37610017
Validated Cost Models for Parallel OQL Query Processing
Query cost models are widely used, both for performance analysis and for comparing execution plans during query optimisation. In essence, a cost modelp redicts where time is being spent during query evaluation. Although many cost models have been proposed, for serial, parallel and distributed database systems, surprisingly few of these have been validated against real systems. This paper presents cost models for the parallel evaluation of ODMG OQL queries, which have been compared with experimental results obtained using the Polar object database system. The paper describes the validation of the cost model for a collection of queries, using three join algorithms over the OO7 benchmark database. The results show that the cost model generally both ranks alternative plans appropriately, and gives a useful indication of the response times that can be expected from a plan. The paper also illustrates the application of the cost model by highlighting the contributions of different features and operations to query response times.
40619191
The Design, Implementation and Evaluation of an ODMG Compliant, Parallel Object Database Server
This paper describes the design, implementation and evaluation of a parallel object database server. While a number of research groups and companies now provide object database servers designed to run on uniprocessors, there has been surprisingly little work on the exploitation of parallelism to provide scalable performance in Object Database Management Systems (ODBMS). The work described in this paper takes as its starting-point the Object Database Management Group (ODMG) standard for object databases, thereby allowing the project to focus on research into parallelism, rather than on the ODBMS interfaces. The system is designed to run on a distributed memory parallel machine, and the paper describes the key issues and design decisions including: parallel query optimisation and execution, flow control, support for user-defined operations in queries, object distribution, cache management and navigational client access. The work shows that the significant differences between the object and relational database paradigms lead to significant differences in the designs of parallel servers to support these two paradigms. The paper presents an extensive performance analysis of the prototype systems which shows that good performance can be achieved on a cluster of linux PCs.
5328327
Fault-tolerant parallel applications using queues and actions
There are many techniques supporting execution of large computations over a network of workstations (NOW) but data intensive computations are usually run on high performance parallel machines. A NOW comprising individual user's machines typically has a low performance interconnect and suffers arbitrary changes of availability. Exploiting such resources to execute data intensive computations is difficult but even in a more constrained environment there is an unfulfilled need for fault-tolerance. The structuring approach presented fulfills this need. Performance exceeding 100 Mflop/s is demonstrated for large fault-tolerant out of core examples of matrix multiplication and Cholesky factorisation using five 133 MHz Pentium compute machines.
8773742
On Synchronisation in Fault-Tolerant Data and Compute Intensive Programs over a Network of Workstations
An application structured as a fault-tolerant bag of tasks adapts easily to changing resources. To be represented by a single bag of tasks, a computation must decompose into purely independent tasks. The work summarised here investigates performance of structuring approaches applicable where this ideal is not possible, partly through analysis and partly through measurements of a realistic fault-tolerant computation.
16057397
Measuring and modelling the performance of a parallel ODMG compliant object database server
Object database management systems (ODBMSs) are now established as the database management technology of choice for a range of challenging data intensive applications. Furthermore, the applications associated with object databases typically have stringent performance requirements, and some are associated with very large data sets. An important feature for the performance of object databases is the speed at which relationships can be explored. In queries, this depends on the effectiveness of different join algorithms into which queries that follow relationships can be compiled. This paper presents a performance evaluation of the Polar parallel object database system, focusing in particular on the performance of parallel join algorithms. Polar is a parallel, shared-nothing implementation of the Object Database Management Group (ODMG) standard for object databases. The paper presents an empirical evaluation of queries expressed in the ODMG Query Language (OQL), as well as a cost model for the parallel algebra that is used to evaluate OQL queries. The cost model is validated against the empirical results for a collection of queries using four different join algorithms, one that is value based and three that are pointer based. Copyright © 2005 John Wiley & Sons, Ltd.
13121681
A parallel algebra for object databases
The paper describes an algebra for use with parallel object databases, and in particular ODMG compliant databases with OQL. Although there have been many proposals for parallel relational database systems, there has been much less work on parallel object databases, and on parallel query processing for object databases. The parallel algebra presented in the paper is an extension of an existing physical algebra for OQL, and has an important role during query optimization, and for describing execution plans. The paper presents not only the algebra, but also its role in the architecture of a parallel database.
10989722
Practical Problems with a Cryptographic Protection Scheme
Z is a software system designed to provide media-transparent net- work services on a collection of UNIX® machines. These services are comprised of file transfer and command execution; Z preserves file own- ership on remote transfer, and more significantly, owner and group iden- tity when executing commands remotely. In order to secure known vulner- abilities in the system, enhancements were made. In particular, a cryptographically-derived checksum was added to the messages. After the initial implementation of the checksumming scheme, several iterations of performance improvement occurred. The result was unsatisfactory to the user community, so the checksum was removed. Instead, vulnerabilities were reduced by improved monitoring and maintenance procedures.
3042095
Measuring the Robustness of Resource Allocations in a Stochastic Dynamic Environment
Heterogeneous distributed computing systems often must operate in an environment where system parameters are subject to uncertainty. Robustness can be defined as the degree to which a system can function correctly in the presence of parameter values different from those assumed. We present a methodology for quantifying the robustness of resource allocations in a dynamic environment where task execution times are stochastic. The methodology is evaluated through measuring the robustness of three different resource allocation heuristics within the context of a stochastic dynamic environment. A Bayesian regression model is fit to the combined results of the three heuristics to demonstrate the correlation between the stochastic robustness metric and the presented performance metric. The correlation results demonstrated the significant potential of the stochastic robustness metric to predict the relative performance of the three heuristics given a common objective function.
18578929
Joint adaptive space and frequency basis selection
We develop a new method for building a representation of an image from a library of basis elements that is facilitated by a joint adaptive space and frequency (JASF) graph. The JASF graph combines partitionable frequency expansion and spatial segmentation of the image, symmetrically. We demonstrate by using a rate-distortion framework for basis selection that the JASF graph improves compression performance over recent wavelet packet and double-tree methods by offering exponentially more bases in which to represent the images.
11001529
A Stochastic Approach to Measuring the Robustness of Resource Allocations in Distributed Systems
Often, parallel and distributed computing systems must operate in an environment replete with uncertainty. Determining a resource allocation that accounts for this uncertainty in a way that can provide a probabilistic guarantee that a given level of quality of service (QoS) is achieved is an important research problem. This paper defines a stochastic methodology for quantifiably determining a resource allocation's ability to satisfy QoS constraints in the midst of uncertainty in system parameters. Uncertainty in system parameters and its impact on system performance are modeled stochastically. This stochastic model is then used to derive a quantitative expression for the robustness of a resource allocation. The paper investigates the utility of the proposed stochastic robustness metric by applying the metric to resource allocations in a simulated distributed system. The simulation results are then compared with deterministically defined metrics from the literature
16664439
An alternative approach for the evaluation of the neocognitron
The neocognitron network is analysed from the point of view of the contribution of the di erent layers to the nal classi cation. A variation to the neocognitron which gives improved performance is suggested. This variant combines the low level feature extraction capabilities of the initial layers with alternative classi ers such as LVQ and Class Based Means Clustering. This is shown to give performance which is superior to the either of those classi ers acting on their own, and to the neocognitron in its standard form on two di erent instances of the letter recognition problem.