Abdelsalam Heddaya

Application-level document caching in the Internet

With the increasing demand for document transfer services such as the World Wide Web comes a need for better resource management to reduce the latency of documents in these systems. To address this need, we analyze the potential for document caching at the application level in document transfer services. We have collected traces of actual executions of Mosaic, reflecting over half a million user requests for WWW documents. Using those traces, we study the tradeoffs between caching at three levels in the system, and the potential for use of application-level information in the caching system. Our traces show that while a high hit rate in terms of URLs is achievable, a much lower hit rate is possible in terms of bytes, because most profitably-cached documents are small. We consider the performance of caching when applied at the level of individual user sessions, at the level of individual hosts, and at the level of a collection of hosts on a single LAN. We show that the performance gain achievable by caching at the session level (which is straightforward to implement) is nearly all of that achievable at the LAN level (where caching is more difficult to implement). However, when resource requirements are considered, LAN level caching becomes muck more desirable, since it can achieve a given level of caching performance using a much smaller amount of cache space. Finally, we consider the use of organizational boundary information as an example of the potential for use of application-level information in caching. Our results suggest that distinguishing between documents produced locally and those produced remotely can provide useful leverage in designing caching policies, because of differences in the potential for sharing these two document types among multiple users.<<ETX>>

WebWave: globally load balanced fully distributed caching of hot published documents

Document publication service over such a large network as the Internet challenges us to harness available server and network resources to meet fast growing demand. We show that large scale dynamic caching can be employed to globally minimize server idle time, and hence maximize the aggregate server throughput of the whole service. To be efficient, scalable and robust, a successful caching mechanism must have three properties: (1) maximize the global throughput of the system; (2) find cache copies without recourse to a directory service, or to a discovery protocol; and (3) be completely distributed in the sense of operating only on the basis of local information. We develop a precise definition, which we call tree load balance (TLB), of what it means for a mechanism to satisfy these three goals. We present an algorithm that computes TLB offline, and a distributed protocol that induces a load distribution that converges quickly to a TLB one. Both algorithms place cache copies of immutable documents on the routing tree that connects the cached documents home server to its clients, thus enabling requests to stumble on cache copies en route to the home server.

Support for Distributed Transactions in the TABS Prototype

The TABS prototype is an experimental facility that provides operating system-level support for distributed transactions that operate on shared abstract types. The facility is designed to simplify the construction of highly available and reliable distributed applications. This paper describes the TABS system model, the TABS prototypes structure, and certain aspects of its operation. The paper concludes with a discussion of the status of the project and a preliminary evaluation.

Mapping parallel iterative algorithms onto workstation networks

For communication-intensive parallel applications, the maximum degree of concurrency achievable is limited by the communication throughput made available by the network. In previous work (Heddaya et al., 1994) we showed experimentally that the performance of certain parallel applications running on a workstation network can be improved significantly if a congestion control protocol is used to enhance network performance. We characterize and analyze the communication requirements of a large class of supercomputing applications that fall under the category of fixed-point problems, amenable to solution by parallel iterative methods. This results in a set of interface and architectural features sufficient for the efficient implementation of the applications over a large-scale distributed system. In particular, we propose a direct link between the application and network layer, supporting congestion control actions at both ends. This in turn enhances the systems responsiveness to network congestion, improving performance. Measurements are given showing the efficacy of our scheme to support large-scale parallel computations.<<ETX>>

An overview of Mermera: a system and formalism for non-coherent distributed parallel memory

The authors give an overview of Mermera, a system that gives the programmer the choice of coherent and noncoherent behavior in the same program. They sketch a formal model that describes Mermeras noncoherent behavior. This model helps identify a new noncoherent behavior called local consistency. Measurements from a pilot implementation on a BBN Butterfly are reported which show the response time of pipelined-RAM operations to be an order of magnitude faster than that of coherent operations. Thus, a new tradeoff is discovered in which the programmer can significantly improve shared memory performance at the cost of tolerating varying degrees of noncoherence.<<ETX>>

Congestion control for asynchronous parallel computing on workstation networks

Asynchronous parallel computing can result in high message generation rates, thus triggering network congestion. We characterize the communication requirements of a large class of supercomputing applications falling under the category of fixed-point problems amenable to solution by parallel iterative methods. In particular, we concentrate on asynchronous iterative algorithms whose communication/computation ratio is especially high resulting in degraded effective throughput if communication is not managed properly. Second, we show the effects of network contention and asynchrony on application performance in a local-area network environment and investigate methods of solution. Our approach is based on a congestion control algorithm called ‘warp control’ whose adaptive properties are exploited to yield significant performance enhancements when network contention is high. Although tested in a LAN environment for experimental control purposes, our solution follows the end-to-end paradigm and refrains from exploiting special MAC-layer properties to achieve applicability to general WAN environments. Third, we provide a framework wherein efficient congestion control can be facilitated, encompassing methods acting at the application layer as well as the transport/network layer, with emphasis on application-driven control. We conclude with a discussion of our experimental results and special issues arising in high-bandwidth ATM networks.

Transaction Optimization Techniques

Replication introduces a tension between query optimization and remote access control in a distributed database system. If we view a transaction as a partially-ordered set of queries and updates, then factors that affect quorum selection for the fragments accessed by a transaction as a whole are currently orthogonal to factors that affect the replica selection during the planning of individual queries. Therefore, the two processes may act at cross-purposes to one another. Query optimization considers an individual query and selects a set of fragments that minimizes the computation and communication cost and allows computation to be pushed into the local site. Transaction management, on the other hand, selects quorums (sets of replicas to retrieve) based on replica availability and on mutual consistency constraints such as quorum intersection among write operations or between read and write operations. Thus, transaction optimization narrows the “optimal” solution space for the queries it contains Hence, transaction management should cooperate with query optimization to optimize transaction processing.