Alessandro Biondi

Schedulability Analysis of Hierarchical Real-Time Systems under Shared Resources

Sharing resources in hierarchical real-time systems implemented with reservation servers requires the adoption of special budget management protocols that preserve the bandwidth allocated to a specific component. In addition, blocking times must be accurately estimated to guarantee both the global feasibility of all the servers and the local schedulability of applications running on each component. This paper presents two new local schedulability tests to verify the schedulability of real-time applications running on reservation servers under fixed priority and EDF local schedulers. Reservation servers are implemented with the BROE algorithm. A simple extension to the SRP protocol is also proposed to reduce the blocking time of the server when accessing global resources shared among components. The performance of the new schedulability tests are compared with other solutions proposed in the literature, showing the effectiveness of the proposed improvements. Finally, an implementation of the main protocols on a lightweight RTOS is described, highlighting the main practical issues that have been encountered.

Exact Interference of Adaptive Variable-Rate Tasks under Fixed-Priority Scheduling

Engine control applications require the execution of tasks activated in relation to specific system variables, such as the crankshaft rotation angle. To prevent possible overload conditions at high rotation speeds, such tasks are designed to vary their functionality (hence their computational requirements) for different speed ranges. Modeling and analyzing such a type of tasks poses new research challenges in the schedulability analysis that are now being addressed in the real-time literature. This paper advances the state of the art by presenting a method for computing the exact worst-case interference of such adaptive variable-rate tasks under fixed priority scheduling, enabling a tight analysis and design of engine control applications.

Response-time analysis for real-time tasks in engine control applications

Engine control systems include computational activities that are triggered at predetermined angular values of the crankshaft, and therefore generate a workload that tends to increase with the engine speed. To cope with overload conditions, a common practice adopted by the automotive industry is to design such angular tasks with a set of modes that switch at given rotation speeds to adapt the computational demand. For this reason, these tasks are referred to as adaptive variable-rate (AVR). This paper presents an exact response time analysis for engine control applications consisting of periodic and AVR tasks scheduled by fixed priority. The proposed analysis is first presented for task sets with a single AVR task, and then extended to consider multiple AVR tasks related to a common rotation source. A number of experimental results are reported to validate the proposed approach and compare it against an existing sufficient test.

A Framework for Supporting Real-Time Applications on Dynamic Reconfigurable FPGAs

Computing platforms are evolving towards heterogeneous architectures including processors of different types and field programmable gate arrays (FPGAs), used as hardware accelerators for speeding up specific functions. The increasing capacity and performance of modern FPGAs, with their partial reconfiguration capabilities, have made them attractive in several application domains, including space applications.This paper proposes a framework for supporting the development of safety-critical real-time systems that exploit hardware accelerators developed through FPGAs with dynamic partial reconfiguration capabilities.A model is first presented and then used to derive a response-time analysis to verify the schedulability of a real-time task set under given constraints and assumptions. Although the analysis is based on a generic model, the proposed framework has been conceived to account for several real-world constraints present on todays platforms and has been practically validated on the Zynq platform, showing that it can actually be supported by state-of-the-art technologies. Finally, a number of experiments are reported to evaluate the worst-case performance of the proposed approach on synthetic workload.

Supporting Component-Based Development in Partitioned Multiprocessor Real-Time Systems

The fast evolution of multicore systems, combined with the need of sharing the same platform for independently developed software, demands for new methodologies and algorithms that allow resource partitioning, while guaranteeing the isolation of concurrent applications. Unfortunately, a major problem that can break the isolation property of concurrent partitions is resource sharing. Although a number of resource access protocols exist for hierarchical uniprocessor systems, no protocols are available today for managing hierarchical partitions implemented on top a multiporcessor platform under partitioned scheduling. This paper presents a framework to support component based design on a multiprocessor platform and proposes a novel reservation server mechanism, called M-BROE, to handle shared resources in multiprocessor systems in the presence of resource reservation scheduling mechanisms.

