789322
Cache Coherency in Location-Dependent Information Services for Mobile Environment
Caching frequently accessed data at the client is an attractive technique for improving access time. In a mobile computing environment, client location becomes a piece of changing information. As such, location-dependent data may become obsolete due to data updates or client movements. Most of the previous work investigated cache invalidation issues related to data updates only, whereas few considered data inconsistency caused by client movements. This paper separates location-dependent data invalidation from traditional cache invalidation. For location-dependent invalidation, several approaches are proposed and their performance is studied by a set of simulation experiments. The results show that the proposed methods substantially outperform the NSI scheme which drops the cache contents entirely when hand-off.
16560540
Optimizing static job scheduling in a network of heterogeneous computers
This paper investigates static job scheduling schemes that distribute workload in a network of computers with different speeds. Static schemes involve very low overhead and complexity compared to dynamic schemes, but can still provide significant performance improvement over the case of no load balancing. Optimization techniques are proposed for workload allocation and job dispatching. Workload allocation is modeled as a non-linear optimization problem and solved mathematically. It is shown that allocating a disproportionately high percentage of jobs to the more powerful computers improves system performance. The proposed job dispatching algorithm is an extension of the traditional round-robin scheme. The objective is to reduce burstiness in the job arrival stream to each computer The schemes are evaluated by simulation experiments. Performance results verify their effectiveness in terms of mean response time, mean response ratio, and fairness. The Optimized Round-Robin (ORR) strategy which combines both techniques outperforms other static scheduling algorithms examined.
12656146
Session-affinity aware request allocation for Web clusters
Persistent connections are increasingly being used in Web retrieval due to wide adoption of HTTP/1.1 standards. With persistent connections, the request allocation algorithm used by Web clusters is often session-grained. This article studies the caching performance of Web clusters under session-grained request allocation. It is shown that although content-based algorithms considerably improve caching performance over content-blind algorithms at the request-grained level, most performance gain is offset by the allocation dependency that arises when the requests are allocated at the session-grained level. The performance loss increases with cluster size and connection holding time. An optimization problem is then formulated for improving the caching effectiveness of session-grained allocation. The problem is proven to be NP-complete. Based on a heuristic approach, a session-affinity aware algorithm is presented that makes use of the correlation between the requests in a session. The new algorithm is shown to significantly outperform the content-based algorithm under session-grained allocation. It is also shown that optimizing session-grained allocation cannot fully compensate for the performance loss caused by allocation dependency.
13652007
Processing Precision-Constrained Approximate Queries in Wireless Sensor Networks
A lot of research efforts have been devoted to improving energy efficiency for wireless sensor networks by exploring distributed data storage and in-network query processing techniques. In this paper, we present a generic two-tier data storage strategy for answering precision-constrained approximate queries in a sensor network. The basic idea is to keep two versions of data in the network. A highprecision version is kept at the sensor node that captures the data while a low-precision version is maintained at the base station. We develop query processing and node refreshment strategies for various types of approximate queries under the two-tier storage. Our extensive experiments show that the two-tier storage strategy outperforms the basic centralized storage scheme by an order of magnitude in terms of network lifetime under various system configur
17927523
An energy-efficient and access latency optimized indexing scheme for wireless data broadcast
Data broadcast is an attractive data dissemination method in mobile environments. To improve energy efficiency, existing air indexing schemes for data broadcast have focused on reducing tuning time only, i.e., the duration that a mobile client stays active in data accesses. On the other hand, existing broadcast scheduling schemes have aimed at reducing access latency through nonflat data broadcast to improve responsiveness only. Not much work has addressed the energy efficiency and responsiveness issues concurrently. This paper proposes an energy-efficient indexing scheme called MHash that optimizes tuning time and access latency in an integrated fashion. MHash reduces tuning time by means of hash-based indexing and enables nonflat data broadcast to reduce access latency. The design of hash function and the optimization of bandwidth allocation are investigated in depth to refine MHash. Experimental results show that, under skewed access distribution, MHash outperforms state-of-the-art air indexing schemes and achieves access latency close to optimal broadcast scheduling
16349513
Analysis of Replica Placement under Expiration-Based Consistency Management
