Real-time Concurrent C: A language for programming dynamic real-time systems

Concurrent C, is a parallel superset of C (and of C++) that provides facilities such as specifying timeouts during process interactions, delaying program execution, accepting messages in a user-specified order, and asynchronous messages that can be used for writing real-time programs. However, Concurrent C does not provide facilities for specifying strict timing constraints, e.g., Concurrent C only ensures that the lower bounds on the specified delay and timeout periods are satisfied. Real-Time Concurrent C extends Concurrent C by providing facilities to specify periodicity or deadline constraints, to seek guarantees that timing constraints will be met, and to perform alternative actions when either the timing constraints cannot be met or the guarantees are not available.In this paper, we will discuss requirements for a real-time programming language, briefly summarize Concurrent C, and motivate and describe the real-time extensions to Concurrent C. We also discuss scheduling and other run-time facilities that have been incorporated to support the real-time extensions. A prototype implementation of Real-Time Concurrent C is nearing completion.     

Efficient scheduling algorithms for real-time multiprocessorsystems

Efficient scheduling algorithms based on heuristic functions are developed for scheduling a set of tasks on a multiprocessor system. The tasks are characterized by worst-case computation times, deadlines, and resources requirements. Starting with an empty partial schedule, each step of the search extends the current partial schedule by including one of the tasks yet to be scheduled. The heuristic functions used in the algorithm actively direct the search for a feasible schedule, i.e. they help choose the task that extends the current partial schedule. Two scheduling algorithms are evaluated by simulation. To extend the current partial schedule, one of the algorithms considers, at each step of the search, all the tasks that are yet to be scheduled as candidates. The second focuses its attention on a small subset of tasks with the shortest deadlines. The second algorithm is shown to be very effective when the maximum allowable scheduling overhead is fixed. This algorithm is hence appropriate for dynamic scheduling in real-time systems     

Allocation and scheduling of precedence-related periodic tasks

This paper discusses a static algorithm for allocating and scheduling components of periodic tasks across sites in distributed systems. Besides dealing with the periodicity constraints, (which have been the sole concern of many previous algorithms), this algorithm handles precedence, communication, as well as replication requirements of subtasks of the tasks. The algorithm determines the allocation of subtasks of periodic tasks to sites, the scheduled start times of subtasks allocated to a site, and the schedule for communication along the communication channel(s). Simulation results show that the heuristics and search techniques incorporated in the algorithm are very effective     

On earliest deadline first scheduling for temporal consistency maintenance

A real-time object is one whose state may become invalid with the passage of time. A temporal validity interval is associated with the object state, and the real-time object is temporally consistent if its temporal validity interval has not expired. Clearly, the problem of maintaining temporal consistency of data is motivated by the need for a real-time system to track its environment correctly. Hence, sensor transactions must be able to execute periodically and also each instance of a transaction should perform the relevant data update before its deadline. Unfortunately, the period and deadline assignment problem for periodic sensor transactions has not received the attention that it deserves. An exception is the More-Less scheme, which uses the Deadline Monotonic (DM) algorithm for scheduling periodic sensor transactions. However, there is no work addressing this problem from the perspective of dynamic priority scheduling. In this paper, we examine the problem of temporal consistency maintenance using the Earliest Deadline First (EDF) algorithm in three steps: First, the problem is transformed to another problem with a sufficient (but not necessary) condition for feasibly assigning periods and deadlines. An optimal solution for the problem can be found in linear time, and the resulting processor utilization is characterized and compared to a traditional approach. Second, an algorithm to search for the optimal periods and deadlines is proposed. The problem can be solved for sensor transactions that require any arbitrary deadlines. However, the optimal algorithm does not scale well when the problem size increases. Hence, thirdly, we propose a heuristic search-based algorithm that is more efficient than the optimal algorithm and is capable of finding a solution if one exists.

The spring scheduling coprocessor: a scheduling accelerator

The spring scheduling coprocessor is a novel very large scale integration (VLSI) accelerator for multiprocessor real-time systems. The coprocessor can be used for static as well as online scheduling. Many different policies and their combinations can be used (e.g., earliest deadline first, highest value first, or resource-oriented policies such as earliest available time first). In this paper, we describe a coprocessor architecture, a CMOS implementation, an implementation of the host/coprocessor interface and a study of the overall performance improvement. We show that the current VLSI chip speeds up the main portion of the scheduling operation by over three orders of magnitude. We also present an overall system improvement analysis by accounting for the operating system overheads and identify the next set of bottlenecks to improve. The scheduling coprocessor includes several novel VLSI features. It is implemented as a parallel architecture for scheduling that is parameterized for different numbers of tasks, numbers of resources, and internal wordlengths. The architecture was implemented using a single-phase clocking style in several novel ways. The 328 000 transistor custom 2-μm VLSI accelerator running with a 100-MHz clock, combined with careful hardware/software co-design results in a considerable performance improvement, thus removing a major bottleneck in real-time systems     

Scalable consistency maintenance in content distribution networks using cooperative leases

We argue that cache consistency mechanisms designed for stand-alone proxies do not scale to the large number of proxies in a content distribution network and are not flexible enough to allow consistency guarantees to be tailored to object needs. To meet the twin challenges of scalability and flexibility, we introduce the notion of cooperative consistency along with a mechanism, called cooperative leases, to achieve it. By supporting /spl Delta/-consistency semantics and by using a single lease for multiple proxies, cooperative leases allow the notion of leases to be applied in a flexible, scalable manner to CDNs. Further, the approach employs application-level multicast to propagate server notifications to proxies in a scalable manner. We implement our approach in the Apache Web server and the Squid proxy cache and demonstrate its efficacy using a detailed experimental evaluation. Our results show a factor of 2.5 reduction in server message overhead and a 20 percent reduction in server state space overhead when compared to original leases albeit at an increased interproxy communication overhead.     

Scheduling algorithms and operating systems support for real-timesystems

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 effort 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 realtime systems. We discuss several research kernels that are currently being built to explicitly meet the needs of real-time applications     

Adaptive Dissemination of Data in Time-Critical Asymmetric Communication Environments

The proliferation of new data-intensive applications in asymmetric communication environments has led to an increasing interest in the development of push-based techniques, in which the information is broadcast to a large population of clients in order to achieve the most efficient use of the limited server and communication resources. It is important to note that quite often the data that is broadcast is time-critical in nature.Most of the related current research focuses on a pure push-based approach (Broadcast Disks model), where the transmission of data is done without allowing explicit requests from the users. More recently, some bidirectional models incorporating a low-capacity uplink channel have been proposed in order to increase the functionality of the Broadcast Disks model. However, the impact of integration of the uplink channel has been investigated using only static client profiles or ignoring the existence of time-sensitive data. None of the existing models integrates all the characteristics needed to perform effectively in a real-world, dynamic time-critical asymmetric communication environment.In this paper we present an adaptive data dissemination model and the associated on-line scheduling algorithms. These improve the functionality and performance of bidirectional broadcast models, maximizing the total number of satisfied users in asymmetric communication environments with dynamic client profiles and time requirements (e.g., mobile systems). This is achieved by means of dynamic adaptation of the broadcast program to the needs of the users, taking into account the bandwidth constraints inherent in asymmetric communication environments and the deadline requirements of the user requests. Performance is evaluated by simulation of a real-time asymmetric communication environment.