Towards Parallel Programming Models for Predictability

Future embedded systems for performance-demanding applications will be massively parallel. High performance tasks will be parallel programs, running on several cores, rather than single threads running on single cores. For hard real-time applications, WCETs for such tasks must be bounded. Low-level parallel programming models, based on concurrent threads, are notoriously hard to use due to their inherent nondeterminism. Therefore the parallel processing community has long considered high-level parallel programming models, which restrict the low-level models to regain determinism. In this position paper we argue that such parallel programming models are beneficial also for WCET analysis of parallel programs. We review some proposed models, and discuss their influence on timing predictability. In particular we identify data parallel programming as a suitable paradigm as it is deterministic and allows current methods for WCET analysis to be extended to parallel code. GPUs are increasingly used for high performance applications: we discuss a current GPU architecture, and we argue that it offers a parallel platform for compute-intensive applications for which it seems possible to construct precise timing models. Thus, a promising route for the future is to develop WCET analyses for data-parallel software running on GPUs

Towards a Time-predictable Dual-Issue Microprocessor: The Patmos Approach

Current processors are optimized for average case performance, often leading to a high worst-case execution time (WCET). Many architectural features that increase the average case performance are hard to be modeled for the WCET analysis. In this paper we present Patmos, a processor optimized for low WCET bounds rather than high average case performance. Patmos is a dual-issue, statically scheduled RISC processor. The instruction cache is organized as a method cache and the data cache is organized as a split cache in order to simplify the cache WCET analysis. To fill the dual-issue pipeline with enough useful instructions, Patmos relies on a customized compiler. The compiler also plays a central role in optimizing the application for the WCET instead of average case performance

parMERASA Multi-Core Execution of Parallelised Hard Real-Time Applications Supporting Analysability

International audienceEngineers who design hard real-time embedded systems express a need for several times the performance available today while keeping safety as major criterion. A breakthrough in performance is expected by parallelizing hard real-time applications and running them on an embedded multi-core processor, which enables combining the requirements for high-performance with timing-predictable execution. parMERASA will provide a timing analyzable system of parallel hard real-time applications running on a scalable multicore processor. parMERASA goes one step beyond mixed criticality demands: It targets future complex control algorithms by parallelizing hard real-time programs to run on predictable multi-/many-core processors. We aim to achieve a breakthrough in techniques for parallelization of industrial hard real-time programs, provide hard real-time support in system software, WCET analysis and verification tools for multi-cores, and techniques for predictable multi-core designs with up to 64 cores

Contention in multicore hardware shared resources: Understanding of the state of the art

The real-time systems community has over the years devoted considerable attention to the impact on execution timing that arises from contention on access to hardware shared resources. The relevance of this problem has been accentuated with the arrival of multicore processors. From the state of the art on the subject, there appears to be considerable diversity in the understanding of the problem and in the “approach” to solve it. This sparseness makes it difficult for any reader to form a coherent picture of the problem and solution space. This paper draws a tentative taxonomy in which each known approach to the problem can be categorised based on its specific goals and assumptions.Postprint (published version

