Luca Abeni

Integrating multimedia applications in hard real-time systems

This paper focuses on the problem of providing efficient run-time support to multimedia applications in a real-time system, where two types of tasks can coexist simultaneously: multimedia soft real-time tasks and hard real-time tasks. Hard tasks are guaranteed based on worst case execution times and minimum interarrival times, whereas multimedia and soft tasks are served based on mean parameters. The paper describes a server-based mechanism for scheduling soft and multimedia tasks without jeopardizing the a priori guarantee of hard real-time activities. The performance of the proposed method is compared with that of similar service mechanisms through extensive simulation experiments and several multimedia applications have been implemented on the HARTIK kernel.

Elastic scheduling for flexible workload management

An increasing number of real-time applications related to multimedia and adaptive control systems require greater flexibility than classical real-time theory usually permits. We present a novel scheduling framework in which tasks are treated as springs with given elastic coefficients to better conform to the actual load conditions. Under this model, periodic tasks can intentionally change their execution rate to provide different quality of service and the other tasks can automatically adapt their periods to keep the system underloaded. The proposed model can also be used to handle overload conditions in a more flexible way and to provide a simple and efficient mechanism for controlling a systems performance as a function of the current load.

Analysis of a reservation-based feedback scheduler

When executing soft real-time tasks in a shared processor, it is important to properly allocate the computational resources such that the quality of service requirements of each task are satisfied. In this paper we propose Adaptive Reservations, based on applying a feedback scheme to a reservation based scheduler After providing a precise mathematical model of the scheduler, we describe how this model can be used for synthesising the controller by applying results from control theory. Finally, we show the effectiveness of our method by simulation and by experiments with an MPEG player running on a modified Linux kernel.

Resource Reservation in Dynamic Real-Time Systems

This paper focuses on the problem of providing efficient run-time support to multimedia applications in a real-time system, where different types of tasks (characterized by different criticality) can coexist. Whereas critical real-time tasks (hard tasks) are guaranteed based on worst-case execution times and minimum interarrival times, multimedia tasks are served based on mean parameters. A novel bandwidth reservation mechanism (the constant bandwidth server) allows real-time tasks to execute in a dynamic environment under a temporal protection mechanism, so that each task will never exceed a predefined bandwidth, independently of its actual requests. The paper also discusses how the proposed server can be used for handling aperiodic tasks efficiently and how a statistical analysis can be applied to perform a probabilistic guarantee of soft tasks. The performance of the proposed method is compared with that of similar service mechanisms (dynamic real-time servers and proportional share schedulers) through extensive simulation experiments.

A new kernel approach for modular real-time systems development

This paper presents a dynamic configurable kernel architecture designed for supporting a simple implementation, integration and evaluation of scheduling algorithms. The main goal of the proposed architecture is to provide a platform for fast prototyping scheduling algorithms both for the CPU and for the devices. The kernel is fully modular in terms of scheduling policies, aperiodic servers, and concurrency control protocols, allowing applications to be developed independently from a particular system configuration. Finally, the system is compliant with the POSIX 1003.13 PSE52 specifications to simplify porting of application code developed for other POSIX compliant kernels.

QoS guarantee using probabilistic deadlines

This paper presents a probabilistic approach to guarantee the performance of a real-time system. While traditional real-time system analysis tends to guarantee that each task instance will complete its execution before its absolute deadline (hard guarantee), our approach permits to estimate the probability that it will happen. Such a statistical guarantee is performed based on inter-arrival and execution times probability distributions, rather than their worst case values. The advantage of a probabilistic approach is a more efficient usage of system resources, allowing to give a certain level of deadline guarantee to task sets that the classical schedulability analysis would reject.

Adaptive bandwidth reservation for multimedia computing

In this paper we present a framework for dynamically allocating the CPU resource to tasks whose execution times are not known a priori. Tasks are partitioned in three classes: the ones that require a uniform execution but do not impose any temporal constraint, periodic tasks that operate on continuous media, and event driven tasks that respond to external interrupts. For the last two classes, we show how to adjust the fraction of the CPU bandwidth assigned to each task using a feedback mechanism.

QoS Management Through Adaptive Reservations

Reservation based (RB) scheduling is a class of scheduling algorithms that is well-suited for a large class of soft real-time applications. They are based on a “bandwidth” abstraction, meaning that a task is given the illusion of executing on a dedicated slower processor. In this context, a crucial design issue is deciding the bandwidth that each task should receive. The point we advocate is that, in presence of large fluctuations on the computation requirements of the tasks, it can be a beneficial choice to dynamically adapt the bandwidth based on QoS measurements and on the subsequent application of feedback control (adaptive reservations).In this paper, we present two novel contributions to this research area. First, we propose three new control algorithms inspired to the ideas of stochastic control. Second, we present a flexible and modular software architecture for adaptive reservations. An important feature of this architecture is that it is realised by means of a minimally invasive set of modifications to the Linux kernel.

Resource sharing in reservation-based systems

In recent years, real-time operating systems have begun to support the resource reservation paradigm. This technique has proved to be very effective in providing QoS to both, real-time and legacy applications, ensuring that the temporal misbehavior of an application does not affect any other. However, resource sharing in a reservation system is still not well understood, and can break the temporal isolation property due to priority inversion. We address this problem, presenting some solution strategies that can be considered extensions to priority inheritance and priority ceiling protocol emulation for reservation systems.

Respecting Temporal Constraints in Virtualised Services

This paper reports some experiences in providing service guarantees to real-time (RT) applications running in virtual Machine (VM), showing how proper scheduling is a necessary condition for a predictable execution. In particular, resource reservation techniques allow to cope with some of the overhead and unpredictabilities experienced when executing multiple VMs on the same host.