Charles Krasic

Quality-adaptive media streaming by priority drop

This paper presents a general design strategy for streaming media applications in best effort computing and networking environments. Our target application is video on demand using personal computers and the Internet. In this scenario, where resource reservations and admission control mechanisms are not generally available, effective streaming must be able to adapt in a responsive and graceful manner. The design strategy we propose is based on a single simple idea, priority data dropping, or priority drop for short. We evaluate the efficacy of priority drop as an adaptation tool in the video and networking domains. Our technical contribution with respect to video is to show how to express adaptation policies and how to do priority-mapping, an automatic translation from adaptation policies to priority assignments on the basic units of video. For the networking domain, we present priority-progress streaming, a real-time best-effort streaming protocol. We have implemented and released a prototype video streaming system that incorporates priority-drop video, priority mapping, and priority-progress streaming. Our system demonstrates a simple encode once, stream anywhere model where a single video source can be streamed across a wide range of network bandwidths, on networks saturated with competing traffic, all the while maintaining real-time performance and gracefully adapting quality.

Specialization tools and techniques for systematic optimization of system software

Specialization has been recognized as a powerful technique for optimizing operating systems. However, specialization has not been broadly applied beyond the research community because current techniques based on manual specialization, are time-consuming and error-prone. The goal of the work described in this paper is to help operating system tuners perform specialization more easily. We have built a specialization toolkit that assists the major tasks of specializing operating systems. We demonstrate the effectiveness of the toolkit by applying it to three diverse operating system components. We show that using tools to assist specialization enables significant performance optimizations without error-prone manual modifications. Our experience with the toolkit suggests new ways of designing systems that combine high performance and clean structure.

A measurement-based analysis of the real-time performance of linux

This paper presents an experimental study of the latency behavior of the Linux OS. We identify major sources of latency in the kernel with the goal of providing real-time performance in a widely used general-purpose operating system. We quantify, each source of latency with a series of micro-benchmarks and also evaluate the effects of latency on a time-sensitive application. Our analysis shows that there are two main causes of latency in the OS: tinier resolution and non-preemptable sections. Our experiments show that in the standard Linux kernel the tinier resolution latency is predominant, and generally hides the non-preemptable section latency We use accurate timers to reduce timer resolution latency and then analyze the non-preemptable section latency for several variants of Linux.

Adaptive live video streaming by priority drop

In this paper we explore the use of priority progress streaming (PPS) for video surveillance applications. PPS is an adaptive streaming technique for the delivery of continuous media over variable bit-rate channels. It is based on the simple idea of reordering media components within a time window into priority order before transmission. The main concern when using PPS for live video streaming is the time delay introduced by reordering. In this paper we describe how PPS can be extended to support live streaming and show that the delay inherent in the approach can be tuned to satisfy a wide range of latency constraints while supporting fine-grain adaptation.

Supporting low latency TCP-based media streams

The dominance of the TCP protocol on the Internet and its success in maintaining Internet stability has led to several TCP-based stored media-streaming approaches. The success of these approaches raises the question whether TCP can be used for low-latency streaming. Low latency streaming allows responsive control operations for media streaming and can make interactive applications feasible. We examined adapting the TCP send buffer size based on TCPs congestion window to reduce application perceived network latency. Our results show that this simple idea significantly improves the number of packets that can be delivered within 200 ms and 500 ms thresholds.

Supporting time-sensitive applications on a commodity OS

Commodity operating systems are increasingly being used for serving time-sensitive applications. These applications require low-latency response from the kernel and from other system-level services. In this paper, we explore various operating systems techniques needed to support time-sensitive applications and describe the design of our Time-Sensitive Linux (TSL) system. We show that the combination of a high-precision timing facility, a well-designed preemptible kernel and the use of appropriate scheduling techniques is the basis for a low-latency response system and such a system can have low overhead. We evaluate the behavior of realistic time-sensitive user- and kernel-level applications on our system and show that, in practice, it is possible to satisfy the constraints of time-sensitive applications in a commodity operating system without significantly compromising the performance of throughput-oriented applications.