Feasibility Analysis of Engine Control Tasks under EDF Scheduling

Engine control applications include software tasks that are triggered at predetermined angular values of the crankshaft, thus generating a computational workload that varies with the engine speed. To avoid overloads at high rotation speeds, these tasks are implemented to self adapt and reduce their computational demand by switching mode at given rotation speeds. For this reason, they are referred to as adaptive variable rate (AVR) tasks. Although a few works have been proposed in the literature to model and analyze the schedulability of such a peculiar type of tasks, an exact analysis of engine control applications has been derived only for fixed priority systems, under a set of simplifying assumptions. The major problem of scheduling AVR tasks with fixed priorities, however, is that, due to engine accelerations, the interarrival period of an AVR task is subject to large variations, therefore there will be several speeds at which any fixed priority assignment is far from being optimal, significantly penalizing the schedulability of the system. This paper proposes for the first time an exact feasibility test under the Earliest Deadline First scheduling algorithm for tasks sets including regular periodic tasks and AVR tasks triggered by a common rotation source. In addition, a set of simulation results are reported to evaluate the schedulability gain achieved in this context by EDF over fixed priority scheduling.

Optimal Design for Reservation Servers under Shared Resources

Modularity and hierarchical-based design are crucial features that need to be supported in complex embedded systems characterized by multiple applications with timing requirements.Resource reservation is a powerful scheduling mechanism for achieving such goals and providing temporal isolation among different real-time applications. When different applications share mutually exclusive resources, a precise feasibility analysis can still be performed in isolation, using specific resource access protocols, taking into account only the application features and the reservation parameters. This paper presents a methodology for selecting the parameters of each reservation in order to guarantee the feasibility of the served applications and minimize the required bandwidth.

Hard Constant Bandwidth Server: Comprehensive formulation and critical scenarios

The Constant Bandwidth Server (CBS) is one of the most used algorithms for implementing resource reservation upon deadline-based schedulers. Although many CBS variants are available in the literature, no proper formalization has been proposed for the CBS in the context of hard reservations, where it is essential to guarantee a bounded-delay service across applications. Existing formulations are affected by a problem that can expose the system to dangerous deadline misses in the presence of blocking. This paper analyzes such a problem and presents a comprehensive and consistent formulation of the CBS for hard reservation scenarios. An overview of the contexts in which a hard CBS can be applied is also provided, focusing on the impact that previous formulations can have on schedulability, when used in conjunction with specific resource sharing protocols or other scheduling mechanisms that may cause a server to block.

OSEK-Like Kernel Support for Engine Control Applications under EDF Scheduling

Engine control applications typically include computational activities consisting of periodic tasks, activated by timers, and engine-triggered tasks, activated at specific angular positions of the crankshaft. Such tasks are typically managed by a OSEK-compliant real-time kernel using a fixed-priority scheduler, as specified in the AUTOSAR standard adopted by most automotive industries. Recent theoretical results, however, have highlighted significant limitations of fixed-priority scheduling in managing engine-triggered tasks that could be solved by a dynamic scheduling policy. To address this issue, this paper proposes a new kernel implementation within the ERIKA Enterprise operating system, providing EDF scheduling for both periodic and engine-triggered tasks. The proposed kernel has been conceived to have an API similar to the AUTOSAR/OSEK standard one, limiting the effort needed to use the new kernel with an existing legacy application. The proposed kernel implementation is discussed and evaluated in terms of run-time overhead and footprint. In addition, a simulation framework is presented, showing a powerful environment for studying the execution of tasks under the proposed kernel.

Engine control: task modeling and analysis

Engine control is characterized by computational activities that are triggered by specific crankshaft rotation angles and are designed to adapt their functionality based on the angular velocity of the engine. Although a few models have been proposed in the literature to handle such tasks, most of them are quite simplistic and do not allow expressing features that are presently used by the automotive industry. This paper proposes a new task model for expressing realistic features of engine control tasks and presents a sufficient real-time analysis for applications consisting of multiple engine control tasks and classical periodic/sporadic tasks scheduled by EDF.