Location-based services (LBS) play an increasing role in mobile systems and applications. In this paper we focus on LBS in a multi-provider network. Using multiple providers reduces dependencies, increases geographic and functional coverage, and allows distribution, load balancing, and fail-over. However, the framework must ensure substitutability of services. We describe the use of a rule engine to dynamically determine a near-optimal provider for each request. The choice is based on performance, reliability, geographic coverage, as well as more intangible considerations. Overall performance, coverage, and distribution can thus be increased at the framework-level.

The location of a mobile user is fundamental to all location-based services. To enable these services the mobile middleware platform needs to access the user's location. A a number of vendors such as Ericsson and Nokia provide mobile positioning services (MPS) that can be used to locate the user's mobile device. Push services, triggered by the user's location, also put heavy demands on the MPS. GPS technology and network based positioning has been researched in detail. Great strides are being made in improving the performance and accuracy of determining the position of a mobile device. This paper looks at performance and scalability issues in accessing positioning information from the middleware platform. We discuss an implementation where we allow the application to specify the accuracy of positioning and then use the user's current location and velocity to determine the location caching policy.

Abstract We study the use of non‐volatile memory for caching in distributed file systems. This provides an advantage over traditional distributed file systems in that the load is reduced at the server without making the data vulnerable to failures. We propose the use of a small non‐volatile cache for writes, at the client and the file server, together with a larger volatile read cache to keep the cost of the caches reasonable. We use a synthetic workload developed from analysis of file I/O traces from commercial production systems and use a detailed simulation of the distributed environment. The service times for the resources of the system were derived from measurements performed on a typical workstation. We show that non‐volatile write caches at the clients and the file server reduce the write response time and the load on the file server dramatically, thus improving the scalability of the system. We examine the comparative benefits of two alternative writeback policies for the non‐volatile write cache. We show that a proposed threshold based writeback policy is more effective than a periodic writeback policy under heavy load. We also investigate the effect of varying the write cache size and show that introducing a small non‐volatile cache at the client in conjunction with a moderate sized non‐volatile server write cache improves the write response time by a factor of four at all load levels.

The authors study the use of non-volatile memory for caching in distributed file systems. This provides an advantage over traditional distributed file systems in that the load is reduced at the server without making the data vulnerable to failures. They show that small non-volatile write caches at the clients and the server are quite effective. They reduce the write response time and the load on the file server dramatically, thus improving the scalability of the system. They show that a proposed threshold based writeback policy is more effective than a periodic writeback policy. They use a synthetic workload developed from analysis of file I/O traces from commercial production systems. The study is based on a detailed simulation of the distributed environment. The service times for the resources of the system were derived from measurements performed on a typical workstation.>

The I/O subsystem in a computer system is becoming the bottleneck as a result of recent dramatic improvements in processor speeds. Disk caches have been effective in closing this gap but the benefit is restricted to the read operations as the write I/Os are usually committed to disk to maintain consistency and to allow for crash recovery. As a result, write I/O traffic is becoming dominant and solutions to alleviate this problem are becoming increasingly important. A simple solution which can easily work with existing tile systems is to use non-volatile disk caches together with a write-behind strategy. In this study, we look at the issues around managing such a cache using a detailed trace driven simulation. Traces from three different commercial sites are used in the analysis of various policies for managing the write cache.We observe that even a simple write-behind policy for the write cache is effective in reducing the total number of writes by over 50%. We further observe that the use of hysteresis in the policy to purge the write cache, with two thresholds, yields substantial improvement over a single threshold scheme. The inclusion of a mechanism to piggyback blocks from the write cache with read miss I/Os further reduces the number of writes to only about 15% of the original total number of write operations. We compare two piggybacking options and also study the impact of varying the write cache size. We briefly looked at the case of a single non-volatile disk cache to estimate the performance impact of statically partitioning the cache for reads and writes.

Improving the performance of the file system is becoming increasingly important to alleviate the effect of I/O bottlenecks in computer systems. To design changes to an existing file system or to architect a new file system it is important to understand current usage patterns. In this paper we analyze file I/O traces of several existing production computer sytems to understand file access behavior.

A comprehensive analysis of file access behavior of several commercial production VAX/VMS environments, based on detailed I/O traces, is presented. The characterization focuses on file control operations (such as opens and closes) which are often expensive in both processing and I/O requirements. The motivation for this analysis is the need to design good data management algorithms for distributed file systems. These algorithms are particularly important for managing client caches. Most of the operations for managing client caches are performed on file opens and closes. The authors present quantitative data on the number of file opens and closes, their timing characteristics, and their declared intent to allow sharing or have exclusive access. The precise dynamic sharing of files is examined.>