Real-Time Analysis of Servers for General Job Arrivals

2015

Real-time systems are traditionally classified into hard real-time and soft real-time: in the first category we have safety critical real-time systems where missing a deadline can have catastrophic consequences, whereas in the second class we find systems or which we need to optimise the Quality of service provided to the user. However, the frontier between these two classes is thinner than one may think, and many systems that were considered as hard real-time in the past should now be reconsidered under a different light. In this paper we shall first recall the fundamental notion of time-predictability and criticality, in order to understand where the real-time deadlines that we use in our theoretical models come from. We shall then introduce the model of a soft real-time system and present one popular method for scheduling hard and soft real-time tasks, the resource reservation framework. Finally, we shall show how resource reservation techniques can be successfully applied to the...

Resource reservations have proven an effective technique to support hard and soft real-time appli-cations in open systems, and some implementations for Linux have already been proposed in the past. However, such implementations generally focus on providing guarantees to real-time applications, dis-regarding the performance of non real-time activities. In this paper, the problems encountered using a reservation-based scheduling algorithm in Linux (the Constant Bandwidth Server) are described, showing why the original algorithm is not suitable for scheduling non real-time activities. Then, the properties required for properly scheduling non real-time tasks are described, and a novel algorithm (called HGRUB) is analysed, showing how it effectively addresses the presented issues. The performance of HGRUB are then evaluated (by using our implementation in Linux) and compared with the performance of traditional reservation systems.

Electronic Notes in Theoretical Computer Science, 2008

The Flexible Resource Manager (FRM) is a dynamic resource management approach that allows a better utilization of the available resources. However, it necessitates an atomic reconfiguration process that must not violate hard timing constraints. This paper exploits the deadline assignment rule of the Total Bandwidth Server (TBS) to schedule reconfiguration, and it formally shows that there exists a minimum task period for which atomicity and schedulability can be guaranteed. With this solution, real-time system engineers have the tools at hand to design their tasks to exploit the benefits of the FRM with hard real-time constraints. a a This work was developed in the course of the Collaborative Research Center 614-Self-Optimizing Concepts and Structures in Mechanical Engineering-Paderborn University, and was published on its behalf and funded by the Deutsche Forschungsgemeinschaft.

1997

We investigate into the validity of the rate monotonic analysis techniques for distributed hard real time systems. A recent paper has shown that the algorithm developed by K. Tindell and J. Clark (1994) for the analysis of this kind of system was incomplete because it did not test all the possible cases. We prove that the algorithm is valid as it is stated and that it effectively obtains an upper bound for the worst case response times to external events in distributed systems, since the longest response always occurs within the cases that are currently tested by this algorithm. In addition, we extend the analysis technique to determine an upper bound for the local response times of particular actions in a response to an event, thus allowing the definition and verification of local deadlines for elementary actions in distributed systems

Turkish Journal of Electrical Engineering and Computer Science, 2010

In this article we present an analysis for task models having random resource needs and different arrival patterns. In hard real-time environments like avionic systems or nuclear reactors, the inputs to the system are obtained from real world by using sensors. And it is highly possible for a task to have different resource needs for each period according to these changing conditions of real world. We made an analysis of schedulers for task models having random resources in each period. Since feasibility tests for usual task models are just limited to some specific schedulers and arrival patterns, we made our analysis using different preemptive schedulers with different arrival patterns. Another issue that this paper presents is that for task sets having processor utilization factors of below 100%, a new scheduler assigns more resource values to the tasks than they originally have by extending and shrinking the resource values. This provides a better utilization of the resources as well as providing all the tasks to finish in their timing constraints and never assigning lower resource values than the initial resources for all tasks. We exposed a dynamic priority driven scheduling scheme that implements the newly developed Steady Scheme with Bounded Utilization for changing resources and simulated to reach the results depicted.

