The 𝑘-center problem requires the selection of 𝑘 points (centers) from a given metric pointset 𝑊 so to minimize the maximum distance of any point of 𝑊 from the closest center.This paper focuses on a fair variant of the problem, known as fair center, where each input point belongs to some category and each category may contribute a limited number of points to the center set.We present the first space-efficient streaming algorithm for fair center in general metrics, under the sliding window model.At any time 𝑡, the algorithm is able to provide a solution for the current window whose quality is almost as good as the one guaranteed by the best, polynomial-time sequential algorithms run on the entire window, and exhibits space and time requirements independent of the window size.Our theoretical results are backed by an extensive set of experiments on both real-world and synthetic datasets, which provide evidence of the significantly better performance/quality tradeoffs attained by our algorithm with respect to the those achievable by running the state-of-the-art sequential baselines on the entire window.

We present a novel technique for computing a center-based approximation of a drifting distribution. Given $k \geq 1$ and a stream of data, whose distribution is changing over time, the goal is to compute, at each step, the best $k$ centers representation of the current distribution, despite possibly having only a single sample from the most recent distribution. In data mining, this is traditionally attempted through the sliding-window mechanism, where the analysis is performed on the most recent fixed-size segment of the data. The problems with this approach are twofold: (1) setting the correct window size is challenging; and (2) a fixed window size cannot effectively track changes in the distribution happening at variable speed. In this paper, we propose a new methodology that dynamically adjusts the window size based on the recent drift of the data. The challenge is that it is not possible to explicitly estimate the drift, as we may have only a single data point from each distribution. Our main contribution lies in providing a rigorous mathematical analysis, establishing both an upper bound via a dynamic window size algorithm, and a lower bound that shows the tightness of our approach.

We present approximation algorithms for some variants of k -center clustering and diversity maximization in a fully dynamic setting, where the active pointset evolves through arbitrary insertions and deletions. All algorithms employ a coreset-based strategy and rely on the use of the cover tree data structure, which we crucially augment to maintain, at any time, some additional information enabling the efficient extraction of the solution for the specific problem. For all the problems under consideration, our algorithms compute \((\alpha+\varepsilon)\) -approximate solutions, where \(\alpha\) is the best-known approximation attainable in polynomial time in the standard static setting, and \(\varepsilon > 0\) is a user-provided accuracy parameter. Remarkably, and unlike previous works, the (cover tree) data structure used by our algorithms and the running times of the update procedures are both independent of the accuracy parameter \(\varepsilon\) and, for the k -center variants, also of parameter k . The analysis is performed in terms of the doubling dimension of the metric space which the points belong to, and it shows that, for spaces of bounded doubling dimension, the times required to extract solutions to the above problems are dramatically smaller than those that would be required to recompute solutions on the entire active pointset from scratch. To the best of our knowledge, ours are the first solutions for the matroid center and diversity maximization problems in the fully dynamic setting. The theoretical results are complemented by an extensive set of experiments, which demonstrate the efficiency and effectiveness of our algorithms for k -center without and with outliers against previously known ones.

We study the classic k-center clustering problem under the additional constraint that each cluster should be fair. In this setting, each point is marked with one or more colors, which can be used to model protected attributes (e.g., gender or ethnicity). A cluster is deemed fair if, for every color, the fraction of its points marked with that color is within some prespecified range. We present a coreset-based approach to fair k-center clustering for general metric spaces which attains almost the best approximation quality of the current state of the art solutions, while featuring running times which can be orders of magnitude faster for large datasets of low doubling dimension. We devise sequential, streaming and MapReduce implementations of our approach and conduct a thorough experimental analysis to provide evidence of their practicality, scalability, and effectiveness.

