Rate Monotonic Scheduling Research Articles

EDF message scheduling on controller area network

The controller area network (CAN) has a maximum transmission rate of 1 Mbit/s and its fixed priorities-based medium access control (MAC) limits the maximum bus utilisation when timeliness guarantees are required. An implementation of earliest deadline first (EDF) message scheduling on the CAN, based on the FTT-CAN (flexible time-triggered communication on CAN) protocol, is presented, which allows higher utilisation factors with timeliness guarantees. The advantages of using EDF instead of rate-monotonic scheduling on the FTT-CAN are highlighted, and a comparison with other implementations of EDF scheduling on the CAN is presented.

Parametric approach to the specification and analysis of real-time scheduling based on ACSR-VP

This paper describes an approach to the specification and analysis of scheduling problems of real-time systems. The method is based on Algebra of Communicating Shared Resources with Value-Passing (ACSR-VP), which is an extension of ACSR, a real-time process algebra, with value-passing capabilities and parameterized processes. ACSR-VP is used to describe an instance of a scheduling problem as a process that has parameters of the problem as free variables. The specification is analyzed by means of a symbolic algorithm, and a boolean expression with free variables is produced as an outcome of the analysis. The solution to a boolean expression identifies under what values of the unknown parameters the system becomes schedulable. The paper presents the theory of ACSR-VP briefly and an example of the period assignment problem for rate-monotonic scheduling. We also explain our current tool implementation effort and plan for incorporating it into the existing toolset, PARAGON.

Optimal reward-based scheduling for periodic real-time tasks

Reward-based scheduling refers to the problem in which there is a reward associated with the execution of a task. In our framework, each real-time task comprises a mandatory and an optional part. The mandatory part must complete before the task's deadline, while a nondecreasing reward function is associated with the execution of the optional part, which can be interrupted at any time. Imprecise computation and Increased-Reward-with-Increased-Service models fall within the scope of this-framework. In this paper, we address the reward-based scheduling problem for periodic tasks. An optimal schedule is one where mandatory-parts complete in a timely manner and the weighted average reward is maximized. For linear and concave reward functions, which are most common, we 1) show the existence of an optimal schedule where the optional service time of a task is constant at every instance and 2) show how to efficiently compute this service time. We also prove the optimality of Rate Monotonic Scheduling (with harmonic periods), Earliest Deadline First, and Least Laxity First policies for the case of uniprocessors when used with the optimal service times we computed. Moreover, we extend our result by showing that any policy which can fully utilize all the processors is also optimal for the multiprocessor periodic reward-based scheduling. To show-that our optimal solution is pushing the limits of reward-based scheduling, we further prove that, when the reward functions are convex, the problem becomes NP-Hard. Our static optimal solution, besides providing considerable reward improvements over the previous suboptimal strategies, also has a major practical benefit. Run-time overhead is eliminated and existing scheduling disciplines may be used without modification with the computed optimal service times.

Liu and Layland's schedulability test revisited

In this paper, we study the problem of scheduling hard real-time periodic task sets on a single processor with the rate monotonic scheduler. We are concerned with the feasibility test given by Liu and Layland, based on the least upper bound of the utilization factor. We show that the result is incomplete and that the argument is incorrect. We complete and correct the result.

