Bryan C. Ward

Supporting Nested Locking in Multiprocessor Real-Time Systems

This paper presents the first real-time multiprocessor locking protocol that supports fine-grained nested resource requests. This locking protocol relies on a novel technique for ordering the satisfaction of resource requests to ensure a bounded duration of priority inversions for nested requests. This technique can be applied on partitioned, clustered, and globally scheduled systems in which waiting is realized by either spinning or suspending. Furthermore, this technique can be used to construct fine-grained nested locking protocols that are efficient under spin-based, suspension-oblivious or suspension-aware analysis of priority inversions. Locking protocols built upon this technique perform no worse than coarse-grained locking mechanisms, while allowing for increased parallelism in the average case (and, depending upon the task set, better worst-case performance).

Safe: simulation automation framework for experiments

The workflow of a network simulation study requires adherence to best practices in methodology so that results are credible and reproducible by third parties. The opportunities for one to introduce errors start at model description and permeate the process through to the reporting of results. The literature indicates that even publications in respected venues include inadvertent mistakes and poor application of methodology. When experts are liable to fail, it is unreasonable to expect that students would fare any better. This paper presents a system designed to provide guidance for inexperienced users of the popular ns-3 network simulator. SAFE automates the workflow from the initialization of model parameters, to the parallelized execution of experiments, to the processing and persistent storage of output data, and to graphical visualization of results. We discuss the architecture and the implementation of the system in the context of similar contributions in the literature.

Fair lateness scheduling: reducing maximum lateness in G-EDF-like scheduling

In prior work on soft real-time (SRT) multiprocessor scheduling, tardiness bounds have been derived for a variety of scheduling algorithms, most notably, the global earliest-deadline-first (G-EDF) algorithm. In this paper, we devise G-EDF-like (GEL) schedulers, which have identical implementations to G-EDF and therefore the same overheads, but that provide better tardiness bounds. We discuss how to analyze these schedulers and propose methods to determine scheduler parameters to meet several different tardiness bound criteria. We employ linear programs to adjust such parameters to optimize arbitrary tardiness criteria, and to analyze lateness bounds (lateness is related to tardiness). We also propose a particular scheduling algorithm, namely the global fair lateness (G-FL) algorithm, to minimize maximum absolute lateness bounds. Unlike the other schedulers described in this paper, G-FL only requires linear programming for analysis. We argue that our proposed schedulers, such as G-FL, should replace G-EDF for SRT applications.

Outstanding Paper Award: Making Shared Caches More Predictable on Multicore Platforms

In safety-critical cyber-physical systems, the usage of multicore platforms has been hampered by problems due to interactions across cores through shared hardware. The inability to precisely characterize such interactions can lead to worst-case execution time pessimism that is so great, the extra processing capacity of additional cores is entirely negated. In this paper, several techniques are proposed and analyzed for dealing with such interactions in the context of shared caches. These techniques are applied in a mixed-criticality scheduling framework motivated by the needs of next-generation unmanned air vehicles.

On the automation of computer network simulators

Simulation has been an important resource for functional and performance analyses of computer networks. Although the number of widely adopted network simulators is small, new ones continue to be created to address gaps in the functionality of existing tools. It can be argued, however, that the greatest need of the scientific community is to raise the credibility of published simulation studies. In this paper, we show that this need can be addressed by enabling network simulators to provide fool-proof automation of the experimental process. Ideally, the simulators interface would provide users with an environment to minimize set up time for experiments and to guarantee their reproducibility, and to safeguard the statistical rigor of data analysis. We demonstrate that advances toward this goal have been made by three different tools. Our contributions in this paper culminate with the derivation of requirements for automation tools from recent literature and from our own experience in tool construction. Once these requirements are fulfilled, network simulation tools can have a stronger impact in education, in carrying out large simulation studies, and in enhancing the credibility of simulation results.

