Task Mapping and Partition Allocation for Mixed-Criticality Real-Time Systems

Abstract—In this paper we address the mapping of mixed-criticality hard real-time applications on distributed embedded architectures. We assume that the architecture provides both spa-tial and temporal partitioning, thus enforcing enough separation between applications. With temporal partitioning, each application runs in a separate partition, and each partition is allocated several time slots on the processors where the application is mapped. The sequence of time slots for all the applications on a processor are grouped within a Major Frame, which is repeated periodically. We assume that the applications are scheduled using static-cyclic scheduling. We are interested to determine the task mapping to processors, and the sequence and size of the time slots within the Major Frame on each processor, such that the applications are schedulable. We have proposed a Tabu Search-based approach to solve this optimization problem. The proposed algorithm has been evaluated using several synthetic and real-life benchmarks. I

Schedulability Analysis for Certification-friendly Multicore Systems

This paper presents a new schedulability test for safety-critical software undergoing a transition from single-core to multicore systems - a challenge faced by multiple industries today. Our migration model, consisting of a schedulability test and execution model, is distinguished by three aspects consistent with reducing transition cost. First, it assumes externally-driven scheduling parameters, such as periods and deadlines, remain fixed (and thus known), whereas exact computation times are not. Second, it adopts a globally synchronized conflict-free I/O model that leads to a decoupling between cores, simplifying the schedulability analysis. Third, it employs global priority assignment across all tasks on each core, irrespective of application, where budget constraints on each application ensure isolation. These properties enable us to obtain a utilization bound that places an allowable limit on total task execution times. Evaluation results demonstrate the advantages of our scheduling model over competing resource partitioning approaches, such as Periodic Server and TDMA.Ope

Design Optimization of Mixed-Criticality Real-Time Applications on Cost-Constrained Partitioned Architectures

Abstract—In this paper we are interested to implement mixed-criticality hard real-time applications on a given heterogeneous distributed architecture. Applications have different criticality levels, captured by their Safety-Integrity Level (SIL), and are scheduled using static-cyclic scheduling. Mixed-criticality tasks can be integrated onto the same architecture only if there is enough spatial and temporal separation among them. We consider that the separation is provided by partitioning, such that applications run in separate partitions, and each partition is allocated several time slots on a processor. Tasks of different SILs can share a partition only if they are all elevated to the highest SIL among them. Such elevation leads to increased development costs. We are interested to determine (i) the mapping of tasks to processors, (ii) the assignment of tasks to partitions, (iii) the sequence and size of the time slots on each processor and (iv) the schedule tables, such that all the applications are schedulable and the development costs are minimized. We have proposed a Tabu Search-based approach to solve this optimization problem. The proposed algorithm has been evaluated using several synthetic and real-life benchmarks. I

Adaptive Mid-term and Short-term Scheduling of Mixed-criticality Systems

A mixed-criticality real-time system is a real-time system having multiple tasks classified according to their criticality. Research on mixed-criticality systems started to provide an effective and cost efficient a priori verification process for safety critical systems. The higher the criticality of a task within a system and the more the system should guarantee the required level of service for it. However, such model poses new challenges with respect to scheduling and fault tolerance within real-time systems. Currently, mixed-criticality scheduling protocols severely degrade lower criticality tasks in case of resource shortage to provide the required level of service for the most critical ones. The actual research challenge in this field is to devise robust scheduling protocols to minimise the impact on less critical tasks. This dissertation introduces two approaches, one short-term and the other medium-term, to appropriately allocate computing resources to tasks within mixed-criticality systems both on uniprocessor and multiprocessor systems. The short-term strategy consists of a protocol named Lazy Bailout Protocol (LBP) to schedule mixed-criticality task sets on single core architectures. Scheduling decisions are made about tasks that are active in the ready queue and that have to be dispatched to the CPU. LBP minimises the service degradation for lower criticality tasks by providing to them a background execution during the system idle time. After, I refined LBP with variants that aim to further increase the service level provided for lower criticality tasks. However, this is achieved at an increased cost of either system offline analysis or complexity at runtime. The second approach, named Adaptive Tolerance-based Mixed-criticality Protocol (ATMP), decides at runtime which task has to be allocated to the active cores according to the available resources. ATMP permits to optimise the overall system utility by tuning the system workload in case of shortage of computing capacity at runtime. Unlike the majority of current mixed-criticality approaches, ATMP allows to smoothly degrade also higher criticality tasks to keep allocated lower criticality ones

Optimization of Time-Partitions for Mixed-Criticality Real-Time Distributed Embedded Systems

Abstract—In this paper we are interested in mixed-criticality embedded real-time applications mapped on distributed hetero-geneous architectures. The architecture provides both spatial and temporal partitioning, thus enforcing enough separation for the critical applications. With temporal partitioning, each application is allowed to run only within predefined time slots, allocated on each processor. The sequence of time slots for all the applications on a processor are grouped within a Major Frame, which is repeated periodically. We assume that the safety-critical applications (on all criticality levels) are scheduled using static-cyclic scheduling and the non-critical applications are scheduled using fixed-priority preemp-tive scheduling. We consider that each application runs in a separate partition, and each partition is allocated several time slots on the processors where the application is mapped. We are interested to determine the sequence and size of the time slots within the Major Frame on each processor such that both the safety-critical and non-critical applications are schedulable. We have proposed a Simulated Annealing-based approach to solve this optimization problem. The proposed algorithm has been evaluated using several synthetic and real-life benchmarks. Keywords-mixed-criticality; real-time systems; temporal-partitioning I

Escalonar sistemas de tempo-real de alta críticalidade

Cyclic executives are used to schedule safety-critical real-time systems because of their determinism, simplicity, and efficiency. One major challenge of the cyclic executive model is to produce the cyclic scheduling timetable. This problem is related to the bin-packing problem [34] and is NP-Hard in the strong sense. Unnecessary context switches within the scheduling table can introduce significant overhead; in IMA (Integrated Modular Avionics), cache-related overheads can increase task execution times up to 33% [18]. Developed in the context of the Software Engineering Master’s Degree at ISEP, the Polytechnic Institute of Engineering in Porto Portugal, this thesis contains two contributions to the scheduling literature. The first is a precise and exact approach to computing the slack of a job set that is schedule policy independent. The method introduces several operations to update and maintain the slack at runtime, ensuring the slack of all jobs is valid and coherent. The second contribution is the definition of a state-of-the-art preemptive scheduling algorithm focused on minimizing the number of system preemptions for real-time safety-critical applications within a reasonable amount of time. Both contributions have been implemented and extensively tested in scala. Experimental results suggest our scheduling algorithm has similar non-preemptive schedulability ratio than Chain Window RM [69], yet lower ratio in high utilizations than Chain Window EDF [69] and BB-Moore [68]. For ask sets that failed to be scheduled non-preemptively, 98-99% of all jobs are scheduled without preemptions. Considering the fact that our scheduler is preemptive, being able to compete with non-preemptive schedulers is an excellent result indeed. In terms of execution time, our proposal is multiple orders of magnitude faster than the aforementioned algorithms. Both contributions of this work are planned to be presented at future conferences such as RTSS@Work and RTAS