Physical Sciences and Mathematics Commons^™

Absence Of Open Strings In A Lattice-Free Simulation Of Cosmic String Formation, Julian Borrill

Dartmouth Scholarship

Lattice-based string formation algorithms can, at least in principle, be reduced to the study of the statistics of the corresponding aperiodic random walk. Since in three or more dimensions such walks are transient, this approach necessarily generates a population of open strings. To investigate whether open strings are an artifact of the lattice we develop an alternative lattice-free simulation of string formation. Replacing the lattice with a graph generated by a minimal dynamical model of a first-order phase transition we obtain results consistent with the hypothesis that the energy density in string is due to a scale-invariant Brownian distribution of …

An Optical And X-Ray Study Of Abell 576, A Galaxy Cluster With A Cold Core, Joseph J. S. Mohr, Margaret J. Geller, Daniel G. Fabricant, Gary Wegner, John Thorstensen, Douglas O. Richstone

Dartmouth Scholarship

We analyze the galaxy population and dynamics of the galaxy cluster A576; the observational constraints include 281 redshifts (230 new), R- band CCD galaxy photometry over a 2 h^-1^ Mpc x 2 h^-1^ Mpc region centered on the cluster, an Einstein IPC X-ray image, and an Einstein MPC X-ray spectrum. We focus on an 86% complete magnitude-limited sample (R_23.5_ < 17) of 169 cluster galaxies. The cluster galaxies with emission lines in their spectra have a larger velocity dispersion and are significantly less clustered on this 2 h^-1^ Mpc scale than galaxies without emission lines. We show that excluding the emission-line galaxies from the cluster sample decreases the velocity dispersion by 18% and the virial mass estimate by a factor of 2. The central cluster region contains a nonemission galaxy population and an intracluster medium which is significantly cooler (σ_core_ = 387_-105_^+250^ km s^-1^ and T_x_ = 1.6_-0.3_^+0.4^ keV at 90% confidence) than the global populations (σ = 977_-96_^+124^ km s^- 1^ for the nonemission population and T_X_ > 4 keV at 90% confidence). Because (1) the low-dispersion galaxy population is no more luminous than the global population and (2) the evidence for a cooling flow is weak, we suggest that the core of A576 may contain the remnants of …

The Expected Lifetime Of Single-Address-Space Operating Systems, David Kotz, Preston Crow

Dartmouth Scholarship

Trends toward shared-memory programming paradigms, large (64-bit) address spaces, and memory-mapped files have led some to propose the use of a single virtual-address space, shared by all processes and processors. To simplify address-space management, some have claimed that a 64-bit address space is sufficiently large that there is no need to ever re-use addresses. Unfortunately, there has been no data to either support or refute these claims, or to aid in the design of appropriate address-space management policies. In this paper, we present the results of extensive kernel-level tracing of the workstations on our campus, and discuss the implications for …

Low-Degree Spanning Trees Of Small Weight, Samir Khuller, Balaji Raghavachari, Neal Young

Dartmouth Scholarship

Given n points in the plane, the degree-K spanning-tree problem asks for a spanning tree of minimum weight in which the degree of each vertex is at most K. This paper addresses the problem of computing low-weight degree-K spanning trees for $K > 2$. It is shown that for an arbitrary collection of n points in the plane, there exists a spanning tree of degree 3 whose weight is at most 1.5 times the weight of a minimum spanning tree. It is shown that there exists a spanning tree of degree 4 whose weight is at most 1.25 times …

File-Access Characteristics Of Parallel Scientific Workloads, Nils Nieuwejaar, David Kotz, Apratim Purakayastha, Carla Schlatter Ellis, Michael Best

Computer Science Technical Reports

Phenomenal improvements in the computational performance of multiprocessors have not been matched by comparable gains in I/O system performance. This imbalance has resulted in I/O becoming a significant bottleneck for many scientific applications. One key to overcoming this bottleneck is improving the performance of parallel file systems.

The design of a high-performance parallel file system requires a comprehensive understanding of the expected workload. Unfortunately, until recently, no general workload studies of parallel file systems have been conducted. The goal of the CHARISMA project was to remedy this problem by characterizing the behavior of several production workloads, on different machines, at …

A Queuing Analysis Of Bandwidth Allocation Schemes For Compressed Video, Saurab Nog, Carl J. Beckmann

Computer Science Technical Reports

Video and audio compression techniques allow continuous media streams to be transmitted at bit rates that are a function of the delivered quality of service. Digital networks will be increasingly used for the transmission of such continuous media streams. This paper describes an admission control policy in which the quality of service is negotiated at stream initiation, and is a function of both the desired quality of service and the available bandwidth resources. The advantage of this approach is the ability to robustly service large numbers of users, while providing increased quality of service during low usage periods. Several simple …

The Expected Lifetime Of "Single-Address-Space" Operating Systems, David Kotz, Preston Crow

Computer Science Technical Reports

Trends toward shared-memory programming paradigms, large (64-bit) address spaces, and memory-mapped files have led some to propose the use of a single virtual-address space, shared by all processes and processors. Typical proposals require the single address space to contain all process-private data, shared data, and stored files. To simplify management of an address space where stale pointers make it difficult to re-use addresses, some have claimed that a 64-bit address space is sufficiently large that there is no need to ever re-use addresses. Unfortunately, there has been no data to either support or refute these claims, or to aid in …

