Multimedia applications have timing requirements that cannot generally be satisfied using the time-sharing scheduling algorithms of general purpose operating systems. Our approach is to provide the predictability of real-time systems while retaining the flexibility of a timesharing system. We designed a processor capacity reservation mechanism that isolates programs from the timing and execution characteristics of other programs in the same way that a memory protection system isolates them from outside memory accesses. In this paper, we describe a scheduling framework that supports reservation and admission control, and we introduce a novel reserve abstraction, specifically designed for the microkernel architecture, for measuring and controlling processor usage. We have implemented processor capacity reserves in Real-Time Mach, and we describe the performance of our system on several types of applications. 1

AbstractÐReal-time middleware services must guarantee predictable performance under specified load and failure conditions, and ensure graceful degradation when these conditions are violated. Guaranteed predictable performance typically entails reservation of resources and use of admission control. Graceful degradation, on the other hand, requires dynamic reallocation of resources to maximize the application-perceived system utility while coping with unanticipated overload and failures. We propose a model for quality-of-service (QoS) negotiation in building real-time services to meet both of the above requirements. QoS negotiation is shown to 1) outperform ªbinaryº admission control schemes (either guaranteeing the required QoS or rejecting the service request), 2) achieve higher application-perceived system utility, and 3) deal with violations of the load and failure hypotheses. We incorporated the proposed QoS-negotiation model into an example real-time middleware service, called RTPOOL, which manages a distributed pool of shared computing resources (processors) to guarantee timeliness QoS for real-time applications. In order to guarantee timeliness QoS, the resource pool is encapsulated with its own schedulability analysis, admission control, and load-sharing support. This support differs from others in that it adheres to the proposed QoS-negotiation model. The efficacy and power of QoS negotiation are demonstrated for an automated flight control system implemented on a network of PCs running RTPOOL. This system is used to fly an F-16 fighter aircraft modeled using the Aerial Combat (ACM) F-16 Flight Simulator. Experimental results indicate that QoS negotiation, while maintaining real-time guarantees, enables graceful QoS degradation under conditions in which traditional schedulability analysis and admission control schemes fail. Index TermsÐQuality-of-service (QoS), QoS negotiation, QoS levels and rewards, schedulability analysis and admission control, automated flight systems. 1

This paper summarizes the state of the real-time field in the areas of scheduling and operating system kernels. Given the vast amount of work that has been done by both the operations research and computer science communities in the scheduling area, we discuss four paradigms underlying the scheduling approaches and present several exemplars of each. The four paradigms are: static table-driven scheduling, static priority preemptive scheduling, dynamic planning-based scheduling, and dynamic best efSort scheduling. In the operating system context, we argue that most of the proprietary commercial kernels as well as real-time extensions to time-sharing operating system kernels do not fit the needs of predictable real-time systems. We discuss several research kernels that are currently being built to explicitly meet the needs of real-time applications. I.

Scheduling theory holds great promise as a means to a priori validate timing correctness of real-time applications. However, there currently exists a wide gap between scheduling theory and its implementation in operating system kernels running on specific hardware platforms. The implementation of any particular scheduling algorithm introduces overheadand blocking components which must be accounted for in the timing correctness validation process. This paper presents a methodology for incorporating the costs of scheduler implementation within the context of fixed priority scheduling algorithms. Both event-driven and timerdriven scheduling implementations are analyzed. We show that for the timer-driven scheduling implementations the selection of the timer interrupt rate can dramatically affect the schedulability of a task set, and we present a method for determining the optimal timer rate. We analyzed both randomly generated and two well defined task sets and found that their schedulabil...

