Results 1  10
of
67
Probabilistic Approximation of Metric Spaces and its Algorithmic Applications
 In 37th Annual Symposium on Foundations of Computer Science
, 1996
"... The goal of approximating metric spaces by more simple metric spaces has led to the notion of graph spanners [PU89, PS89] and to lowdistortion embeddings in lowdimensional spaces [LLR94], having many algorithmic applications. This paper provides a novel technique for the analysis of randomized ..."
Abstract

Cited by 326 (29 self)
 Add to MetaCart
The goal of approximating metric spaces by more simple metric spaces has led to the notion of graph spanners [PU89, PS89] and to lowdistortion embeddings in lowdimensional spaces [LLR94], having many algorithmic applications. This paper provides a novel technique for the analysis of randomized algorithms for optimization problems on metric spaces, by relating the randomized performance ratio for any metric space to the randomized performance ratio for a set of "simple" metric spaces. We define a notion of a set of metric spaces that probabilisticallyapproximates another metric space. We prove that any metric space can be probabilisticallyapproximated by hierarchically wellseparated trees (HST) with a polylogarithmic distortion. These metric spaces are "simple" as being: (1) tree metrics. (2) natural for applying a divideandconquer algorithmic approach. The technique presented is of particular interest in the context of online computation. A large number of online al...
Implementing Global Memory Management in a Workstation Cluster
"... Advances in network and processor technology have greatly changed the communication and computational power of localarea workstation clusters. However, operating systems still treat workstation clusters as a collection of looselyconnected processors, where each workstation acts as an autonomous an ..."
Abstract

Cited by 160 (14 self)
 Add to MetaCart
(Show Context)
Advances in network and processor technology have greatly changed the communication and computational power of localarea workstation clusters. However, operating systems still treat workstation clusters as a collection of looselyconnected processors, where each workstation acts as an autonomous and independent agent. This operating system structure makes it difficult to exploit the characteristics of current clusters, such as lowlatency communication, huge primary memories, and highspeed processors, in order to improve the performance of cluster applications. This paper describes the design and implementation of global memory management in a workstation cluster. Our objective is to use a single, unified, but distributed memory management algorithm at the lowest level of the operating system. By managing memory globally at this level, all system and higherlevel software, including VM, file systems, transaction systems, and user applications, can benefit from available cluster memory. We have implemented our algorithm in the OSF/1 operating system running on an ATMconnected cluster of DEC Alpha workstations. Our measurements show that on a suite of memoryintensive programs, our system improves performance by a factor of 1.5 to 3.5. We also show that our algorithm has a performance advantage over others that have been proposed in the past.
Competitive Distributed File Allocation
, 1993
"... This paper deals with the file allocation problem [BFR92] concerning the dynamic optimization of communication costs to access data in a distributed environment. We develop a dynamic file reallocation strategy that adapts online to a sequence of read and write requests whose location and relative ..."
Abstract

Cited by 108 (12 self)
 Add to MetaCart
This paper deals with the file allocation problem [BFR92] concerning the dynamic optimization of communication costs to access data in a distributed environment. We develop a dynamic file reallocation strategy that adapts online to a sequence of read and write requests whose location and relative frequencies are completely unpredictable. This is achieved by replicating the file in response to read requests and migrating the file in response to write requests while paying the associated communications costs, so as to be closer to processors that access it frequently. We develop first explicit deterministic online strategy assuming existence of global information about the state of the network; previous (deterministic) solutions were complicated and more expensive. Our solution has (optimal) logarithmic competitive ratio. The paper also contains the first explicit deterministic data migration [BS89] algorithm achieving the best known competitive ratio for this problem. Using somewhat ...
Competitive Algorithms for Distributed Data Management
 In Proceedings of the 24th Annual ACM Symposium on Theory of Computing
"... We deal with the competitive analysis of algorithms for managing data in a distributed environment. We deal with the file allocation problem ([DF], [ML]), where copies of a file may be be stored in the local storage of some subset of processors. Copies may be replicated and discarded over time so ..."
Abstract