Real-time applications are becoming increasingly popular in distributed environments. These real-time applications range from hard real-time applications with periodic or aperiodic tasks and intertask relative timing constraints to soft real-time applications with best effort timing requirements. This paper introduces a complete system model for scheduling and dispatching hard as well as soft real-time tasks with intertask temporal dependencies in distributed environments. The model uses a dynamic time based off-line scheduler to verify the feasibility of a distributed hard real-time task set, and a parametric runtime kernel that guarantees the temporally determinate dispatching of hard real-time task instances and best effort performance for soft real-time task instances. The use of the dynamic time based scheduling, provides off-line guarantees for all the timing requirements of the hard real-time tasks while the parametric dispatching mechanism maintains a flexible run-time envir...

1995

In this paper we present a study of the effects caused in distributed real-time systems by jitter in the activation of tasks and messages. We show that although jitter has usually a small impact in the schedulability of singleprocessor systems, in distributed architectures the worstcase response times are significantly delayed. Reducing or eliminating jitter in these systems can increase the schedulability of the system up to 50% more than when jitter is permitted. Jitter can be prevented by using a bandwidth-preserving scheduling algorithm such as the sporadic server. Since this kind of scheduling policy is not designed for communication networks, in this paper we describe how to adapt and implement the sporadic server algorithm for communication networks. Using the sporadic server both in the processors and networks, we can build distributed systems with up to 100% utilization of the CPUs and communication resources, while still guaranteeing that hard real-time requirements are met.

ic.unicamp.br

1999

This paper proposes e cient scheduling algorithms for the joint scheduling of hard aperiodic, sporadic and periodic real time tasks, in systems based on preemptive, xed-priority dispatching. Our scheme guarantees or rejects hard aperiodic real time tasks without any prior knowledge of their attributes, by managing the idle processor capacity dynamically. The method assigns xed priorities to periodic tasks based on the Deadline Monotonic (DM) scheme and analyzes their schedule o -line. We derive closed form solutions for the idle processor capacity process Z(t) within a schedule. In the absence of pending dynamic tasks, periodic tasks execute in their earliest possible schedule S F , called the Fixed-Priority First (FPF). Upon the arrival of a non-periodic task Ja, the scheduler directly determines its admissibility, based on closed form equation of the available processor time in the current schedule, until the deadline of Ja. If Ja cannot be guaranteed under FPF, the scheduler evaluates the idle processor capacity of an alternative schedule S L , where periodic tasks are delayed to execute at their latest possible times, called the Latest Deadline Last (LDL). If LDL o ers su cient idle capacity, the scheduler switches all periodic tasks from FPF to LDL, assigns Ja the lowest priority and admits it into the system. Otherwise, it immediately rejects Ja. We develop the theoretical framework and derive e cient algorithms to compute the idle processors capacity Z(a; b) within a time interval a; b], and maintain it when the schedule is adjusted. The algorithms can also reclaim unused capacity from guaranteed tasks. Our admission control procedure has computational complexity (n) when the non-periodic task queue is serviced in FIFO order, with n periodic tasks. Previously proposed methods have pseudopolynomial time and space complexity. Experimental results show that with n = 160 periodic tasks, the actual computation time for the admission control procedure is less than 90 -secs on a SUN Ultra-Sparc I, 143MHz machine and less than 30 -secs on a SGI Origin 2000, 250MHz workstation. Experiments on well known task sets show overheads which are below 10 -secs even for the slowest machine. The proposed methodology easily extends to algorithms that minimize total task tardiness and number of tardy tasks.

SICS Software-Intensive Cyber-Physical Systems, 2021

We investigate the mathematical properties of event bound functions as they are used in the worst-case response time analysis and utilization tests. We figure out the differences and similarities between the two approaches. Based on this analysis, we derive a more general form do describe events and event bounds. This new unified approach gives clear new insights in the investigation of real-time systems, simplifies the models and will support algebraic proofs in future work. In the end, we present a unified analysis which allows the algebraic definition of any scheduler. Introducing such functions to the real-time scheduling theory will lead two a more systematic way to integrate new concepts and applications to the theory. Last but not least, we show how the response time analysis in dynamic scheduling can be improved.