Damir Isovic

Efficient scheduling of sporadic, aperiodic, and periodic tasks with complex constraints

Many industrial applications with real-time demands are composed of mixed sets of tasks with a variety of requirements. These can be in the form of standard timing constraints, such as periods and deadlines, or complex, e.g. to express application-specific or nontemporal constraints, reliability, performance, etc. Arrival patterns determine whether tasks are treated as periodic, sporadic or aperiodic. As many algorithms only focus on specific sets of task types and constraints, system design has to focus on those supported by a particular algorithm, at the expense of the rest. In this paper, we present an algorithm to deal with a combination of mixed sets of tasks and constraints: periodic tasks with complex and simple constraints, soft and firm aperiodic tasks and sporadic tasks. Instead of providing an algorithm tailored to a specific set of constraints, we propose an EDF (earliest deadline first) based runtime algorithm and the use of an offline scheduler for complexity reduction to transform complex constraints into the EDF model. At runtime, an extension to EDF, two-level EDF, ensures the feasible execution of tasks with complex constraints in the presence of additional tasks or overloads. We present an algorithm for handling offline guaranteed sporadic tasks, with minimum inter-arrival times, in this context, which keeps track of arrivals of instances of sporadic tasks to reduce pessimism about future sporadic arrivals and to improve the response times and acceptance of firm aperiodic tasks. A simulation study underlines the effectiveness of the proposed approach.

Timing constraints of MPEG-2 decoding for high quality video: misconceptions and realistic assumptions

Decoding MPEG-2 video streams imposes hard real-time constraints for consumer devices such as TV sets. The freedom of encoding choices provided by the MPEG-2 standard results in high variability inside streams, in particular with respect to frame structures and their sizes. In this paper, we identify realistic timing constraints demanded by MPEG-2 video decoding. We present results from a study of realistic MPEG-2 video streams to analyze the validity of common assumptions for software decoding and identify a number of misconceptions. Furthermore, we identify constraints imposed by frame buffer handling and discussed their implications on decoding architecture and timing constraints.

Quality aware MPEG-2 stream adaptation in resource constrained systems

A number of algorithms have been presented for handling software decoding of MPEG-2 streams based on buffering or rate adjustment focusing on providing good average quality. The potentially arising drops in quality are tolerated, e.g., in transmissions over the Internet; they cannot be accepted in high quality consumer products: these mandate real-time methods. When resources, such as processing power or network bandwidth, are limited and not all frames can be handled, best effort decoders incur unnecessary quality decrease while wasting resources. In this paper, we present a method for quality aware frame selection for MPEG decoding under limited resources, based on realistic timing constraints for the decoding of MPEG streams. Given that not all frames can be processed, it selects those which provide the best picture quality while matching the available resources, starting only such decoding, which is guaranteed to be completed. We formulate the method as real-time scheduling problem and present its application in an example scheduling algorithm. Results from study based on realistic MPEG-2 video underline the effectiveness of our approach.

Handling sporadic tasks in off-line scheduled distributed real-time systems

Many industrial applications mandate the use of a time-triggered paradigm and consequently the use of off-line scheduling for reasons such as predictability, certification, cost, or product reuse. The construction of an off-line schedule requires complete knowledge about all temporal aspects of the application. The acquisition of this information may involve unacceptable cost or be impossible. Often, only partial information is available from the controlled environment. In this paper we present an algorithm to handle event-triggered sporadic tasks, i.e., with unknown arrival times, but known maximum arrival frequencies, in the context of distributed, off-line scheduled systems. Sporadic tasks are guaranteed during design time, allowing rescheduling or redesign in the failure case. At run-time, the sporadic tasks are scheduled dynamically, allowing the reuse of resources reserved for, but not consumed by the sporadic tasks. We provide an off-line schedulability test for sporadic tasks and apply the method to perform on-line scheduling on top of off-line schedules. Since the major part of preparations is performed off-line, the involved on-line mechanisms are simple. The on-line reuse of resources allows for high resource utilization.