Cited by 100 (8 self)
 Add to MetaCart
(Show Context)
We deal with the competitive analysis of algorithms for managing data in a distributed environment. We deal with the file allocation problem ([DF], [ML]), where copies of a file may be be stored in the local storage of some subset of processors. Copies may be replicated and discarded over time so as to optimize communication costs, but multiple copies must be kept consistent and at least one copy must be stored somewhere in the network at all times. We deal with competitive algorithms for minimizing communication costs, over arbitrary sequences of reads and writes, and arbitrary network topologies. We define the constrained file allocation problem to be the solution of many individual file allocation problems simultaneously, subject to the constraints of local memory size. We give competitive algorithms for this problem on the uniform network topology. We then introduce distributed competitive algorithms for online data tracking (a generalization of mobile user tracking [AP1...
Flashdb: dynamic selftuning database for nand flash
 In IPSN
, 2007
"... FlashDB is a selftuning database optimized for sensor networks using NAND flash storage. In practical systems flash is used in different packages such as onboard flash chips, compact flash cards, secure digital cards and related formats. Our experiments reveal nontrivial differences in their acce ..."
Abstract

Cited by 69 (5 self)
 Add to MetaCart
(Show Context)
FlashDB is a selftuning database optimized for sensor networks using NAND flash storage. In practical systems flash is used in different packages such as onboard flash chips, compact flash cards, secure digital cards and related formats. Our experiments reveal nontrivial differences in their access costs. Furthermore, databases may be subject to different types of workloads. We show that existing databases for flash are not optimized for all types of flash devices or for all workloads and their performance is thus suboptimal in many practical systems. FlashDB uses a novel selftuning index that dynamically adapts its storage structure to workload and underlying storage device. We formalize the selftuning nature of an index as a twostate task system and propose a 3competitive online algorithm that achieves the theoretical optimum. We also provide a framework to determine the optimal size of an index node that minimizes energy and latency for a given device. Finally, we propose optimizations to further improve the performance of our index. We prototype and compare different indexing schemes on multiple flash devices and workloads, and show that our indexing scheme outperforms existing schemes under all workloads and flash devices we consider.
Distributed Paging for General Networks
, 1996
"... Distributed paging [BFR92, ABF93b, AK95] deals with the dynamic allocation of copies of files in a distributed network as to minimize the total communication cost over a sequence of read and write requests. Most previous work deals with the file allocation problem [BS89, West91, CLRW93, ABF93a, ..."
Abstract

Cited by 59 (5 self)
 Add to MetaCart
Distributed paging [BFR92, ABF93b, AK95] deals with the dynamic allocation of copies of files in a distributed network as to minimize the total communication cost over a sequence of read and write requests. Most previous work deals with the file allocation problem [BS89, West91, CLRW93, ABF93a, WY93, Koga93, AK94, LRWY94] where infinite nodal memory capacity is assumed. In contrast the distributed paging problem makes the more realistic assumption that nodal memory capacity is limited. Former work on distributed paging deals with the problem only in the case of a uniform network topology. This paper gives the first distributed paging algorithm for general networks. The algorithm is competitive in storage and communication. The competitive ratios are polylogarithmic in the total number of network nodes and the diameter of the network. Johns Hopkins University and Lab. for Computer Science, MIT. Supported by Air Force Contract TNDGAFOSR860078, ARO contract DAAL0386K0171, NSF contract 9114440CCR, DARPA contract N00014J 921799, and a special grant from IBM. EMail: baruch@theory.lcs.mit.edu. y Department of Computer Science, School of Mathematics, TelAviv University, TelAviv 69978, Israel. Supported by a grant from the Israeli Academy of Sciences. Email: yairb@math.tau.ac.il, fiat@math.tau.ac.il 0 1
NUMA policies and their relation to memory architecture
 In Architectural Support for Programming Languages and Operating Systems
, 1991
"... Multiprocessor memory reference traces provide a wealth of information on the behavior of parallel programs. We have used this information to explore the relationship between kernelbased NUMA management policies and multiprocessor memory architecture. Our trace analysis techniques employ an offlin ..."
Abstract