Clustering is a pivotal primitive for unsupervised learning and data analysis. A popular variant is the (k,ℓ)-clustering problem, where, given a pointset P from a metric space, one must determine a subset S of k centers minimizing the sum of the ℓ-th powers of the distances of points in P from their closest centers. This formulation covers the well-studied k-median (ℓ=1) and k-means (ℓ=2) clustering problems. A more general variant, introduced to deal with noisy pointsets, features a further parameter z and allows up to z points of P (outliers) to be disregarded when computing the sum. We present a distributed coreset-based 3-round approximation algorithm for the (k,ℓ)-clustering problem with z outliers, using MapReduce as a computational model. An important feature of our algorithm is that it obliviously adapts to the intrinsic complexity of the dataset, captured by its doubling dimension D. Remarkably, for D=O(1), our algorithm requires sublinear local memory per reducer, and yields a solution whose approximation ratio is an additive term O(γ) away from the one achievable by the best known sequential (possibly bicriteria) algorithm, where γ can be made arbitrarily small. To the best of our knowledge, no previous distributed approaches were able to attain similar quality-performance tradeoffs for metrics with constant doubling dimension.

An important step in cluster analysis is the evaluation of the quality of a given clustering through structural measures of goodness.Measures that do not require additional information for their evaluation (but the clustering itself), called internal measures, are commonly used because of their generality.The most widely used internal measure is the silhouette coefficient, whose naïve computation requires a quadratic number of distance calculations, unfeasible for massive datasets.Surprisingly, there are no known general methods to efficiently approximate the silhouette coefficient of a clustering with rigorously provable high accuracy.In this paper, we present the first scalable algorithm to compute such a rigorous approximation for the evaluation of clusterings based on any metric distances.Our algorithm approximates the silhouette coefficient within a mere additive error O (ε) with probability 1 -δ using a very small number of distance calculations, for any fixed ε, δ ∈ (0, 1).We also provide a distributed implementation of the algorithm using the MapReduce model, which runs in constant rounds and requires only sublinear local space at each worker, thus making our estimation approach applicable to big data scenarios.An extensive experimental evaluation provides evidence that our algorithm returns highly accurate silhouette estimates, unlike competing heuristics, while running in a fraction of the time required by the exact computation.

In this paper we present a novel streaming algorithm for the k-center clustering problem for general metric spaces under the sliding window model. The algorithm maintains a small coreset which, at any time, allows to compute a solution to the k-center problem on the current window with an approximation quality that can be made arbitrarily close to the best approximation attainable by a sequential algorithm running on the entire window. Remarkably, the size of our coreset is independent of the window size and can be upper bounded by a function of k, of the desired accuracy, and of the doubling dimension of the metric space induced by the stream. For streams of bounded doubling dimension, the coreset size is merely linear in k. One of the major strengths of our algorithm is that it is fully oblivious to the doubling dimension of the stream, and it adapts to the characteristics of each individual window. Also, unlike previous works, the algorithm can be made oblivious to the aspect ratio of the metric space, a parameter related to the spread of distances. We also provide experimental evidence of the practical viability of the approach and its superiority over the current state of the art.

Diversity maximization is a fundamental problem in web search and data mining. For a given dataset S of n elements, the problem requires to determine a subset of S containing k ≪ n “representatives” which maximize some diversity function expressed in terms of pairwise distances, where distance models dissimilarity. An important variant of the problem prescribes that the solution satisfy an additional orthogonal requirement, which can be specified as a matroid constraint (i.e., a feasible solution must be an independent set of size k of a given matroid). While unconstrained diversity maximization admits efficient coreset-based strategies for several diversity functions, known approaches dealing with the additional matroid constraint apply only to one diversity function (sum of distances), and are based on an expensive, inherently sequential, local search over the entire input dataset. We devise the first coreset-based algorithms for diversity maximization under matroid constraints for various diversity functions, together with efficient sequential, MapReduce, and Streaming implementations. Technically, our algorithms rely on the construction of a small coreset, that is, a subset of S containing a feasible solution which is no more than a factor 1−ɛ away from the optimal solution for S . While our algorithms are fully general, for the partition and transversal matroids, if ɛ is a constant in (0,1) and S has bounded doubling dimension, the coreset size is independent of n and it is small enough to afford the execution of a slow sequential algorithm to extract a final, accurate, solution in reasonable time. Extensive experiments show that our algorithms are accurate, fast, and scalable, and therefore they are capable of dealing with the large input instances typical of the big data scenario.