Abstract: This paper describes the Chimera II Real-time Operating System, which has been developed for advanced sensor-based control applications. It has been designed as a local operating system, to be used in conjunction with a global operating system. It executes on one or more single board computers in a VMEbus-based system. Advanced sensor-based control systems are both statically and dynamically reconfigurable. As a result, they require many special features, which are currently not found in commercial real-time operating systems. In this paper, we present several design issues for such systems, and we also present the features we have developed and implemented as part of Chimera II. These features include a real-time kernel with dynamic scheduling, global error handling, user signals, and two levels of device drivers; an enhanced collection of interprocessor communication mechanisms, including global shared memory, spin-locks, remote semaphores, priority message passing, global state variable tables, multiprocessor servo task control, and host workstation integration; and several support utilities, including a UNIX C and math libraries, a matrix library, a command interpreter library, and a configuration file library. Chimera II is currently being used with a variety of systems, including the CMU Direct Drive Arm II, the CMU Reconfigurable Modular Manipulator System, the Troikabot System for Rapid Assembly, and the Self-Mobile Space Manipulator. I.

Portable RK is a portable implementation of a resource kernel, a resource-centric approach to build a real-time kernel that provides explicit timely, guaranteed and enforced access by applications to system resources [13]. Portable RK is designed to work with widely available operating systems with minimal changes. This facilitates experimentation in a familiar software environment and helps the faster deployment of research results. Execution in resource kernels is directly based on OS-enforced resource reservation [7]. As a result, an application can request the reservation of a certain amount of a resource, and the kernel can guarantee that the requested amount is exclusively available to that application. In this paper, we describe the design and implementation of Portable RK called Linux/RK that resides within the Linux operating system. The evaluation results show that Portable RK in the form of Linux/RK gives direct control over timely resource utilization by applications and that its overhead costs are small enough to be negligible. 1

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This paper seeks to bridge the gap between theory and practice of real-time scheduling in the domain of high speed multimedia networking. We show that the strict preemptive nature of real-time scheduling leads to more context switching, and requires system calls for concurrency control. We present our scheduling scheme called rate-monotonic with delayed preemption (rmdp) and show how it reduces both these overheads. We then develop the analytical framework to analyze rmdp and other scheduling schemes that lie in the region between strict (immediate) preemption and no preemption. Our idealized scheduler simulation methodology accounts for the blocking introduced by these schemes under the usual assumption that the time for context switching and preemption is zero. We derive simpler schedulability tests for non-preemptive scheduling, and prove a variant of rate-monotonic scheduling that has fewer preemptions. Our measurements on Sparc and Pentium platforms, show that for the workloads w...

The advent of performance-critical services such as online brokerage and e-commerce, as well as QoS-sensitive services such as streaming multimedia, makes existing FIFO servers incapable of meeting application QoS requirements. Re-designing server code to support QoS provisioning, on the other hand, is costly and time-consuming. To remedy this problem, we propose a new QoS-provisioning approach that does not require modification of server and OS code. We develop a middleware, called qContracts, that can be transparently interposed between the server process and the operating system to achieve performance differentiation and soft QoS guarantees. The middleware enables reuse of existing legacy software in QoS-sensitive contexts, and off-loads QoS management concerns from future real-time service programmers. As an example, we show how the Apache [9] web server is endowed with QoS support using qContracts on UNIX. Experimental results show the efficacy of the middleware in achieving the c...

EMERALDS (Extensible Microkernel for Embedded, ReAL-time, Distributed Systems) is a real-time microkernel designed for small-memory embedded applications. These applications must run on slow (15--25MHz) processors with just 32--128 kbytes of memory, either to keep production costs down in mass-produced systems or to keep weight and power consumption low. To be feasible for such applications, the OS must not only be small in size (less than 20 kbytes) but also have low-overhead kernel services. Unlike commercial embedded OSs which rely on carefully-optimized code to achieve efficiency, EMERALDS takes the approach of redesigning the basic OS services of task scheduling, synchronization, communication, and system call mechanism by using characteristics found in small-memory embedded systems such as small code size and a priori knowledge of task execution & communication patterns. With these new schemes, the overheads of various OS services are reduced 20--40% without compromising any OS functionality.