Handling mixed sets of tasks in combined offline and online scheduled real-time systems

Many industrial applications with real-time demands are composed of mixed sets of tasks with a variety of requirements. These can be in the form of standard timing constraints, such as period and deadline, or complex, e.g., to express application specific or nontemporal constraints, reliability, performance, etc. As many algorithms focus on specific sets of task types and constraints only, system design has to focus on those supported by a particular algorithm, at the expense of the rest.In this paper, we present a method to deal with a combination of mixed sets of tasks and constraints: periodic tasks with complex and simple constraints, soft and firm aperiodic, and sporadic tasks. We propose the use of an offline scheduler to manage complex timing and resource constraints of periodic tasks and transform these into a simple EDF model with start-times and deadlines. At run-time, the execution of the offline scheduled tasks is flexibly shifted in order to allow for feasible inclusion of dynamically arriving sporadic and aperiodic tasks. Sporadic tasks are guaranteed offline based on their worst-case activation frequencies. At run-time, this pessimism is reduced by the online algorithm which uses the exact knowledge about sporadic arrivals to reclaim resources and improve response times and acceptance of firm aperiodic tasks.

Integrated global and local quality-of-service adaptation in distributed, heterogeneous systems

In this paper we have developed a method for an efficient Quality-of-Service provision and adaptation in dynamic, heterogeneous systems, based on our Matrix framework for resource management. It integrates local QoS mechanisms of the involved devices that deal mostly with short-term resource fluctuations, with a global adaptation mechanism that handles structural and long-term load variations on the system level. We have implemented the proposed approach and demonstrated its effectiveness in the context of video streaming.

User-friendly H.264/AVC for remote browsing

With the growing popularity of variable network technologies, it is highly desirable to enable effective and quick browsing of remote multimedia content. In this paper we present a method for quick access of remote video content as an initial step towards a full digital Video Cassette Recording functionality in multimedia streaming applications such as Video-On-Demand, video broadcasting and remote video editing.We propose a transcoding scheme for H.264/AVC video that fully utilizes the benefits of recently proposed SP- and SI-frames to facilitate user-friendly remote stream browsing and editing. The transcoding parameters can be adaptively changed and optimized to support different characteristics of H.264 video streams.

Limited preemptive scheduling of mixed time-triggered and event-triggered tasks

Many embedded systems have complex timing constraints and, at the same time, have flexibility requirements which prohibit offline planning of the entire system. To support a mixture of time-triggered and event-triggered tasks, some industrial systems deploy a table-driven dispatcher for time-triggered tasks complemented with a preemptive scheduler to allocate the free time slots to event-driven tasks. Contrary to fully preemptive scheduling, limiting the preemptions of tasks to fixed preemptions points may reduce memory requirements and it alleviates the preemption costs in the system. We revisit slotshifting, which at run time mixes time-triggered and event-triggered tasks in a preemptive EDF schedule. In this paper, we extend slotshifting with limited-preemptive execution of event-triggered tasks. We present a synchronization protocol to arbitrate the executions of non-preemptive regions, so that time-triggered tasks keep meeting their timing constraints implicitly. Furthermore, we investigate how to disable preemptions of event-triggered tasks during the execution of the offline-scheduled time-triggered tasks, while keeping the feature of slotshifting to reallocate their slots of execution dynamically at run time.

Real-Time Control and Scheduling Co-Design for Efficient Jitter Handling

In this paper we propose an integrated approach for control design and real-time scheduling, suitable for both discrete-time and continuous-time controllers. It guarantees system performance by accepting a certain minimum value of jitter for control tasks and feasibly schedules them together with other tasks in the system. Results from comparison with other approaches from real-time and control theory domains underline the effectiveness of our method.

RTOS Support for Mixed Time-triggered and Event-triggered Task Sets

Many embedded systems have complex timing constraints and, at the same time, have flexibility requirements which prohibit offline planning of the entire system. To support a mixture of time-triggered and event-triggered tasks, some industrial systems deploy an RTOS with a table-driven dispatcher complemented with a preemptive scheduler to allocate the free time slots to event-driven tasks. Rather than allocating dedicated time-slots to time-triggered tasks, in this work we provide RTOS support to dynamically re-allocate time-slots of time-triggered tasks within pre-computed time ranges to maximize the availability of the processing capacity for event-triggered tasks. Although the concept - called slot shifting - is not new, we are the first to extend a commercial RTOS with such support. In addition, we extend slot shifting with a run-time mechanism to reclaim resources of time-triggered tasks when their reserved capacities are unused. This mechanism eliminates over-provisioning of capacities to tasks that have been converted into periodic tasks to resolve interdependencies during off-line synthesis, but by nature are event-triggered. This allows, for example, for a resource-efficient implementation of a polling task. After implementing our unique RTOS extensions, we investigate the run-time overheads for the corresponding scheduling mechanisms. Measurements show that the increase in terms of absolute run-time overhead is minor compared to an off-the-shelf micro-kernel with a fixed-priority scheduler.