A Multi-core processor for hard real-time systems

The increasing demand for new functionalities in current and future hard real-time embedded systems, like the ones deployed in automotive and avionics industries, is driving an increment in the performance required in current embedded processors. Multi-core processors represent a good design solution to cope with such higher performance requirements due to their better performance-per-watt ratio while maintaining the core design simple. Moreover, multi-cores also allow executing mixed-criticality level workloads composed of tasks with and without hard real-time requirements, maximizing the utilization of the hardware resources while guaranteeing low cost and low power consumption. Despite those benefits, current multi-core processors are less analyzable than single-core ones due to the interferences between different tasks when accessing hardware shared resources. As a result, estimating a meaningful Worst-Case Execution Time (WCET) estimation - i.e. to compute an upper bound of the application's execution time - becomes extremely difficult, if not even impossible, because the execution time of a task may change depending on the other threads running at the same time. This makes the WCET of a task dependent on the set of inter-task interferences introduced by the co-running tasks. Providing a WCET estimation independent from the other tasks (time composability property) is a key requirement in hard real-time systems. This thesis proposes a new multi-core processor design in which time composability is achieved, hence enabling the use of multi-cores in hard real-time systems. With our proposals the WCET estimation of a HRT is independent from the other co-running tasks. To that end, we design a multi-core processor in which the maximum delay a request from a Hard Real-time Task (HRT), accessing a hardware shared resource can suffer due to other tasks is bounded: our processor guarantees that a request to a shared resource cannot be delayed longer than a given Upper Bound Delay (UBD). In addition, the UBD allows identifying the impact that different processor configurations may have on the WCET by determining the sensitivity of a HRT to different resource allocations. This thesis proposes an off-line task allocation algorithm (called IA3: Interference-Aware Allocation Algorithm), that allocates tasks in a task set based on the HRT's sensitivity to different resource allocations. As a result the hardware shared resources used by HRTs are minimized, by allowing Non Hard Real-time Tasks (NHRTs) to use the rest of resources. Overall, our proposals provide analyzability for the HRTs allowing NHRTs to be executed into the same chip without any effect on the HRTs. The previous first two proposals of this thesis focused on supporting the execution of multi-programmed workloads with mixed-criticality levels (composed of HRTs and NHRTs). Higher performance could be achieved by implementing multi-threaded applications. As a first step towards supporting hard real-time parallel applications, this thesis proposes a new hardware/software approach to guarantee a predictable execution of software pipelined parallel programs. This thesis also investigates a solution to verify the timing correctness of HRTs without requiring any modification in the core design: we design a hardware unit which is interfaced with the processor and integrated into a functional-safety aware methodology. This unit monitors the execution time of a block of instructions and it detects if it exceeds the WCET. Concretely, we show how to handle timing faults on a real industrial automotive platform.La creciente demanda de nuevas funcionalidades en los sistemas empotrados de tiempo real actuales y futuros en industrias como la automovilística y la de aviación, está impulsando un incremento en el rendimiento necesario en los actuales procesadores empotrados. Los procesadores multi-núcleo son una solución eficiente para obtener un mayor rendimiento ya que aumentan el rendimiento por vatio, manteniendo el diseño del núcleo simple. Por otra parte, los procesadores multi-núcleo también permiten ejecutar cargas de trabajo con niveles de tiempo real mixtas (formadas por tareas de tiempo real duro y laxo así como tareas sin requerimientos de tiempo real), maximizando así la utilización de los recursos de procesador y garantizando el bajo consumo de energía. Sin embargo, a pesar los beneficios mencionados anteriormente, los actuales procesadores multi-núcleo son menos analizables que los de un solo núcleo debido a las interferencias surgidas cuando múltiples tareas acceden simultáneamente a los recursos compartidos del procesador. Como resultado, la estimación del peor tiempo de ejecución (conocido como WCET) - es decir, una cota superior del tiempo de ejecución de la aplicación - se convierte en extremadamente difícil, si no imposible, porque el tiempo de ejecución de una tarea puede cambiar dependiendo de las otras tareas que se estén ejecutando concurrentemente. Determinar una estimación del WCET independiente de las otras tareas es un requisito clave en los sistemas empotrados de tiempo real duro. Esta tesis propone un nuevo diseño de procesador multi-núcleo en el que el tiempo de ejecución de las tareas se puede componer, lo que permitirá el uso de procesadores multi-núcleo en los sistemas de tiempo real duro. Para ello, diseñamos un procesador multi-núcleo en el que la máxima demora que puede sufrir una petición de una tarea de tiempo real duro (HRT) para acceder a un recurso hardware compartido debido a otras tareas está acotado, tiene un límite superior (UBD). Además, UBD permite identificar el impacto que las diferentes posibles configuraciones del procesador pueden tener en el WCET, mediante la determinación de la sensibilidad en la variación del tiempo de ejecución de diferentes reservas de recursos del procesador. Esta tesis propone un algoritmo estático de reserva de recursos (llamado IA3), que asigna tareas a núcleos en función de dicha sensibilidad. Como resultado los recursos compartidos del procesador usados por tareas HRT se reducen al mínimo, permitiendo que las tareas sin requerimiento de tiempo real (NHRTs) puedas beneficiarse del resto de recursos. Por lo tanto, las propuestas presentadas en esta tesis permiten el análisis del WCET para tareas HRT, permitiendo así mismo la ejecución de tareas NHRTs en el mismo procesador multi-núcleo, sin que estas tengan ningún efecto sobre las tareas HRT. Las propuestas presentadas anteriormente se centran en el soporte a la ejecución de múltiples cargas de trabajo con diferentes niveles de tiempo real (HRT y NHRTs). Sin embargo, un mayor rendimiento puede lograrse mediante la transformación una tarea en múltiples sub-tareas paralelas. Esta tesis propone una nueva técnica, con soporte del procesador y del sistema operativo, que garantiza una ejecución analizable del modelo de ejecución paralela software pipelining. Esta tesis también investiga una solución para verificar la corrección del WCET de HRT sin necesidad de ninguna modificación en el diseño de la base: un nuevo componente externo al procesador se conecta a este sin necesidad de modificarlo. Esta nueva unidad monitorea el tiempo de ejecución de un bloque de instrucciones y detecta si se excede el WCET. Esta unidad permite detectar fallos de sincronización en sistemas de computación utilizados en automóviles