An Rpc Mechanism For Transportable Agents, Saurab Nog, Sumit Chawla, David Kotz

Computer Science Technical Reports

Transportable agents are autonomous programs that migrate from machine to machine, performing complex processing at each step to satisfy client requests. As part of their duties agents often need to communicate with other agents. We propose to use remote procedure call (RPC) along with a flexible interface definition language (IDL), to add structure to inter-agent communication. The real power of our Agent RPC comes from a client-server binding mechanism based on flexible IDL matching and from support for multiple simultaneous bindings. Our agents are programmed in Agent Tcl; we describe how the Tcl implementation made RPC particularly easy to implement. …

Flexibility And Performance Of Parallel File Systems, David Kotz, Nils Nieuwejaar

Dartmouth Scholarship

Many scientific applications for high-performance multiprocessors have tremendous I/O requirements. As a result, the I/O system is often the limiting factor of application performance. Several new parallel file systems have been developed in recent years, each promising better performance for some class of parallel applications. As we gain experience with parallel computing, and parallel file systems in particular, it becomes increasingly clear that a single solution does not suit all applications. For example, it appears to be impossible to find a single appropriate interface, caching policy, file structure, or disk management strategy. Furthermore, the proliferation of file-system interfaces and abstractions …

Fast Compression Of Transportable Tcl Scripts, Robert S. Gray

Computer Science Technical Reports

An information agent is charged with the task of searching a collection of electronic resources for information that is relevant to the user's current needs. These resources are often distributed across a network and can contain tremendous quantities of data. One of the paradigms that has been suggested for allowing efficient access to such resources is transportable agents -- the agent is sent to the machine that maintains the information resource; the agent executes on this remote machine and then returns its results to the local machine. We have implemented a transportable agent system that uses the Tool Command Language …

A Performance Comparison Of Tcp/Ip And Mpi On Fddi, Fast Ethernet, And Ethernet, Saurab Nog, David Kotz

Computer Science Technical Reports

Communication is a very important factor affecting distributed applications. Getting a close handle on network performance (both bandwidth and latency) is thus crucial to understanding overall application performance. We benchmarked some of the metrics of network performance using two sets of experiments, namely roundtrip and datahose. The tests were designed to measure a combination of network latency, bandwidth, and contention. We repeated the tests for two protocols (TCP/IP and MPI) and three networks (100 Mbit FDDI (Fiber Distributed Data Interface), 100 Mbit Fast Ethernet, and 10 Mbit Ethernet). The performance results provided interesting insights into the behaviour of these networks …

Transportable Information Agents, Robert Gray, Daniela Rus, David Kotz

Computer Science Technical Reports

We have designed and implemented autonomous software agents. Autonomous software agents navigate independently through a heterogeneous network. They are capable of sensing the network configuration, monitoring software conditions, and interacting with other agents. Autonomous agents are implemented as transportable programs, e.g., programs that are capable of suspending execution, moving to a different machine, and starting from where they left off. We illustrate the intelligent behavior of autonomous agents in the context of distributed information-gathering tasks.

Compositional Reasoning Is Not Possible In Determining The Solvability Of Consensus, Prasad Jayanti

Computer Science Technical Reports

Consensus, which requires processes with different input values to eventually agree on one of these values, is a fundamental problem in fault-tolerant computing. We study this problem in the context of asynchronous shared-memory systems. In our model, shared-memory consists of a sequence of cells and supports a specific set of operations. Prior research on consensus focussed on its solvability in shared-memories supporting specific operations. In this paper, we investigate the following general question: Let OP1 and OP2 be any two sets of operations such that each set includes read and write operations. Suppose there is no consensus protocol for N …

Early Spectra Of The Supernova 1987f, Gary Wegner, Steven R. Swanson

Dartmouth Scholarship

Spectroscopy is presented of the peculiar Type n supernova 1987F, sometimes called the 'Type I Seyfert imposter'. This supernova differed from others of Type n in the slow evolution in its light curve and the strengths and profiles of the spectral lines. The time covered is 1987 April 2 to June 8, when the spectrum of the supernova evolved from a simple nearly featureless blue continuum with superimposed narrow emission lines of H I, He I, [N II], [0 III], and [S II] to a complicated combination of broad and narrow emission components in H I and He I plus …

Introduction To Multiprocessor I/O Architecture, David Kotz

Dartmouth Scholarship

The computational performance of multiprocessors continues to improve by leaps and bounds, fueled in part by rapid improvements in processor and interconnection technology. I/O performance thus becomes ever more critical, to avoid becoming the bottleneck of system performance. In this paper we provide an introduction to I/O architectural issues in multiprocessors, with a focus on disk subsystems. While we discuss examples from actual architectures and provide pointers to interesting research in the literature, we do not attempt to provide a comprehensive survey. We concentrate on a study of the architectural design issues, and the effects of different design alternatives.

The Orbital Period Of Bk Lyncis (Pg 0917 + 342), F. A. Ringwald, J. R. Thorstensen, R. K. Honeycutt, J. W. Robertson

Dartmouth Scholarship

Long-term light curves of the cataclysmic variable BK Lyn = PG 0917 + 342 from the Indiana Automated CCD photometric telescope (‘RoboScope’) and the Harvard College Observatory plate archive reveal no dwarf nova outbursts. Two radial velocity studies show its orbital period to be 107.97 ;1 0.07 min, confirming that it does have an orbital period shorter than the period gap for cataclysmic variables. Whether this is the first nova-like variable below the period gap or a dwarf nova with rare outbursts resembling WZ Sge is still unclear, however.