Christian Poellabauer

Analysis of a window-constrained scheduler for real-time and best-effort packet streams

Describes how dynamic window-constrained scheduling (DWCS) can guarantee real-time service to packets from multiple streams with different performance objectives. We show that: (1) DWCS can guarantee that no more than x packets miss their deadlines for every y consecutive packets requiring service, as long as the minimum aggregate bandwidth requirement of all real-time packet streams does not exceed the available bandwidth; (2) using DWCS, the delay of service to real-time packer streams is bounded even when the scheduler is overloaded; (3) DWCS can ensure that the delay bound of any given stream is independent of other streams; and (4) a fast response time for best-effort packet streams, in the presence of real-time packet streams, is possible. Furthermore, if a feasible schedule exists, each stream is guaranteed a minimum fraction of available bandwidth over a finite window of time.

Energy-aware traffic shaping for wireless real-time applications

Sleep modes of wireless network cards are used to switch these cards into low-power state when idle, but large timeout periods and frequent wake-ups can reduce the utility of this approach. Modern processors offer the ability to switch CPU voltages or clock frequencies and therefore reduce CPU energy consumption, however, that can reduce the sleep durations of a network device, adversely affecting the achievable energy savings. This paper describes an approach in which multiple resource managers cooperate to reduce a mobile devices energy consumption. This system-level approach is based on the integrated management of a real-time CPU scheduler, the frequency scaling capabilities of a modern processor, a QoS packet scheduler, and the low-power sleep mode of a wireless network card.

Cooperative run-time management of adaptive applications and distributed resources

This paper presents Q-fabric, which is a set of lightweight, kernel-level abstractions for cooperative, distributed resource management and system/application adaptation. The basis of Q-fabric is its kernel-level, anonymous, asynchronous event service. With this mechanism, (1) applications can monitor and manage the local and remote resources they are using, (2) system-level resource managers can customize their actions to meet the needs of individual applications, and (3) policies can be developed that combine application adaptation with distributed resource management. Results presented in this paper demonstrate the Q-fabrics ability to effectively adapt and manage the resources of a distributed multimedia application. In this application, media streams are adapted at application-level via data down-sampling, and their resources are managed at system-level (e.g., task scheduling) to cope with run-time variations in resource availability. The Q-fabric is implemented as kernel modules on standard Linux platforms.

Lessons learned from the netsense smartphone study

Over the past few years, smartphones have emerged as one of the most popular mechanisms for accessing content across the Internet driving considerable research to improve wireless performance. A key foundation for such research efforts is the proper understanding of user behavior. However, the gathering of live smartphone data at scale is often difficult and expensive. The focus of this paper is to explore the lessons learned from a two year study of two hundred smart phone users at the University of Notre Dame. In this paper, we offer commentary with regards to the entire process of the study covering aspects including funding considerations, technical architecture design, lessons learned, and recommendations for future efforts gathering live user data.

EMBARC: error model based adaptive rate control for vehicle-to-vehicle communications

Channel congestion is one of the major challenges for deployment of collision avoidance systems based on DSRC (Dedicated Short Range Communication) in large scale networks. If vehicles do not adapt to congestion conditions, DSRC transmissions could encounter extensive packet losses in areas of high vehicle density, leading to degradation in the performance of safety applications. In this paper, we propose a novel congestion control algorithm called Error Model Based Adaptive Rate Control (EMBARC) which adapts a vehicles transmission rate as a function of channel load and vehicular dynamics. In particular, we extend Linear Integrated Message Rate Control (LIMERIC) algorithms message rate adaptation with the capability to preemptively schedule messages based on the vehicles movement. This leads to more transmission opportunities for vehicles with higher dynamics. The determination of a preemptive scheduling event is based on a novel suspected tracking error technique. Since LIMERIC maintains the channel load around a specific value, vehicles moving less dynamically will adapt to slightly reduced transmission rates in EMBARC. The extra transmit opportunities for highly dynamic vehicles reduce incidences of large tracking error compared to a pure LIMERIC approach. At the same time, EMBARCs use of adaptive rate control provides tracking error advantages over systems that transmit largely independent of channel load. We use simulations of a road with a winding segment to compare EMBARC with algorithms that do not take both channel load and vehicle dynamics into account. The results show that EMBARC has the best tracking accuracy among these algorithms over a wide range of node densities.