Fast concurrent dynamic linking for an adaptive operating system

The need for customizable and application-specific operating systems has been recognized for many years. A customizable operating system is one that can adapt to some particular circumstance to gain some functional or performance benefits. Microkernels have attempted to address this problem, but suffer performance degradation due to the cost of inter-process protection barriers. Commercial operating systems that can efficiently adapt themselves to changing circumstances have failed to appear, in part due to the difficulty of providing an interface that is efficient to invoke, provides a protection barrier, and can be dynamically reconfigured. Providing such a safe, efficient, and dynamic interface in a concurrent operating system requires an effective concurrency control mechanism to prevent conflicts between system components proposing to execute specialized components, and those components responsible for dynamically replacing specialized components. We outline our basic approach to specialization of operating systems, and detail our dynamic replacement mechanism and its concurrency control features.

Quality of Service Semantics for Multimedia Database Systems

Quality of service (QoS) support has been a hot research topic in multimedia databases, and multimedia systems in general, for the past several years. However, there remains little consensus on how QoS support should be provided. At the resource-management level, systems designers are still debating the suitability of reservation-based versus adaptive QoS management. The design of higher system layers is less clearly understood, and the specification of QoS requirements in domain-specific terms is still an open research topic. To address these issues, we propose a QoS model for multimedia databases. The model covers the specification of user-level QoS preferences and their relationship to QoS control at the resource-management level, and is applicable to adaptive and reservation-based systems. In this paper we present the model, discuss the implications it has for multimedia database design, and describe a practical implementation of it.

Low-latency adaptive streaming over tcp

Media streaming over TCP has become increasingly popular because TCPs congestion control provides remarkable stability to the Internet. Streaming over TCP requires adapting to bandwidth availability, but unforunately, TCP can introduce significant latency at the application level, which causes unresponsive and poor adaptation. This article shows that this latency is not inherent in TCP but occurs as a result of throughput-optimized TCP implementations. We show that this latency can be minimized by dynamically tuning TCPs send buffer. Our evaluation shows that this approach leads to better application-level adaptation and it allows supporting interactive and other low-latency applications over TCP.

Priority-progress streaming for quality-adaptive multimedia

The Internets ubiquity amply motivates us to harness it for video distribution, however, its best-effort service model is in direct conflict with videos inherent timeliness requirements. Today, the Internet is unrivaled in its rich composition, consisting of an unparalleled assortment of networks and hosts. This richness is the result of an architecture that emphasizes interoperability over predictable performance. From the lowest levels, the Internet architecture prefers the best effort service model. We feel current solutions for media-streaming have yet to adequately address this conflict between timeliness and best-effort service.We propose that streaming-media solutions targetted at the Internet must fully embrace the notion of graceful degradation, they must be architected with the expectation that they operate within a continuum of service levels, adjusting quality-resource trade-offs as necessary to achieve timeliness requirements. In the context of the Internet, the continuum of service levels spans across a number oftime scales, ranging from sub-second timescales to timescales as long as months and years. We say sub-second timescales in relation to short-term dynamics such as network traffic and host workloads, while timescales of months and years relate to the continuous deployment of improving network, compute and storage infrastructure.We support our thesis with a proposal for a streaming model which we claim is simple enough to use end-to-end, yet expressive enough to tame the conflict between real-time and best-effort personalities of Internet streaming. The model is called Priority-Progress streaming. In this proposal, we will describe the main features of Priority-Progress streaming, which we have been implemented in a software-based streaming video system, called the Quasar pipeline.Our work is primarily concerned with the class of streaming applications. To prevent confusion, we now clarify the important distinction between streaming and other forms of distribution, namely download. For a video, we assume download is defined so that the transfer of the video must complete before the video is viewed. Transfer and viewing are temporally sequential. With this definition, it is a simple matter to employ Quality-adaptive video. One algorithm would be to deliver the entire video in the order from low to high quality components. The user may terminate the download early, and the incomplete video will automatically have as high quality as was possible. Thus, Quality-adaptive download can be implemented in an entirely best-effort, time-insensitive, fashion. On the other hand, we assume streaming means that the user views the video at the same time that the transfer occurs. Transfer and viewing are concurrent. There are timeliness requirements inherent in this definition, which can only be reconciled with best-effort delivery through a time-sensitive adaptive approach.