Fault-tolerant rate-monotonic first-fit scheduling in hard-real-time systems

Hard-real-time systems require predictable performance despite the occurrence of failures. In this paper, fault tolerance is implemented by using a novel duplication technique where each task scheduled on a processor has either an active backup copy or a passive backup copy scheduled on a different processor. An active copy is always executed, while a passive copy is executed only in the case of a failure. First, the paper considers the ability of the widely-used rate-monotonic scheduling algorithm to meet the deadlines of periodic tasks in the presence of a processor failure. In particular, the completion time test is extended so as to check the schedulability on a single processor of a task set including backup copies. Then, the paper extends the well-known rate-monotonic first-fit assignment algorithm, where all the task copies, included the backup copies, are considered by rate-monotonic priority order and assigned to the first processor in which they fit. The proposed algorithm determines which tasks must use the active duplication and which can use the passive duplication. Passive duplication is preferred whenever possible, so as to overbook each processor with many passive copies whose primary copies are assigned to different processors. Moreover, the space allocated to active copies is reclaimed as soon as a failure is detected. Passive copy overbooking and active copy deallocation allow many passive copies to be scheduled sharing the same time intervals on the same processor, thus reducing the total number of processors needed. Simulation studies reveal a remarkable saving of processors with respect to those needed by the usual active duplication approach in which the schedule of the non-fault-tolerant case is duplicated on two sets of processors.

Parametric Approach to the Specification and Analysis of Real-time System Designs based on ACSR-VP

This paper describes an approach to the specification and analysis of scheduling problems of real-time systems. The method is based on ACSR-VP, which is an extension of ACSR, a real-time process algebra, with value-passing capabilities and parameterized processes. ACSR-VP is used to describe an instance of a scheduling problem as a process that has parameters of the problem as free variables. The specification is analyzed by means of a symbolic algorithm, and a boolean expression with free variables is produced as an outcome of the analysis. The solution to a boolean expression identifies under what values of the unknown parameters the system becomes schedulable. The paper presents the theory of ACSR-VP briefly and an example of the period assignment problem for rate-monotonic scheduling. We also explain our current tool implementation effort and plan for incorporating it into the existing toolset, PARAGON.

Point-to-Point Real-Time Communication Over a Backplane Bus

This paper suggests two scheduling methods for a real-time point-topoint communication over a backplane bus which has limited priorities for the bus mastership. Fixed priority assignment(FPA) algorithm based on the rate monotonic scheduling is suggested for the periodic hard real-time applications. Dynamic priority assignment(DPA) algorithm based on the earliest deadline first scheduling is suggested for the sporadic soft real-time applications. The worst communication latency for the proposed methods is analyzed and then the schedulability is evaluated using computer simulation.

Fault-Tolerant Rate-Monotonic Scheduling

Due to the critical nature of the tasks in hard real-time systems, it is essential that faults be tolerated. In this paper, we present a scheme which can be used to tolerate faults during the execution of preemptive real-time tasks. We describe a recovery scheme which can be used to re-execute tasks in the event of single and multiple transient faults and discuss conditions that must be met by any such recovery scheme. We then extend the original Rate Monotonic Scheduling (RMS) scheme and the exact characterization of RMS to provide tolerance for single and multiple transient faults. We derive schedulability bounds for sets of real-time tasks given the desired level of fault tolerance for each task or subset of tasks. Finally, we analyze and compare those bounds with existing bounds for non-fault-tolerant and other variations of RMS.

A Feasibility Decision Algorithm for Rate Monotonic andDeadline Monotonic Scheduling

Rate monotonic and deadline monotonic scheduling are commonly used for periodic real-time task systems. This paper discusses a feasibility decision for a given real-time task system when the system is scheduled by rate monotonic and deadline monotonic scheduling. The time complexity of existing feasibility decision algorithms depends on both the number of tasks and maximum periods or deadlines when the periods and deadlines are integers. This paper presents a new necessary and sufficient condition for a given task system to be feasible and proposes a new feasibility decision algorithm based on that condition. The time complexity of this algorithm depends solely on the number of tasks. This condition can also be applied as a sufficient condition for a task system using priority inheritance protocols to be feasible with rate monotonic and deadline monotonic scheduling.

