Service differentiation that provides prioritized service qualities to multiple classes of client requests can effectively utilize available server resources. This paper studies how demand-driven service differentiation in terms of end-user performance can be supported in cluster-based network servers. Our objective is to deliver better services to high priority request classes without over-sacrificing low priority classes. To achieve this objective, we propose a dynamic scheduling scheme, called DDSD that adapts to fluctuating request resource demands by periodically repartitioning servers. This scheme also employs priority-based admission control to drop excessive user requests and achieve soft performance guarantees. For each scheduling period, our scheme monitors the system status and uses a queuing model to approximate server behaviors and guide resource allocation. Our experiments show that the proposed technique achieves demand-driven service differentiation while maximizing resource utilization and that it can substantially outperform static server partitioning.

Caching dynamic pages at a server site is beneficial in reducing server resource demands and it also helps dynamic page caching at proxy sites. Previous work has used fine-grain dependence graphs among individual dynamic pages and underlying data sets to enforce result consistency. This paper proposes a complementary solution for applications that require coarse-grain cache management. The key idea is to partition dynamic pages into classes based on URL patterns so that an application can specify page identification and data dependence, and invoke invalidation for a class of dynamic pages. To make this scheme time-efficient with small space requirement, lazy invalidation is used to minimize slow disk accesses when IDs of dynamic pages are stored in memory with a digest format. Selective precomputing is further proposed to refresh stale pages and smoothen load peaks. A data structure is developed for efficient URL class searching during lazy or eager invalidation. This paper also presents design and implementation of a caching system called Cachuma which integrates the above techniques, runs in tandem with standard Web servers, and allows Web sites to add dynamic page caching capability with minimal changes. The experimental results show that the proposed techniques are effective in supporting coarse-grain cache management and reducing server response times for tested applications.

Clustering support with a single-system image for large-scale Web servers is important to improve the system scalability in processing a large number of concurrent requests from Internet, especially when those requests involve resource-intensive dynamic content generation.This paper proposes scheduling optimization for a Web server cluster with a master/slave architecture which separates static and dynamic content processing.Our experimental results show that the proposed optimization using reservation-based scheduling can produce up to a 68% performance improvement.

This paper investigates load balancing strategies for clustered Alexandria digital library (ADL) servers. The ADL system, which provides on-line information searching and browsing of spatially-referenced materials through the World Wide Web, involves intensive database I/O and heterogeneous CPU activities. Clustering servers can improve the scalability of the ADL system in response to a large number of simultaneous access requests. One difficulty addressed is that clustered workstation nodes may be non-uniform in terms of CPU and I/O speeds. An optimization scheme is proposed in this paper to dynamically monitor the resource availability, use a low-cost communication strategy for updating load information among nodes, and schedule requests based on both I/O and computation load indices. Since the accurate cost estimation for processing database-searching requests is difficult, a sampling and prediction scheme is used to identify the relative efficiency of nodes for satisfying I/O and CPU demands of these requests. A set of experiments using the ADL traces have been conducted to verify the effectiveness of the proposed strategies.