Center-based clustering is a fundamental primitive for data analysis and becomes very challenging for large datasets. In this paper, we focus on the popular k-median and k-means variants which, given a set P of points from a metric space and a parameter k<|P|, require to identify a set S of k centers minimizing, respectively, the sum of the distances and of the squared distances of all points in P from their closest centers. Our specific focus is on general metric spaces, for which it is reasonable to require that the centers belong to the input set (i.e., S subseteq P). We present coreset-based 3-round distributed approximation algorithms for the above problems using the MapReduce computational model. The algorithms are rather simple and obliviously adapt to the intrinsic complexity of the dataset, captured by the doubling dimension D of the metric space. Remarkably, the algorithms attain approximation ratios that can be made arbitrarily close to those achievable by the best known polynomial-time sequential approximations, and they are very space efficient for small D, requiring local memory sizes substantially sublinear in the input size. To the best of our knowledge, no previous distributed approaches were able to attain similar quality-performance guarantees in general metric spaces.

Center-based clustering is a fundamental primitive for data analysis and becomes very challenging for large datasets. In this paper, we focus on the popular k -center variant which, given a set S of points from some metric space and a parameter k < | S |, requires to identify a subset of k centers in S minimizing the maximum distance of any point of S from its closest center. A more general formulation, introduced to deal with noisy datasets, features a further parameter z and allows up to z points of S (outliers) to be disregarded when computing the maximum distance from the centers. We present coreset-based 2-round MapReduce algorithms for the above two formulations of the problem, and a 1-pass Streaming algorithm for the case with outliers. For any fixed ϵ > 0, the algorithms yield solutions whose approximation ratios are a mere additive term ϵ away from those achievable by the best known polynomial-time sequential algorithms, a result that substantially improves upon the state of the art. Our algorithms are rather simple and adapt to the intrinsic complexity of the dataset, captured by the doubling dimension D of the metric space. Specifically, our analysis shows that the algorithms become very space-efficient for the important case of small (constant) D . These theoretical results are complemented with a set of experiments on real-world and synthetic datasets of up to over a billion points, which show that our algorithms yield better quality solutions over the state of the art while featuring excellent scalability, and that they also lend themselves to sequential implementations much faster than existing ones.

Max-sum diversity is a fundamental primitive for web search and data mining. For a given set S of n elements, it returns a subset of k«l n representatives maximizing the sum of their pairwise distances, where distance models dissimilarity. An important variant of the primitive prescribes that the desired subset of representatives satisfies an additional orthogonal requirement, which can be specified as a matroid constraint (i.e., a feasible solution must be an independent set of size k). While unconstrained max-sum diversity admits efficient coreset-based strategies, the only known approaches dealing with the additional matroid constraint are inherently sequential and are based on an expensive local search over the entire input set. We devise the first coreset constructions for max-sum diversity under various matroid constraints, together with efficient sequential, MapReduce and Streaming implementations. By running the local-search on the coreset rather than on the entire input, we obtain the first practical solutions for large instances. Technically, our coresets are subsets of S containing a feasible solution which is no more than a factor 1-ε away from the optimal solution, for any fixed ε <1, and, for spaces of bounded doubling dimension, they have a small size independent of n. Extensive experiments show that, with respect to full-blown local search, our coreset-based approach yields solutions of comparable quality, with improvements of up to two orders of magnitude in the running time, also for input sets of unknown dimensionality.

An uncertain graph 𝒢 = (V, E, p : E → (0, 1]) can be viewed as a probability space whose outcomes (referred to as possible worlds ) are subgraphs of 𝒢 where any edge e ε E occurs with probability p ( e ), independently of the other edges. These graphs naturally arise in many application domains where data management systems are required to cope with uncertainty in interrelated data, such as computational biology, social network analysis, network reliability, and privacy enforcement, among the others. For this reason, it is important to devise fundamental querying and mining primitives for uncertain graphs. This paper contributes to this endeavor with the development of novel strategies for clustering uncertain graphs. Specifically, given an uncertain graph 𝒢 and an integer k , we aim at partitioning its nodes into k clusters, each featuring a distinguished center node, so to maximize the minimum/average connection probability of any node to its cluster's center, in a random possible world. We assess the NP-hardness of maximizing the minimum connection probability, even in the presence of an oracle for the connection probabilities, and develop efficient approximation algorithms for both problems and some useful variants. Unlike previous works in the literature, our algorithms feature provable approximation guarantees and are capable to keep the granularity of the returned clustering under control. Our theoretical findings are complemented with several experiments that compare our algorithms against some relevant competitors, with respect to both running-time and quality of the returned clusterings.