Minimum achievable utilization for fault-tolerant processing of periodic tasks

The Rate Monotonic Scheduling (RMS) policy is a widely accepted scheduling strategy for real-time systems due to strong theoretical foundations and features attractive to practical uses. For a periodic task set of n tasks with deadlines at the end of task periods, it guarantees a feasible schedule on a single processor as long as the utilization factor of the task set is below n(2/sup 1/n/-1) which converges to 0.69 for large n. We analyze the schedulability of a set of periodic tasks that is scheduled by the RMS policy and is susceptible to a single fault. The recovery action is the reexecution of all uncompleted tasks. The priority of the RMS policy is maintained even during recovery. Under these conditions, we guarantee that no task will miss a single deadline, even in the presence of a fault, if the utilization factor on the processor does not exceed 0.5. Thus, 0.5 is the minimum achievable utilization that permits recovery from faults before the expiration of the deadlines of the tasks. This bound is better than the trivial bound of 0.6912=0.345 that would be obtained if computation times were doubled to provide for reexecutions in the RMS analysis. Our result provides scheduling guarantees for tolerating a variety of intermittent and transient hardware and software faults that can be handled simply by reexecution. In addition, we demonstrate how permanent faults can be tolerated efficiently by maintaining common spares among a set of processors that are independently executing periodic tasks.

Providing deterministic delay guarantees in ATM networks

B-ISDN is expected to carry various types of traffic for which stringent quality of service (QoS) requirements must be met. In particular, per-packet (per-cell) delay guarantees are an important QoS requirement for real-time applications. Although several methods have been proposed to provide delay guarantees in packet-switching networks, they are either too complex for ATM networks which allow only very simple operations to achieve high bandwidths (hence, high-speed switching and routing), or too inefficient (in using network resources) to be cost effective. In this paper, we propose a cell-multiplexing scheme for the real-time communication service in ATM networks. The proposed scheme achieves an efficiency close to that of packet-by-packet generalized processor sharing, and incurs an implementation cost similar to that of rate controlled static priority queueing. It employs the leaky bucket model as the input traffic description model and regulates the input traffic at the user network interface to follow the input traffic specification. Inside the network, it consists of two components, traffic controller and scheduler. The function of the traffic controller is to shape real-time traffic to have the same input pattern at every switch along the path. The end-to-end delay is bounded by the scheduler which employs a nonpreemptive rate-monotonic priority scheduling policy at each switch on the path. The proposed scheme is compared to three other popular schemes with MPEG-coded movie clips. Finally, we present a hardware implementation of the proposed scheme based on a systolic array priority queue.

Schedulability Analysis in a Real-Time Fieldbus Network

The theory of real-time task scheduling can be applied, in many situations, to real-time message scheduling. Particularly, it can be advantageously applied in real-time fieldbus networks such as the FIP fieldbus (Factory Instrumentation Protocol). In this paper the Rate-Monotonic Scheduling theory is used to derive a suficient schedulability condition for the FIP fieldbus with a modified scheduler. Such condition may enhance the usability of FIP-like fieldbuses in dynamic environments.

802.4 Rate Monotonic scheduling in hard real-time environments: Setting the medium access control parameters

This paper presents an analysis of the Rate Monotonic scheduling of an 802.4 network operating in a hard real-time environment. The worst-case number of priority inversions is determined. With that, an iterative algorithm is derived to partition the set of nodes in priority classes and to assign the two fundamental medium access control parameters, Target Token Rotation Time and High Priority Token Holding Time, in order to meet the hard real-time constraints.

Rate-monotonic scheduling for hard-real-time systems

Hard-real-time computing systems are widely used in our society, for example, in nuclear and industrial plants, telecommunications, avionics and robotics. In such systems, almost all tasks occur infinitely often and have time deadlines, namely, their correctness relies not only on their logical results, but also on the time at which the results are available. A scheduling algorithm specifies an order in which all the tasks are to be executed, in a way that all the time deadlines are met. This paper provides a review on deterministic scheduling algorithms for hard-real-time systems, focusing mainly on fixed priority, preemptive scheduling of periodic tasks on a single processor and, in particular, on the Rate-Monotonic algorithm. After presenting some basic results, several generalisations, aimed at relaxing some constraints and facing more realistic cases, are described. Issues covered include uniprocessor and multiprocessor systems, periodic and non-periodic tasks, restricted and arbitrary deadlines, fixed and dynamic priorities, independent and synchronised tasks, as well as fault-free and fault-tolerant systems.

