Ragunathan Rajkumar

Priority inheritance protocols: an approach to real-time synchronization

An investigation is conducted of two protocols belonging to the priority inheritance protocols class; the two are called the basic priority inheritance protocol and the priority ceiling protocol. Both protocols solve the uncontrolled priority inversion problem. The priority ceiling protocol solves this uncontrolled priority inversion problem particularly well; it reduces the worst-case task-blocking time to at most the duration of execution of a single critical section of a lower-priority task. This protocol also prevents the formation of deadlocks. Sufficient conditions under which a set of periodic tasks using this protocol may be scheduled is derived. >

Cyber-physical systems: the next computing revolution

Cyber-physical systems (CPS) are physical and engineered systems whose operations are monitored, coordinated, controlled and integrated by a computing and communication core. Just as the internet transformed how humans interact with one another, cyber-physical systems will transform how we interact with the physical world around us. Many grand challenges await in the economically vital domains of transportation, health-care, manufacturing, agriculture, energy, defense, aerospace and buildings. The design, construction and verification of cyber-physical systems pose a multitude of technical challenges that must be addressed by a cross-disciplinary community of researchers and educators.

A resource allocation model for QoS management

Quality of service (QoS) has been receiving wide attention in many research communities including networking, multimedia systems, real-time systems and distributed systems. In large distributed systems such as those used in defense systems, on-demand service and inter-networked systems, applications contending for system resources must satisfy timing, reliability and security constraints as well as application-specific quality requirements. Allocating sufficient resources to different applications in order to satisfy various requirements is a fundamental problem in these situations. A basic yet flexible model for performance-driven resource allocations can therefore be useful in making appropriate tradeoffs. We present an analytical model for QoS management in systems which must satisfy application needs along multiple dimensions such as timeliness, reliable delivery schemes, cryptographic security and data quality. We refer to this model as Q-RAM (QoS-based Resource Allocation Model). The model assumes a system with multiple concurrent applications, each of which can operate at different levels of quality based on the system resources available to it. The goal of the model is to be able to allocate resources to the various applications such that the overall system utility is maximized under the constraint that each application can meet its minimum needs. We identify resource profiles of applications which allow such decisions to be made efficiently and in real-time. We also identify application utility functions along different dimensions which are composable to form unique application requirement profiles. We use a video-conferencing system to illustrate the model.

Real-time synchronization protocols for multiprocessors

The authors investigate the synchronization problem in the context of priority-driven preemptive scheduling on shared-memory multiprocessors. Unfortunately, a direct application of synchronization mechanisms such as the Ada rendezvous, semaphores, or monitors can lead to uncontrolled priority inversion: a high job being blocked by a lower priority job for an indefinite period of time. A task allocation scheme based on the generalized protocol is outlined.<<ETX>>

Generalized rate-monotonic scheduling theory: a framework for developing real-time systems

Real-time computing systems are used to control telecommunication systems, defense systems, avionics, and modern factories. Generalized rate-monotonic scheduling theory, is a recent development that has had large impact on the development of real-time systems and open standards. In this paper we provide an up-to-date and self-contained review of generalized rate-monotonic scheduling theory. We show how this theory can be applied in practical system development, where special attention must be given to facilitate concurrent development by geographically distributed programming teams and the reuse of existing hardware and software components. >

A scalable solution to the multi-resource QoS problem

The problem of maximizing system utility by allocating a single finite resource to satisfy discrete Quality of Service (QoS) requirements of multiple applications along multiple QoS dimensions was studied previously. In this paper we consider the more complex problem of apportioning multiple finite resources to satisfy the QoS needs of multiple applications along multiple QoS dimensions. In other words, each application, such as video-conferencing, needs multiple resources to satisfy its QoS requirements. We evaluate and compare three strategies to solve this provably NP-hard problem. We show that dynamic programming and mixed integer programming compute optimal solutions to this problem but exhibit very long running times. We then adapt the mixed integer programming problem to yield near-optimal results with smaller running times. Finally, we present an approximation algorithm based on a local search technique that is less than 5% away from the optimal solution but which is more than two orders of magnitude faster. Perhaps more significantly, the local search technique turns out to be very scalable and robust as the number of resources required by each application increases.

A real-time locking protocol

The authors examine a priority driven two-phase lock protocol called the read/write priority ceiling protocol. It is shown that this protocol leads to freedom from mutual deadlock. In addition, a high-priority transactions can be blocked by lower priority transactions for at most the duration of a single embedded transaction. These properties can be used by schedulability analysis to guarantee that a set of periodic transactions using this protocol can always meet its deadlines. Finally, the performance of this protocol is examined for randomly arriving transactions using simulation studies. >

Real-time synchronization protocols for shared memory multiprocessors

A priority-based synchronization protocol that explicitly uses shared-memory primitives is defined and analyzed. A solution that has been proposed for bounding and minimizing synchronization delays in real-time systems is briefly reviewed. The waiting times introduced by synchronization requirements in multiple-processor environments are identified, and a set of goals for priority-based multiprocessor synchronization protocols is derived. The underlying priority consideration for a shared memory synchronization protocol are studied and priority assignments to be used by the protocol are derived.<<ETX>>

Scheduling Parallel Real-Time Tasks on Multi-core Processors

Massively multi-core processors are rapidly gaining market share with major chip vendors offering an ever increasing number of cores per processor. From a programming perspective, the sequential programming model does not scale very well for such multi-core systems. Parallel programming models such as OpenMP present promising solutions for more effectively using multiple processor cores. In this paper, we study the problem of scheduling periodic real-time tasks on multiprocessors under the fork join structure used in OpenMP. We illustrate the theoretical best-case and worst-case periodic fork-join task sets from a processor utilization perspective. Based on our observations of these task sets, we provide a partitioned preemptive fixed-priority scheduling algorithm for periodic fork-join tasks. The proposed multiprocessor scheduling algorithm is shown to have a resource augmentation bound of 3.42, which implies that any task set that is feasible on m unit speed processors can be scheduled by the proposed algorithm on m processors that are 3:42 times faster.

Mode Change Protocols for Priority-Driven Preemptive Scheduling

In many real-time applications, the set of tasks in the system, as well as the characteristics of the tasks, change during system execution. Specifically, the system moves from one mode of execution to another as its mission progresses. A mode change is characterized by the deletion of some tasks, addition of new tasks, or changes in the parameters of certain tasks, for example, increasing the sampling rate to obtain a more accurate result. This paper discusses how mode changes can be accommodated within a given framework of priority driven real-time scheduling.