Wireless reliability: Rethinking 802.11 packet loss

Wireless enabled devices are ubiquitous in todaypsilas computing environment. Businesses, universities, and home users alike are taking advantage of the easy deployment of wireless devices to provide network connectivity without the expense associated with wired connections. Unfortunately, the wireless medium is inherently unreliable resulting in significant work having been performed to better understand the characteristics of the wireless environment. Notably, many works attribute the primary source of wireless losses to errors in the physical medium. In contrast, our work shows that the wireless device itself plays a significant role in 802.11 packet loss. In our experiments, we found that the correlation of loss between multiple closely located (within one lambda) receivers is low with the majority of loss instances only occurring at one of the receivers. We conducted extensive experiments on the individual loss characteristics of five common wireless cards, showing that while the cards behave similarly on the macro-level (e.g. similar overall loss rates), the cards perform quite differently on the micro-level (e.g. burstiness, correlation, and consistency).

Resource-aware stream management with the customizable dproc distributed monitoring mechanisms

Monitoring the resources of distributed systems is essential to the successful deployment and execution of grid applications, particularly when such applications have well-defined QoS requirements. The dproc system-level monitoring mechanisms implemented for standard Linux kernels have several key components. First, utilizing the familiar /proc filesystem, dproc extends this interface with resource information collected from both local and remote hosts. Second, to predictably capture and distribute monitoring information, dproc uses a kernel-level group communication facility, termed KECho, which is based on events and event channels. Third and the focus of this paper is dprocs run-time customizability for resource monitoring, which includes the generation and deployment of monitoring functionality within remote operating system kernels. Using dproc, we show that: (a) data streams can be customized according to a clients resource availabilities (dynamic stream management); (b) by dynamically varying distributed monitoring (dynamic filtering of monitoring information), appropriate balance can be maintained between monitoring overheads and application quality; and (c) by performing monitoring at kernel-level, the information captured enables decision making that takes into account the multiple resources used by applications.

Energy-aware media transcoding in wireless systems

In distributed systems, transcoding techniques have been used to customize multimedia objects, utilizing trade-offs between the quality and sizes of these objects to provide differentiated services to clients. Our research uses transcoding techniques in wireless systems to customize video streams to the requirements of users, while minimizing the energy costs. We introduce an approach to dynamically determine which transcoders to execute and where to execute them (e.g., client or server). The goal is to select appropriate transcoders (a) to provide clients with the quality of service they desire while (b) minimizing the energy consumption of the end-hosts in accordance with application-specific global energy management directives. This paper investigates sample transcoder functions for video streaming on handheld devices and introduces a mechanism for selecting the most appropriate transcoders and transcoder parameters.

Power-aware video decoding using real-time event handlers

Multimedia applications have to receive sufficient resource allocations to maintain their desired levels of Quality of Service (QoS). On the other hand, in mobile environments, the devices on which these applications must run have to minimize power consumption to prolong battery life. Our work focuses on the QoS issues in the event-driven distribution of multimedia streams between mobile users, where a source provides interactive video in the form of streams of data events to multiple remote sinks. This paper addresses the power-aware execution of event handlers at such event sinks. In particular, an adaptive approach to the dynamic selection of a suitable CPU clock frequency of a mobile device is shown superior to non-adaptive power management. This approach (a) minimizes power consumption while also (b) guaranteeing that a given event handler finishes its execution within application-specific timing constraints. This is realized by dynamically measuring the progress of event handler functions and then using this information to re-adjust the clock frequency for the current event and to select appropriate clock frequencies for future events.

Lightweight kernel/user communication for real-time and multimedia applications

Operating system enhancements to support real-time and multimedia appl ications often include specializations and extensions of kernel functionality, as with the kernel HTTP daemon (khttpd) in Linux, for instance. To enable efficient and flexible interactions of such extensions with user-level functionality, we have developed ECalls, a lightweight, bidirectional kernel/user event delivery facility, which not only supports the timely delivery of events, but it also reduces the cost and frequency of kernel/user boundary crossings. ECalls is a communication tool that allows (a) kernel extensions to register their offered services and (b) applications to register their interest in these services. Using ECalls, applications use lightweight system calls to generate events, while kernel extensions raise real-time signals or invoke handler functions (residing in either user or kernel space), or they may use kernel threads to handle events on behalf of applications. ECalls can also influence the CPU scheduler such that a process with pending events is given preference over other processes. To demonstrate its utility, this paper implements an I/O event delivery mechanism using ECalls. This mechanism is shown to improve the performance of two applications: a distributed video player and a web server.