Given a dataset of points in a metric space and an integer k , a diversity maximization problem requires determining a subset of k points maximizing some diversity objective measure, e.g., the minimum or the average distance between two points in the subset. Diversity maximization is computationally hard, hence only approximate solutions can be hoped for. Although its applications are mainly in massive data analysis, most of the past research on diversity maximization focused on the sequential setting. In this work we present space and pass/round-efficient diversity maximization algorithms for the Streaming and MapReduce models and analyze their approximation guarantees for the relevant class of metric spaces of bounded doubling dimension. Like other approaches in the literature, our algorithms rely on the determination of high-quality core-sets, i.e., (much) smaller subsets of the input which contain good approximations to the optimal solution for the whole input. For a variety of diversity objective functions, our algorithms attain an ( α + ε )-approximation ratio, for any constant ε > 0, where α is the best approximation ratio achieved by a polynomial-time, linear-space sequential algorithm for the same diversity objective. This improves substantially over the approximation ratios attainable in Streaming and MapReduce by state-of-the-art algorithms for general metric spaces. We provide extensive experimental evidence of the effectiveness of our algorithms on both real world and synthetic datasets, scaling up to over a billion points.

We present a space and time efficient practical parallel algorithm for approximating the diameter of massive weighted undirected graphs on distributed platforms supporting a MapReduce-like abstraction. The core of the algorithm is a weighted graph decomposition strategy generating disjoint clusters of bounded weighted radius. Theoretically, our algorithm uses linear space and yields a polylogarithmic approximation guarantee, moreover, for important practical classes of graphs, it runs in a number of rounds asymptotically smaller than those required by the natural approximation provided by the state-of-the-art Δ-stepping SSSP algorithm, which is its only practical linear-space competitor in the aforementioned computational scenario. We complement ourtheoretical findings with an extensive experimental analysis on large benchmark graphs, which demonstrates that our algorithm attains substantial improvements on a number of key performance indicators with respect to the aforementioned competitor, while featuring a similar approximation ratio (a small constant less than 1.4, as opposed to the polylogarithmic theoretical bound).

A framework is proposed for the design and analysis of network-oblivious algorithms, namely algorithms that can run unchanged, yet efficiently, on a variety of machines characterized by different degrees of parallelism and communication capabilities. The framework prescribes that a network-oblivious algorithm be specified on a parallel model of computation where the only parameter is the problem’s input size, and then evaluated on a model with two parameters, capturing parallelism granularity and communication latency. It is shown that for a wide class of network-oblivious algorithms, optimality in the latter model implies optimality in the decomposable bulk synchronous parallel model, which is known to effectively describe a wide and significant class of parallel platforms. The proposed framework can be regarded as an attempt to port the notion of obliviousness, well established in the context of cache hierarchies, to the realm of parallel computation. Its effectiveness is illustrated by providing optimal network-oblivious algorithms for a number of key problems. Some limitations of the oblivious approach are also discussed.

We develop a novel parallel decomposition strategy for unweighted, undirected graphs, based on growing disjoint connected clusters from batches of centers progressively selected from yet uncovered nodes. With respect to similar previous decompositions, our strategy exercises a tighter control on both the number of clusters and their maximum radius. We present two important applications of our parallel graph decomposition: (1) $k$-center clustering approximation; and (2) diameter approximation. In both cases, we obtain algorithms which feature a polylogarithmic approximation factor and are amenable to a distributed implementation that is geared for massive (long-diameter) graphs. The total space needed for the computation is linear in the problem size, and the parallel depth is substantially sublinear in the diameter for graphs with low doubling dimension. To the best of our knowledge, ours are the first parallel approximations for these problems which achieve sub-diameter parallel time, for a relevant class of graphs, using only linear space. Besides the theoretical guarantees, our algorithms allow for a very simple implementation on clustered architectures: we report on extensive experiments which demonstrate their effectiveness and efficiency on large graphs as compared to alternative known approaches.