Expiration-based consistency management is widely used to keep replicated contents up-to-date in the Internet. The effectiveness of replication can be characterized by the communication costs of client accesses and consistency management. Both costs depend on the locations of the replicas. This paper investigates the problem of placing replicas in a network where replica consistency is managed by the expiration-based scheme. Our objective is to minimize the total cost of client accesses and consistency management. By analyzing the communication cost of recursive validations for cascaded replicas, we prove that in the optimal placement scheme, the nodes not assigned replicas induce a connected subgraph that includes the origin server. Our results are generic in that they apply to any request arrival patterns. Based on the analysis, an O(D)-time algorithm is proposed to compute the optimal placement of the replicas, where D is the sum of the number of descendants over all nodes in the routing tree
47107463
Using Motion Planning to Study RNA Folding Kinetics
2027823
Evolutionary Search for Faces from Line Drawings
Single 2D line drawing is a straightforward method to illustrate 3D objects. The faces of an object depicted by a line drawing give very useful information for the reconstruction of its 3D geometry. Two recently proposed methods for face identification from line drawings are based on two steps: finding a set of circuits that may be faces and searching for real faces from the set according to some criteria. The two steps, however, involve two combinatorial problems. The number of the circuits generated in the first step grows exponentially with the number of edges of a line drawing. These circuits are then used as the input to the second combinatorial search step. When dealing with objects having more faces, the combinatorial explosion prevents these methods from finding solutions within feasible time. This paper proposes a new method to tackle the face identification problem by a variable-length genetic algorithm with a novel heuristic and geometric constraints incorporated for local search. The hybrid GA solves the two combinatorial problems simultaneously. Experimental results show that our algorithm can find the faces of a line drawing having more than 30 faces much more efficiently. In addition, simulated annealing for solving the face identification problem is also implemented for comparison.
6097645
Simulating Protein Motions with Rigidity Analysis
Protein motions, ranging from molecular flexibility to large-scale conformational change, play an essential role in many biochemical processes. Despite the explosion in our knowledge of structural and functional data, our understanding of protein movement is still very limited. In previous work, we developed and validated a motion planning based method for mapping protein folding pathways from unstructured conformations to the native state. In this paper, we propose a novel method based on rigidity theory to sample conformation space more effectively, and we describe extensions of our framework to automate the process and to map transitions between specified conformations. Our results show that these additions both improve the accuracy of our maps and enable us to study a broader range of motions for larger proteins. For example, we show that rigidity-based sampling results in maps that capture subtle folding differences between protein G and its mutants, NuG1 and NuG2, and we illustrate how our technique can be used to study large-scale conformational changes in calmodulin, a 148 residue signaling protein known to undergo conformational changes when binding to Ca(2+). Finally, we announce our web-based protein folding server which includes a publicly available archive of protein motions: (http://parasol.tamu.edu/foldingserver/).
1083190
Streaming media caching algorithms for transcoding proxies
Streaming media is expected to become one of the most popular types of Web content in the future. Due to increasing variety of client devices and the range of access speeds to the Internet, multimedia contents may be required to be transcoded to match the client's capability. With transcoding, both the network and the proxy CPU are potential bottlenecks for streaming media delivery. This paper discusses and compares various caching algorithms designed for transcoding proxies. In particular we propose a new adaptive algorithm that dynamically selects an appropriate metric for adjusting the management policy. Experimental results show that the proposed algorithm significantly outperforms those that cache only untranscoded or only transcoded objects. Moreover motivated by the characteristics of many video compression algorithms, we investigate partitioning a video object into sections based on frame type and handling them individually for proxy caching. It is found that partitioning improves performance when CPU power rather than network bandwidth is the limiting resource, particularly when the reference pattern is not highly skewed.
8798483
Planning with Reachable Distances: Fast Enforcement of Closure Constraints
Motion planning for closed-chain systems is particularly difficult due to additional closure constraints placed on the system. In fact, the probability of randomly selecting a set of joint angles that satisfy the closure constraints is zero. We propose planning with reachable distance (PRD) to overcome this challenge by first precomputing the subspace satisfying the closure constraints, then directly sampling in it. To do so, we represent the chain as a hierarchy of sub-chains. Then we calculate the "closure" sub-space as appropriate reachable distance ranges of sub-chains satisfying the closure constraints. This provides two distinct advantages over traditional approaches: (1) configurations are quickly sampled and converted to joint angles using basic trigonometry functions instead of more expensive inverse kinematics solvers, and (2) configurations are guaranteed to be closed. In this paper, we describe this hierarchical chain representation and give a sampling algorithm with complexity linear in the number of links. We provide the necessary motion planning primitives for most sampling-based motion planners. Our experimental results show our method is fast, making sampling closed configurations comparable to sampling open chain configurations that ignore closure constraints. Our method is general, easy to implement, and also extends to other distance-related constraints besides the ones demonstrated here