11611563
Sequentiality and prefetching in database systems
Sequentiality of access is an inherent characteristic of many database systems. We use this observation to develop an algorithm which selectively prefetches data blocks ahead of the point of reference. The number of blocks prefetched is chosen by using the empirical run length distribution and conditioning on the observed number of sequential block references immediately preceding reference to the current block. The optimal number of blocks to prefetch is estimated as a function of a number of “costs,” including the cost of accessing a block not resident in the buffer (a miss), the cost of fetching additional data blocks at fault times, and the cost of fetching blocks that are never referenced. We estimate this latter cost, described as memory pollution, in two ways. We consider the treatment (in the replacement algorithm) of prefetched blocks, whether they are treated as referenced or not, and find that it makes very little difference. Trace data taken from an operational IMS database system is analyzed and the results are presented. We show how to determine optimal block sizes. We find that anticipatory fetching of data can lead to significant improvements in system operation.
12460438
Evaluation of cache consistency algorithm performance
As more computer systems turn to multiprocessing for improved performance, additional research is needed to evaluate and improve the performance of cache consistency protocols. In this study, we use trace-driven simulation to examine the performance of several consistency protocols, including some new adaptive protocols which have not been examined in prior research. This study uses a wider variety of traces than have been previously analyzed, including some production applications from a vector mini-supercomputer system, and presents a wider variety of analyses than have been previously presented for a given workload. We find that the sharing characteristics of application programs have a large bearing on the relative performance of the different protocols. Update-based protocols outperform invalidate-based protocols when accesses so shared data are highly interleaved among different processors (fine-grain sharing), while invalidate-based protocols are superior if one processor performs all accesses to shared data over long periods of time (coarse-grain sharing). Adaptive protocols provide the best overall performance across all applications; we present a new protocol called Update-Once, which yields the highest average performance. In even the best cases, however, estimated processor utilizations are unacceptably low due to the overhead to maintain consistent caches. To extract good performance from multiprocessor systems, existing application programs must be recoded to reduce sharing between processors.
14423694
Characteristics of I/O traffic in personal computer and server workloads
Understanding the characteristics of I/O traffic is increasingly important as the performance gap between the processor and disk-based storage continues to widen. Moreover, recent advances in technology, coupled with market demands, have led to new and exciting developments in storage systems, particularly network storage, storage utilities, and intelligent self-optimizing storage. In this paper, we empirically examine the physical I/O traffic of a wide range of server and personal computer (PC) workloads, focusing on how these workloads will be affected by the recent developments in storage systems. As part of our analysis, we compare our results with historical data and re-examine some rules of thumb (e.g., one bit of I/O per second for each instruction per second of processing power) that have been widely used for designing computer systems. We find that the I/O traffic is bursty and appears to exhibit self-similar characteristics. Our analysis also indicates that there is little cross-correlation between traffic volumes of server workloads, which suggests that aggregating these workloads will likely help to smooth out the traffic and enable more efficient utilization of resources. We discover that there is significant potential for harnessing "free" system resources to perform background tasks such as optimization of disk block layout. In general, we observe that the characteristics of the I/O traffic are relatively insensitive to the extent of upstream caching, and thus our results still apply, on a qualitative level, when the upstream cache is increased in size.
3191261
Multiprocessor memory organization and memory interference
ture of shared memory in a multiprocessor computer system is examined with particular attention to noninterleaved memory. Alternative memory organizations are compared and it is shown that a home memory organization, in which each processor is associated with one or more memories in which its address space is concentrated, is quite effective in reducing memory interference. Home memory organization is shown to be particularly suited to certain specialized computational problems as well as to possess advantages in terms of interference and reliability for general purpose computation. Results for interleaved memory are drawn from previous work and are used for comparison. Trace-driven simulations are used to verify the conclusions of the analysis.
11584462
The automatic improvement of locality in storage systems
Disk I/O is increasingly the performance bottleneck in computer systems despite rapidly increasing disk data transfer rates. In this article, we propose Automatic Locality-Improving Storage (ALIS), an introspective storage system that automatically reorganizes selected disk blocks based on the dynamic reference stream to increase effective storage performance. ALIS is based on the observations that sequential data fetch is far more efficient than random access, that improving seek distances produces only marginal performance improvements, and that the increasingly powerful processors and large memories in storage systems have ample capacity to reorganize the data layout and redirect the accesses so as to take advantage of rapid sequential data transfer. Using trace-driven simulation with a large set of real workloads, we demonstrate that ALIS considerably outperforms prior techniques, improving the average read performance by up to 50% for server workloads and by about 15% for personal computer workloads. We also show that the performance improvement persists as disk technology evolves. Since disk performance in practice is increasing by only about 8% per year, the benefit of ALIS may correspond to as much as several years of technological progress.