This work explores fundamental modeling and algorithmic issues arising in the well-established MapReduce framework. First, we formally specify a computational model for MapReduce which captures the functional flavor of the paradigm by allowing for a flexible use of parallelism. Indeed, the model diverges from a traditional processor-centric view by featuring parameters which embody only global and local memory constraints, thus favoring a more data-centric view. Second, we apply the model to the fundamental computation task of matrix multiplication presenting upper and lower bounds for both dense and sparse matrix multiplication, which highlight interesting tradeoffs between space and round complexity. Finally, building on the matrix multiplication results, we derive further space-round tradeoffs on matrix inversion and matching.

As advances in technology allow for the collection, storage, and analysis of vast amounts of data, the task of screening and assessing the significance of discovered patterns is becoming a major challenge in data mining applications. In this work, we address significance in the context of frequent itemset mining. Specifically, we develop a novel methodology to identify a meaningful support threshold s * for a dataset, such that the number of itemsets with support at least s * represents a substantial deviation from what would be expected in a random dataset with the same number of transactions and the same individual item frequencies. These itemsets can then be flagged as statistically significant with a small false discovery rate. We present extensive experimental results to substantiate the effectiveness of our methodology.

Motivated by the growing interest in mobile systems, we study the dynamics of information dissemination between agents moving independently on a plane. Formally, we consider k mobile agents performing independent random walks on an n-node grid. At time 0, each agent is located at a random node of the grid and one agent has a rumor. The spread of the rumor is governed by a dynamic communication graph process {Gt(r)|t ≥ 0}, where two agents are connected by an edge in Gt(r) iff their distance at time t is within their transmission radius r. Modeling the physical reality that the speed of radio transmission is much faster than the motion of the agents, we assume that the rumor can travel throughout a connected component of Gt before the graph is altered by the motion. We study the broadcast time TB of the system, which is the time it takes for all agents to know the rumor. We focus on the sparse case (below the percolation point rc ≈ √n/k) where, with high probability, no connected component in Gt has more than a logarithmic number of agents and the broadcast time is dominated by the time it takes for many independent random walks to meet one other. Quite surprisingly, we show that for a system below the percolation point, the broadcast time does not depend on the transmission radius. In fact, we prove that TB = Θ(n/√k) for any 0 ≤ r < rc, even when the transmission range is significantly larger than the mobility range in one step, giving a tight characterization up to logarithmic factors. Our result complements a recent result of Peres et al. (SODA 2011) who showed that above the percolation point the broadcast time is polylogarithmic in k.

As advances in technology allow for the collection, storage, and analysis of vast amounts of data, the task of screening and assessing the significance of discovered patterns is becoming a major challenge in data mining applications. In this work, we address significance in the context of frequent itemset mining. Specifically, we develop a novel methodology to identify a meaningful support threshold s* for a dataset, such that the number of itemsets with support at least s* represents a substantial deviation from what would be expected in a random dataset with the same number of transactions and the same individual item frequencies. These itemsets can then be flagged as statistically significant with a small false discovery rate.

Abstract We study the connectivity properties of a family of random graphs that closely model the Bluetooth's device discovery process, where each device tries to connect to other devices within its visibility range in order to establish reliable communication channels yielding a connected topology. Specifically, we provide both analytical and experimental evidence that when the visibility range of each node (i.e. device) is limited to a vanishing function ofn, the total number of nodes in the system, full connectivity can still be achieved with high probability by letting each node connect only to a ‘small’ number of visible neighbors. Our results extend previous studies, where connectivity properties were analyzed only for the case of a constant visibility range, and provide evidence that Bluetooth can indeed be used for establishing largead hocnetworks. Copyright © 2008 John Wiley & Sons, Ltd.

