On Strong and Weak Sustainability, with an Application to Self-Suspending Real-Time Tasks

Motivated by an apparent contradiction regarding whether certain scheduling policies are sustainable, we revisit the topic of sustainability in real-time scheduling and argue that the existing definitions of sustainability should be further clarified and generalized. After proposing a formal, generic sustainability theory, we relax the existing notion of (strongly) sustainable scheduling policy to provide a new classification called weak sustainability. Proving weak sustainability properties allows reducing the number of variables that must be considered in the search of a worst-case schedule, and hence enables more efficient schedulability analyses and testing regimes even for policies that are not (strongly) sustainable. As a proof of concept, and to better understand a model for which many mistakes were found in the literature, we study weak sustainability in the context of dynamic self-suspending tasks, where we formalize a generic suspension model using the Coq proof assistant and provide a machine-checked proof that any JLFP scheduling policy is weakly sustainable with respect to job costs and variable suspension times

A Response-Time Analysis for Non-Preemptive Job Sets under Global Scheduling

An effective way to increase the timing predictability of multicore platforms is to use non-preemptive scheduling. It reduces preemption and job migration overheads, avoids intra-core cache interference, and improves the accuracy of worst-case execution time (WCET) estimates. However, existing schedulability tests for global non-preemptive multiprocessor scheduling are pessimistic, especially when applied to periodic workloads. This paper reduces this pessimism by introducing a new type of sufficient schedulability analysis that is based on an exploration of the space of possible schedules using concise abstractions and state-pruning techniques. Specifically, we analyze the schedulability of non-preemptive job sets (with bounded release jitter and execution time variation) scheduled by a global job-level fixed-priority (JLFP) scheduling algorithm upon an identical multicore platform. The analysis yields a lower bound on the best-case response-time (BCRT) and an upper bound on the worst-case response time (WCRT) of the jobs. In an empirical evaluation with randomly generated workloads, we show that the method scales to 30 tasks, a hundred thousand jobs (per hyperperiod), and up to 9 cores

Response-Time Analysis of Limited-Preemptive Parallel DAG Tasks Under Global Scheduling

Most recurrent real-time applications can be modeled as a set of sequential code segments (or blocks) that must be (repeatedly) executed in a specific order. This paper provides a schedulability analysis for such systems modeled as a set of parallel DAG tasks executed under any limited-preemptive global job-level fixed priority scheduling policy. More precisely, we derive response-time bounds for a set of jobs subject to precedence constraints, release jitter, and execution-time uncertainty, which enables support for a wide variety of parallel, limited-preemptive execution models (e.g., periodic DAG tasks, transactional tasks, generalized multi-frame tasks, etc.). Our analysis explores the space of all possible schedules using a powerful new state abstraction and state-pruning technique. An empirical evaluation shows the analysis to identify between 10 to 90 percentage points more schedulable task sets than the state-of-the-art schedulability test for limited-preemptive sporadic DAG tasks. It scales to systems of up to 64 cores with 20 DAG tasks. Moreover, while our analysis is almost as accurate as the state-of-the-art exact schedulability test based on model checking (for sequential non-preemptive tasks), it is three orders of magnitude faster and hence capable of analyzing task sets with more than 60 tasks on 8 cores in a few seconds

nDimNoC: Real-Time D-dimensional NoC

The growing demand of powerful embedded systems to perform advanced functionalities led to a large increase in the number of computation nodes integrated in Systems-on-chip (SoC). In this context, network-on-chips (NoCs) emerged as a new standard communication infrastructure for multi-processor SoCs (MPSoCs). In this work, we present nDimNoC, a new D-dimensional NoC that provides real-time guarantees for systems implemented upon MPSoCs. Specifically, (1) we propose a new router architecture and a new deflection-based routing policy that use the properties of circulant topologies to ensure bounded worst-case communication delays, and (2) we develop a generic worst-case communication time (WCCT) analysis for packets transmitted over nDimNoC. In our experiments, we show that the WCCT of packets decreases when we increase the dimensionality of the NoC using nDimNoC 19s topolgy and routing policy. By implementing nDimNoC in Verilog and synthesizing it for an FPGA platform, we show that a 3D-nDimNoC requires "485-times less silicon than routers that use virtual channels (VC). We computed the maximum operating frequency of a 3D-nDimNoC with Xilinx Vivado. Increasing the number dimensions in the NoC improves WCCT at the cost of a more complex routing logic that may result in a reduced operating clock frequency.info:eu-repo/semantics/publishedVersio

Design and implementation of an FPGA-based NoC for Real Time Systems

In order to communicate, cores of a multi-core platform traditionally relied on shared busses. However, with the increasing number of computation nodes integrated in multi- and many-core platforms, Network-on-Chips (NoCs) emerged as a new alternative communication medium in Systems-on-Chips (SoCs). Hoplite-RT is a new NoC design that was recently proposed. Hoplite-RT is a compact design easy to analyze and with a low-cost implementation that was specifically tailored for FPGA. In this work, we introduce priority-based routing to Hoplite-RT and change the network topology so as to improve its timing behavior, i.e., its Worst-Case Traversal Time (WCTT).info:eu-repo/semantics/publishedVersio

The SRP Resource Sharing Protocol for Self-Suspending Tasks

Motivated by the increasingly wide adoption of realtime workload with self-suspending behaviors, and the relevance of mechanisms to handle mutually-exclusive shared resources, this paper takes a new look at locking protocols for self-suspending tasks under uniprocessor fixed-priority scheduling. Pitfalls when integrating the widely-adopted Stack Resource Policy (SRP) with self-suspending tasks are firstly illustrated, and then a new finegrained SRP analysis is presented. Next, a new locking protocol, named SRP-SS, is proposed to overcome the limitations of the original SRP. The SRP-SS is a generalization of the SRP to cope with the specificities of self-suspending tasks. It therefore reduces to the SRP under some configurations and hence theoretically dominates the SRP. It also ensures backward compatibility for applications developed specifically for the SRP. The SRP-SS comes with its own schedulability analysis and configuration algorithm. The performances of the SRP and SRP-SS are finally studied by means of large-scale schedulability experiments.info:eu-repo/semantics/publishedVersio