36713689
Performance Characterization of Optimizing Compilers
Optimizing compilers have become an essential component in achieving high levels of performance. Various simple and sophisticated optimizations are implemented at different stages of compilation to yield significant improvements, but little work has been done in characterizing the effectiveness of optimizers, or in understanding where most of this improvement comes from. We study the performance impact of optimization in the context of our methodology for CPU performance characterization based on the abstract machine model. The model considers all machines to be different implementations of the same high level language abstract machine; in previous research, the model has been used as a basis to analyze machine and benchmark performance. We show that our model can be extended to characterize the performance improvement provided by optimizers and to predict the run time of optimized programs, and measure the effectiveness of several compilers in implementing different optimization techniques. >
10194960
Secure Remote Authentication Using Biometric Data
Biometric data offer a potential source of high-entropy, secret information that can be used in cryptographic protocols provided two issues are addressed: (1) biometric data are not uniformly distributed; and (2) they are not exactly reproducible. Recent work, most notably that of Dodis, Reyzin, and Smith, has shown how these obstacles may be overcome by allowing some auxiliary public information to be reliably sent from a server to the human user. Subsequent work of Boyen has shown how to extend these techniques, in the random oracle model, to enable unidirectional authentication from the user to the server without the assumption of a reliable communication channel. 
 
We show two efficient techniques enabling the use of biometric data to achieve mutual authentication or authenticated key exchange over a completely insecure (i.e., adversarially controlled) channel. In addition to achieving stronger security guarantees than the work of Boyen, we improve upon his solution in a number of other respects: we tolerate a broader class of errors and, in one case, improve upon the parameters of his solution and give a proof of security in the standard model.
12877131
An Australasian Experience of the use of Selected Technologies in the Delivery of a Legal Education Program - Some Lessons for Faculties and Educational Program Planners
3332151
Blue screen matting
A classical problem of imaging—the matting problem—is separation of a non-rectangular foreground image from a (usually) rectangular background image—for example, in a film frame, extraction of an actor from a background scene to allow substitution of a different background. Of the several attacks on this difficult and persistent problem, we discuss here only the special case of separating a desired foreground image from a background of a constant, or almost constant, backing color. This backing color has often been blue, so the problem, and its solution, have been called blue screen matting. However, other backing colors, such as yellow or (increasingly) green, have also been used, so we often generalize to constant color matting. The mathematics of constant color matting is presented and proven to be unsolvable as generally practiced. This, of course, flies in the face of the fact that the technique is commonly used in film and video, so we demonstrate constraints on the general problem that lead to solutions, or at least significantly prune the search space of solutions. We shall also demonstrate that an algorithmic solution is possible by allowing the foreground object to be shot against two constant backing colors—in fact, against two completely arbitrary backings so long as they differ everywhere.
12035419
A Transition Probability Model for Amino Acid Substitutions from Blocks
Substitution matrices have been useful for sequence alignment and protein sequence comparisons. The BLOSUM series of matrices, which had been derived from a database of alignments of protein blocks, improved the accuracy of alignments previously obtained from the PAM-type matrices estimated from only closely related sequences. Although BLOSUM matrices are scoring matrices now widely used for protein sequence alignments, they do not describe an evolutionary model. BLOSUM matrices do not permit the estimation of the actual number of amino acid substitutions between sequences by correcting for multiple hits. The method presented here uses the Blocks database of protein alignments, along with the additivity of evolutionary distances, to approximate the amino acid substitution probabilities as a function of actual evolutionary distance. The PMB (Probability Matrix from Blocks) defines a new evolutionary model for protein evolution that can be used for evolutionary analyses of protein sequences. Our model is directly derived from, and thus compatible with, the BLOSUM matrices. The model has the additional advantage of being easily implemented.
6414828
Local Search Genetic Algorithm for Optimization of Highly Reliable Communications Networks
This paper presents a genetic algorithm (GA) with specialized encoding, initialization and local search genetic operators to optimize communication network topologies. This NPhard problem is often highly constrained so that random initialization and standard genetic operators usually generate infeasible network architectures. Compounding this infeasibility issue is that the fitness function involves calculating the all-terminal reliability of the network, a calculation which is computationally expensive. Therefore, it is imperative that the search balances the need to thoroughly explore the boundary between feasible and infeasible networks, along with calculating fitness on only the most promising candidate networks. The algorithm results are compared to optimum results found by branch and bound and also to GA results without local search operators on a suite of 79 test problems. This GA strategy of employing bounds, simple heuristic checks and problem specific repair and local search operators can be used on other highly constrained combinatorial applications where numerous fitness calculations are prohibitive.