We address the problem of developing a suite of microbenchmarking experiments aimed at providing the basic functionalities of a measurement tool for a P2P-based globally distributed computing platform, usually referred to as overlay computer. We argue that such a measuring system should take into account the communication patterns generated by the applications in order to provide useful performance insights

The design of algorithms that can run unchanged yet efficiently on a variety of machines characterized by different degrees of parallelism and communication capabilities is a highly desirable goal. We propose a framework for network-obliviousness based on a model of computation where the only parameter is the problem's input size. Algorithms are then evaluated on a model with two parameters, capturing parallelism and granularity of communication. We show that, for a wide class of network-oblivious algorithms, optimality in the latter model implies optimality in a block-variant of the decomposable BSP model, which effectively describes a wide and significant class of parallel platforms. We illustrate our framework by providing optimal network-oblivious algorithms for a few key problems, and also establish some negative results.

This paper explores the relation between the structured parallelism exposed by the decomposable BSP (D-BSP) model through submachine locality and locality of reference in multi-level cache hierarchies. Specifically, an efficient cache-oblivious algorithm is developed to simulate D-BSP programs on the ideal cache model (ICM). The effectiveness of the simulation is proved by showing that optimal cache-oblivious algorithms for prominent problems can be obtained from D-BSP algorithms. Finally, a tight relation between optimality in the D-BSP and ICM models is established

Summary form only given. The design of algorithms exhibiting a high degree of temporal and spatial locality of reference is crucial to attain good performance on current and foreseeable computing systems featuring ever deeper memory hierarchies. Previous work has demonstrated that task parallelism can be efficiently transformed into locality of reference in two-level hierarchies. Recently, we moved a step forward and showed how the more structured type of parallelism exposed by submachine locality can be efficiently turned into temporal locality on arbitrarily deep hierarchies. We complete and extend the above result by encompassing also spatial locality. Specifically, we present a scheme to simulate parallel algorithms designed for the decomposable BSP (a BSP variant which captures submachine locality) on the hierarchical memory model with block transfer. The simulation yields good hierarchy-conscious sequential algorithms from parallel ones, and provides evidence of the strict relation between submachine locality in parallel computation and locality of reference (both temporal and spatial) in the hierarchical memory setting.

We present a general deterministic scheme to implement a shared memory abstraction on any distributed-memory machine which exhibits a clustered structure. More specifically, we develop a memory distribution strategy and an access protocol for the Decomposable BSP (D-BSP), a generic machine model whose bandwidth/latency parameters can be instantiated to closely reflect the characteristics of machines that admit a hierarchical decomposition into independent clusters. Our scheme achieves provably optimal slowdown for those machines where delays due to latency dominate over those due to bandwidth limitations. For machines where this is not the case, the slowdown is a mere logarithmic factor away from the natural bandwidth-based lower bound. An important feature of the scheme is that it can be made fully constructive for small memory sizes, while for larger sizes it relies solely on nonconstructive graphs of weak expansion.

We study the routing of messages with multiple destinations on an n-node square mesh (one-to-many routing). The obvious approach of simply replicating each message into the appropriate number of point-to-point messages and routing these independently does not generally yield optimal performance. A standard argument proves that (Ω √ n + cm) time is required to route m ⪇ n messages, where each message is generated by a distinct node and at most c messages must be delivered to any individual node. The lower bound does not depend on the number of destinations per message. We provide both randomized and deterministic algorithms for one-to-many routing, which use constant-size buffers at each node. The randomized algorithm attains optimal performance, while the deterministic algorithm is slower by a factor of Ο (log2 n). We also describe an optimal deterministic algorithm that, however, requires large buffers of size Ο (c).

We study the issue of performance prediction on the SGI-Power Challenge, a typical SMP. On such a platform, the cost of memory accesses depends on their locality and on contention among processors. By running a carefully designed suite of microbenchmarks, we provide quantitative evidence that memory hierarchy effects impact performance far more substantially than other phenomena related to contention. We also fit three cost functions based on variants of the BSP model, which do not account for the hierarchy, and a newly defined function F expressed in terms of hardware counters, which captures both memory hierarchy and contention effects. We test the accuracy of all the functions on both synthetic and application benchmarks showing that, unlike the other functions, F achieves an excellent level of accuracy in all cases. Although hardware counters are only available at run-time, we give evidence that function F can still be employed as a prediction tool by extrapolating values of the counters from pilot runs on small input sizes.