Cache Sharing and Isolation Tradeoffs in Multicore Mixed-Criticality Systems

In mixed-critical applications, tension exists between sharing and isolation with respect to hardware resources: while strong isolation might be required for highly critical tasks, somewhat permissive sharing might be reasonable for less critical tasks to improve throughput or average-case performance. In this paper, this tension is examined as it pertains to shared last-level caches (LLCs) on multicore platforms. In particular, criticality-aware optimization techniques based on linear programming are presented for allocating LLC areas in the context of the previously proposed MC2 (mixed-criticality on multicore) framework. Experiments are also presented that show that these techniques can result in significant schedulability improvements.

Replica-Request Priority Donation: A Real-Time Progress Mechanism for Global Locking Protocols

Real-time locking protocols employ progress mechanism(s) to ensure that resource-holding jobs are scheduled. These mechanisms are required to bound the duration of priority-inversion blocking (pi-blocking) for jobs sharing resources. Examples of such progress mechanisms include priority inheritance and priority donation. Unfortunately, some progress mechanisms can cause any job, including those that never request shared resources, to be blocked upon job release. This paper presents a variant of priority donation for globally-scheduled systems that only causes blocking for jobs waiting for shared resources. Additionally, this variant of priority donation is employed to construct a new suspension-based locking protocol called the replica-request donation global locking protocol (R2DGLP), which is asymptotically optimal for both mutex and k-exclusion (i.e., multi-unit) resources. This work is motivated by multicore systems where tasks may share I/O devices (e.g., GPUs) where critical sections can be long. In such applications, progress mechanisms that cause jobs that do not access I/O devices to be blocked to ensure progress can be detrimental from a schedulability perspective.

Enhancing the Credibility of Wireless Network Simulations with Experiment Automation

The last few years have witnessed a growing consensus around the notion that many papers discussing wireless network simulation are plagued by issues that weaken their scientific value. A number of articles have shown evidence of this crisis of credibility and identified many of its causes. In this paper, we show that the methodology flaws in wireless network simulation can be avoided with the use of a framework for experiment automation. We describe the rationale that drove us to develop tools for component-based simulators intending to guide the experimental process from first to last stages. We conclude that a framework that imposes the right constraints on the experimenter can lead to more credible simulation studies. The framework we present helps the construction of consistent models, the definition of model parameters, the design and the execution of experiments, the analysis of output data, and the preparation of data for the dissemination of results that allow experiments to be reproduced.

Reconciling the Tension Between Hardware Isolation and Data Sharing in Mixed-Criticality, Multicore Systems

Recent work involving a mixed-criticality framework called MC2 has shown that, by combining hardware-management techniques and criticality-aware task provisioning, capacity loss can be significantly reduced when supporting real-time workloads on multicore platforms. However, as in most other prior research on multicore hardware management, tasks were assumed in that work to not share data. Data sharing is problematic in the context of hardware management because it can violate the isolation properties hardware-management techniques seek to ensure. Clearly, for research on such techniques to have any practical impact, data sharing must be permitted. Towards this goal, this paper presents a new version of MC2 that permits tasks to share data within and across criticality levels through shared memory. Several techniques are presented for mitigating capacity loss due to data sharing. The effectiveness of these techniques is demonstrated by means of a large-scale, overhead-aware schedulability study driven by micro-benchmark data.

Optimizing Preemption-Overhead Accounting in Multiprocessor Real-Time Systems

There exist two general techniques to account for preemption-related overheads on multiprocessors. This paper presents a new preemption-related overhead-accounting technique, called analytical redistribution of preemption overheads (ARPO), which integrates the two previous techniques to minimize preemption-overhead-related utilization loss. ARPO is applicable under any job-level fixed priority (JLFP) preemptive scheduler, as well as some limited-preemption schedulers. ARPO is evaluated in a new experimental-design framework for overhead-aware schedulability studies that addresses unrealistic simplifying assumptions made in previous studies, and is shown to improve real-time schedulability.