Joseph Pasquale

Multicast routing for multimedia communication

The authors present heuristics for multicast tree construction for communication that depends on: bounded end-to-end delay along the paths from source to each destination and minimum cost of the multicast tree, where edge cost and edge delay can be independent metrics. The problem of computing such a constrained multicast tree is NP-complete. It is shown that the heuristics demonstrate good average case behavior in terms of cost, as determined by simulations on a large number of graphs. >

The importance of non-data touching processing overheads in TCP/IP

We present detailed measurements of various processing overheads of the TCP/IP and UDP/IP protocol stacks on a DECstation 5000/200 running the Ultrix 4.2a operating system. These overheads include data-touching operations, such as the checksum computation and data movement, which are well known to be major time consumers. In this study, we also considered overheads due to non-data touching operations, such as network buffer manipulation, protocol-specific processing, operating system functions, data structure manipulations (other than network buffers), and error checking. We show that when one considers realistic message size distributions, where the majority of messages are small, the cumulative time consumed by the non-data touching overheads represents the majority of processing time. We assert that it will be difficult to significantly reduce the cumulative processing time due to non-data touching overheads.

Multicasting for multimedia applications

The authors investigate multicast routing for high-bandwidth delay-sensitive applications in a point-to-point network as an optimization problem. They associate an edge cost and an edge delay with each edge in the network. The problem is to construct a tree spanning the destination nodes, such that it has the least cost, and so that the delay on the path from the source to each destination is bounded. Since the problem is computationally intractable, the authors present an efficient approximation algorithm. Experimental results through simulations show that the performance of the heuristic is near optimal.<<ETX>>

A high performance multi-structured file system design

File system I/O is increasingly becoming a performance bottleneck in large distributed computer systems. This is due to the increased file I/O demands of new applications, the inability of any single storage structure to respond to these demands, and the slow decline of, disk access times (latency and seek) relative to the rapid increase in CPU speeds, memory size, and network bandwidth.We present a multi-structured file system designed for high bandwidth I/O and fast response. Our design is based on combining disk caching with three different file storage structures, each implemented on an independent and isolated disk array. Each storage structure is designed to optimize a different set of file system access characteristics such as cache writes, directory searches, file attribute requests or large sequential reads/writes.As part of our study, we analyze the performance of an existing file system using trace data from UNIX disk I/O-intensive workloads. Using trace driven simulations, we show how performance is improved by using separate storage structures as implemented by a multi-structured file system.

Stratified round Robin: a low complexity packet scheduler with bandwidth fairness and bounded delay

Fair queuing is a well-studied problem in modern computer networks. However, there remains a gap between scheduling algorithms that have provably good performance, and those that are feasible and practical to implement in high speed routers. In this paper, we propose a novel packet scheduler called Stratified Round Robin, which has low complexity, and is amenable to a simple hardware implementation. Stratified Robin Robin exhibits good fairness and delay properties that are demonstrated through both analytical results and simulations. In particular, it provides a single packet delay bound that is independent of the number of flows. This property is unique to Stratified Round Robin among all other schedulers of comparable complexity.

The multimedia multicasting problem

Abstract. This paper explores the problems associated with the multicasting of continuous media to support multimedia group applications. The interaction between multicasting and the delivery of multiple time-correlated continuous-media streams with real-time delay requirements poses various new and interesting problems in research on communication protocols and architectures. We describe these problems, and identify where the opportunities are for effective solutions, all in the context of providing an overview of the current state of research in multimedia multicasting. The issues we discuss include quality of service, resource reservations, routing, error and traffic control, heterogeneity, and the use of hierarchical coding and open-loop control techniques.

Analysis of Long-Running Replicated Systems

We address the problem of using replication to reliably maintain state in a distributed system for time spans that far exceed the lifetimes of individual replicas. This scenario is relevant for any system comprised of a potentially large and selectable number of replicated components, each of which may be highly unreliable, where the goal is to have enough replicas to keep the system “alive” (meaning at least one replica is working or available) for a certain expected period of time, i.e., the system’s lifetime. In particular, this applies to recent efforts to build highly available storage systems based on the peer-to-peer paradigm. We model notions of replica loss and replica repair in such systems by a simple Markov chain model, and derive an expression for the lifetime of the replicated state. We then apply this model to study the impact of practical considerations like storage and bandwidth limits on the system, and describe methods to optimally choose system parameters so as to maximize lifetime. Our analysis sheds light on the efficacy of various replication strategies.

Profiling and reducing processing overheads in TCP/IP

This paper presents detailed measurements of processing overheads for the Ultrix 4.2a implementation of TCP/IP network software running on a DECstation 5000/200. The performance results were used to uncover throughput and latency bottlenecks. We present a scheme for improving throughput when sending large messages by avoiding most checksum computations in a relatively safe manner. We also show that for the implementation we studied, reducing latency (when sending small messages) is a more difficult problem because processing overheads are spread over many operations; gaining a significant savings would require the optimization of many different mechanisms. This is especially important because, when processing a realistic workload, we have found that nondata-touching operations consume more time in aggregate than data-touching operations.

An upper bound on delay for the VirtualClock service discipline

Proves that a connection composed of virtual-clock servers provides an upper bound on delay for leaky bucket constrained sessions, i.e., sessions conforming to a token bucket filter. This upper bound on delay is calculated, and it is the same upper bound on delay given by PGPS. The authors also prove that leaky bucket constrained sessions are the only type of sessions for which an upper bound on delay can be provided by servers with an upper bound on link capacity. >

Container shipping: operating system support for I/O-intensive applications

New I/O devices with data rates ranging from 10 to 100 Mbytes per second are becoming available for personal computers and workstations. Along with continual improvements in processor, memory, and bus technology, these devices have enabled I/O-intensive applications for desktop computing that require input, processing, and output of very large amounts of data. We focus on an important aspect of operating system support for these applications: efficient transfer of large data objects between the protection domains in which processes and devices reside. A rapidly growing class of I/O-intensive applications is multimedia computing. After we present an I/O-pipeline model, we analyze issues relevant to the design of an operating system inter-domain data-transfer facility. Then we present the design for such a facility. An I/O pipeline is a model of a dynamic computation structure consisting of a sequence of domains: an input domain followed by one or more intermediate domains, and an output domain.<<ETX>>