This paper describes a scheme to implement a shared address space of size m on an n-node mesh, with m polynomial in n, where each mesh node hosts a processor and a memory module. At the core of the simulation is a hierarchical memory organization scheme (HMOS), which governs the distribution of the shared variables, each replicated into multiple copies, among the memory modules, through a cascade of bipartite graphs. Based on the expansion properties of such graphs, we devise a protocol that accesses any n-tuple of shared variables in worst-case time $O(n^{1/2+\eta})$, for any constant $\eta > 0$, using $O(1/\eta^{1.59})$ copies per variable, or in worst-case time O(n 1/2 log n), using O(log 1.59 n ) copies per variable. In both cases the access time is close to the natural $O(\sqrt{n})$ lower bound imposed by the network diameter. A key feature of the scheme is that it can be made fully constructive when m is not too large, thus providing in this case the first efficient, constructive, deterministic scheme in the literature for bounded-degree processor networks. For larger memory sizes, the scheme relies solely on a nonconstructive graph of weak expansion. Finally, the scheme can be efficiently ported to other architectures, as long as they exhibit certain structural properties. In the paper we discuss the porting to multidimensional meshes and to the pruned butterfly, an area-universal network which is a variant of the fat-tree.

A quantitative comparison of the BSP and LogP models for parallel computation is developed.Very efficient cross simulations between the two models are derived, showing their substantial equivalence for algorithmic design guided by asymptotic analysis.It is also shown that the two models can be implemented with similar performance on most point-to-point networks.In conclusion, within the limits of our analysis that is mainly of asymptotic nature, BSP and LogP can be viewed as closely related variants within the bandwidth-latency framework for modeling parallel computation.BSP seems somewhat preferable due to greater simplicity and portability, and slightly greater power.

We study the most general communication paradigm on a multiprocessor, wherein each processor has a distinct message (of possibly distinct lengths) for each other processor. We study this paradigm, which we call chatting, on multiprocessors that do not allow messages once dispatched ever to be delayed on their routes. By insisting on oblivious routes for messages, we convert the communication problem to a pure scheduling problem. We introduce the notion of a virtual chatting schedule, and we show how efficient chatting schedules can often be produced from efficient virtual chatting schedules. We present a number of strategies for producing efficient virtual chatting schedules on a variety of network topologies.

We present a constructive deterministic simulation of a PRAM with n processors and m = n α shared variables, 1 < α ≤ 2, on an n-node mesh-connected computer where each node hosts a processor and a memory module. At the core of the simulation is a Hierarchical Memory Organization Scheme (HMOS) that governs the distribution of the PRAM variables (each replicated into a number of copies) among the modules. The HMOS consists of a cascade of explicit bipartite graphs whose expansion properties, combined with suitable access and routing protocols, yield a time performance that, for α < 3/2, is close to the Ω(√n) bound imposed by the network's diameter, and that, for α ≥ 3/2, is a function of α never exceeding O(n5/8).

The operations of scattering and gathering in a network of processors involve one processor of the network (P/sub 0/) communicating with all other processors. In scattering, P/sub 0/ sends distinct messages to P/sub 0/. The authors consider networks that are trees of processors. Algorithms for scattering messages from and gathering messages to the processor that resides at the root of the tree are presented. The algorithms are quite general, in that the messages transmitted can differ arbitrarily in length; quite strong, in that they send messages along noncolliding paths, and hence do not require any buffering or queueing mechanisms in the processors; and quite efficient in that algorithms for scattering in general trees are optimal, the algorithm for gathering in a path is optimal and the algorithms for gathering in general trees are nearly optimal. The algorithms can easily be converted using spanning trees to efficient algorithms for scattering and gathering in networks of arbitrary topologies.>

A reliability model is proposed for recovery block structures based on error events which can be observed and distinguished during testing. Strategies are then described for the collection of failure histories needed to estimate the model parameters and obtain dependability predictions. Given that the software goes through different testing stages, the model can be employed at different points of the development cycle to assess or forecast the quality of project choices and the resulting product.>