve a set of six programming problems using C or Pascal. Their solutions were sent by electronic mail to Duke, where their programs …
A Bound Of Data Availability When Networks Partition, Michael Goldweber, Donald B. Johnson
A Bound Of Data Availability When Networks Partition, Michael Goldweber, Donald B. Johnson
Computer Science Technical Reports
Many consistency or replication control schemes that increase data availability in distributed systems exist, and the search for improvements continues, though there have been no good nontrivial upper bound demonstrating how much improvement is possible. We present a new upper bound for data availability under replication for general networks. In addition we also describe a new technique that yields near optimal levels of data availability with respect to this bound.