Multi-Tasking Programming with a Rate-Monotonic Scheduling Based Upon a Visual Data Flow Language

The presented work concerns the multi-tasking programming of real time applications with a visual language founded on the data flow formalism and with a preemptable scheduler. In that context, we have shown that a direct programming of concurrent data flows leads to a non deterministic execution sequence. So, this result being non acceptable in the most process control environment, a scheduler, based on the rate monotonic algorithm, has been implemented as a specific data flow or task that dispatches the processor access to the other tasks of the set. This programming method opens the opportunity of a multi-tasking programming in a visual data flow language environment with the respect a deterministic execution sequence. This work effectively improves the programming with a visual data flow language in the sense of safety that is the main exigency in a process control application.

Bringing real-time scheduling theory and practice closer for multimedia computing

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 we considered, Rmdp increases useful utilization by as much as 8%. Thus our scheduling policies have the potential to improve performance over existing methods.

Allocating fixed-priority periodic tasks on multiprocessor systems

In this paper, we study the problem of allocating a set of periodic tasks on a multiprocessor system such that tasks are scheduled to meet their deadlines on individual processors by the Rate-Monotonic scheduling algorithm. A new schedulability condition is developed for the Rate-Monotonic scheduling that allows us to develop more efficient on-line allocation algorithms. Two on-line allocation algorithms—RM-FF and RM-BF are presented, and shown that their worst-case performance, over the optimal allocation, is upper bounded by 2.33 and lower bounded by 2.28. Then RM-FF and RM-BF are further improved to form two new algorithms: Refined-RM-FF (RRM-FF) and Refined-RM-BF (RRM-BF), both of which have a worst-case performance bound of 2. We also show that when the maximum allowable utilization of a task is small, the worst-case performance of all the new algorithms can be significantly improved. The worst-case performance bounds of RRM-FF and RRM-BF are currently the best bounds in the class of on-line scheduling algorithms proposed to solve the same scheduling problem. Simulation studies show that the average-case performance of the newly proposed algorithms is significantly superior to those in the existing literature.

A tool for allocating periodic real-time tasks to a set of processors

We describe the design and implementation of a tool for applying rate-monotonic scheduling (RMS) technology in distributed environments that serve real-time applications. Our tool, DRMS, enables users to define sets of real-time tasks and the processors to which the tasks will be allocated. The tool finds allocations such that real-time constraints of the tasks are guaranteed. Several feasibility tests can be applied by users to evaluate the feasibility of task allocations. Users can vary the descriptions of tasks and processors so that DRMS is a practical tool for designers of real-time systems.

Semaphore queue priority assignment for real-time multiprocessor synchronization

Prior work on real time scheduling with global shared resources in multiprocessor systems assigns as much blocking as possible to the lowest priority tasks. We show that better schedulability can be achieved if global blocking is distributed according to the blocking tolerance of tasks rather than their execution priorities. We describe an algorithm that assigns global semaphore queue priorities according to blocking tolerance, and we present simulation results demonstrating the advantages of this approach with rate monotonic scheduling. Our simulations also show that a simple FIFO usually provides better real time schedulability with global semaphores than priority queues that use task execution priorities. >

New strategies for assigning real-time tasks to multiprocessor systems

Optimal scheduling of real-time tasks on multiprocessor systems is known to be computationally intractable for large task sets. Any practical scheduling algorithm for assigning real-time tasks to a multiprocessor system presents a trade-off between its computational complexity and its performance. In this study, new schedulability conditions are presented for homogeneous multiprocessor systems where individual processors execute the rate-monotonic scheduling algorithm. The conditions are used to develop new strategies for assigning real-time tasks to processors. The performance of the new strategies is shown to be significantly better than suggested by the existing literature. Under the realistic assumption that the load of each real-time task is small compared to the processing speed of each processor, it is shown that the processors can be almost fully utilized.