OTAWA: An Open Toolbox for Adaptive WCET Analysis

International audienceThe analysis of worst-case execution times has become mandatory in the design of hard real-time systems: it is absolutely necessary to know an upper bound of the execution time of each task to determine a task schedule that insures that deadlines will all be met. The OTAWA toolbox presented in this paper has been designed to host algorithms resulting from research in the domain of WCET analysis so that they can be combined to compute tight WCET estimates. It features an abstraction layer that decouples the analyses from the target hardware and from the instruction set architecture, as well as a set of functionalities that facilitate the implementation of new approaches

Design and Implementation of a Time Predictable Processor: Evaluation With a Space Case Study

Embedded real-time systems like those found in automotive, rail and aerospace, steadily require higher levels of guaranteed computing performance (and hence time predictability) motivated by the increasing number of functionalities provided by software. However, high-performance processor design is driven by the average-performance needs of mainstream market. To make things worse, changing those designs is hard since the embedded real-time market is comparatively a small market. A path to address this mismatch is designing low-complexity hardware features that favor time predictability and can be enabled/disabled not to affect average performance when performance guarantees are not required. In this line, we present the lessons learned designing and implementing LEOPARD, a four-core processor facilitating measurement-based timing analysis (widely used in most domains). LEOPARD has been designed adding low-overhead hardware mechanisms to a LEON3 processor baseline that allow capturing the impact of jittery resources (i.e. with variable latency) in the measurements performed at analysis time. In particular, at core level we handle the jitter of caches, TLBs and variable-latency floating point units; and at the chip level, we deal with contention so that time-composable timing guarantees can be obtained. The result of our applied study with a Space application shows how per-resource jitter is controlled facilitating the computation of high-quality WCET estimates

The potential of programmable logic in the middle: cache bleaching

Consolidating hard real-time systems onto modern multi-core Systems-on-Chip (SoC) is an open challenge. The extensive sharing of hardware resources at the memory hierarchy raises important unpredictability concerns. The problem is exacerbated as more computationally demanding workload is expected to be handled with real-time guarantees in next-generation Cyber-Physical Systems (CPS). A large body of works has approached the problem by proposing novel hardware re-designs, and by proposing software-only solutions to mitigate performance interference. Strong from the observation that unpredictability arises from a lack of fine-grained control over the behavior of shared hardware components, we outline a promising new resource management approach. We demonstrate that it is possible to introduce Programmable Logic In-the-Middle (PLIM) between a traditional multi-core processor and main memory. This provides the unique capability of manipulating individual memory transactions. We propose a proof-of-concept system implementation of PLIM modules on a commercial multi-core SoC. The PLIM approach is then leveraged to solve long-standing issues with cache coloring. Thanks to PLIM, colored sparse addresses can be re-compacted in main memory. This is the base principle behind the technique we call Cache Bleaching. We evaluate our design on real applications and propose hypervisor-level adaptations to showcase the potential of the PLIM approach.Accepted manuscrip