Cited by 51 (17 self)
 Add to MetaCart
(Show Context)
Multiprocessor memory reference traces provide a wealth of information on the behavior of parallel programs. We have used this information to explore the relationship between kernelbased NUMA management policies and multiprocessor memory architecture. Our trace analysis techniques employ an offline, optimal cost policy as a baseline against which to compare online policies, and as a policyinsensitive tool for evaluating architectural design alternatives. We compare the performance of our optimal policy with that of three implementable policies (two of which appear in previous work), on a variety of applications, with varying relative speeds for page moves and local, global, and remote memory references. Our results indicate that a good NUMA policy must be chosen to match its machine, and confirm that such policies can be both simple and effective. They also indicate that programs for NUMA machines must be written with care to obtain the best performance. 1
Randomized Competitive Algorithms for the List Update Problem
 Algorithmica
, 1992
"... We prove upper and lower bounds on the competitiveness of randomized algorithms for the list update problem of Sleator and Tarjan. We give a simple and elegant randomized algorithm that is more competitive than the best previous randomized algorithm due to Irani. Our algorithm uses randomness only d ..."
Abstract

Cited by 42 (2 self)
 Add to MetaCart
(Show Context)
We prove upper and lower bounds on the competitiveness of randomized algorithms for the list update problem of Sleator and Tarjan. We give a simple and elegant randomized algorithm that is more competitive than the best previous randomized algorithm due to Irani. Our algorithm uses randomness only during an initialization phase, and from then on runs completely deterministically. It is the first randomized competitive algorithm with this property to beat the deterministic lower bound. We generalize our approach to a model in which access costs are fixed but update costs are scaled by an arbitrary constant d. We prove lower bounds for deterministic list update algorithms and for randomized algorithms against oblivious and adaptive online adversaries. In particular, we show that for this problem adaptive online and adaptive offline adversaries are equally powerful. 1 Introduction Recently much attention has been given to competitive analysis of online algorithms [7, 20, 22, 25]. Ro...
Online Generalized Steiner Problem
, 1996
"... The Generalized Steiner Problem (GSP) is defined as follows. We are given a graph with nonnegative weights and a set of pairs of vertices. The algorithm has to construct minimum weight subgraph such that the two nodes of each pair are connected by a path. Offline generalized Steiner problem ap ..."
Abstract

Cited by 41 (5 self)
 Add to MetaCart
The Generalized Steiner Problem (GSP) is defined as follows. We are given a graph with nonnegative weights and a set of pairs of vertices. The algorithm has to construct minimum weight subgraph such that the two nodes of each pair are connected by a path. Offline generalized Steiner problem approximation algorithms were given in [AKR91, GW92]. We consider the online generalized Steiner problem, in which pairs of vertices arrive online and are needed to be connected immediately. We give a simple O(log² n) competitive deterministic online algorithm. The previous best algorithm was O( p n log n) competitive [WY93]. We also consider the network connectivity leasing problem which is a generalization of the GSP. Here edges of the graph can be either bought or leased for different costs. We provide simple randomized O(log² n) competitive algorithm based on the online generalized Steiner problem result.
Flashing Up the Storage Layer
, 2008
"... In the near future, commodity hardware is expected to incorporate both flash and magnetic disks. In this paper we study how the storage layer of a database system can benefit from the presence of both kinds of disk. We propose using the flash and the magnetic disk at the same level of the memory hie ..."
Abstract

Cited by 30 (3 self)
 Add to MetaCart
In the near future, commodity hardware is expected to incorporate both flash and magnetic disks. In this paper we study how the storage layer of a database system can benefit from the presence of both kinds of disk. We propose using the flash and the magnetic disk at the same level of the memory hierarchy and placing a data page to only one of these disks according to the workload of the page. Pages with a readintensive workload are placed on the flash disk, while pages with a writeintensive workload are placed on the magnetic disk. We present a family of online algorithms to decide the optimal placement of a page and study their theoretical properties. Our system is selftuning, i.e., our algorithms adapt page placement to changing workloads. We also present a buffer replacement policy that takes advantage of the asymmetric I/O properties of the two types of storage media to reduce the total I/O cost. Our experimental evaluation shows remarkable I/O performance improvement over both flashonly and magneticonly systems. These results, we believe, exhibit both the potential and necessity of such algorithms in future database systems.