The problems you're having may not be the problems you think you're having: results from a latency study of windows NT

ManuscriptThis paper is intended to catalyze discussions on two intertwined systems topics. First, it presents early results from a latency study of Windows NT that identifies some specific causes of long thread scheduling latencies, many of which delay the dispatching of runnable threads for tens of milliseconds. Reasons for these delays, including technical, methodological, and economic are presented and possible solutions are discussed. Secondly, and equally importantly, it is intended to serve as a cautionary tale against believing one's own intuition about the causes of poor system performance. We went into this study believing we understood a number of the causes of these delays, with our beliefs informed more by conventional wisdom and hunches than data. In nearly all cases the reasons we discovered via instrumentation and measurement surprised us. In fact, some directly contradicted "facts" we thought we "knew"

Two case studies in predictable application scheduling using Rialto/NT

Journal ArticleThis paper analyzes the results of two case studies in applying the Rialto/NT scheduler to real Windows 2000 applications. The first study is of a soft modem-a modem whose signal processing work is performed on the host CPU, rather than on a dedicated signal processing chip. The second is of an audio player application. Both of these are frequently used real-time applications-ones running on systems that were not designed to support predictable real-time execution. To function correctly, both applications require that ongoing computations be performed in a timely manner. In both cases, we first measured an original version designed to run on Windows 2000, and then modified the application to take advantage of ongoing CPU Reservations provided by the Rialto/NT scheduler. We report on the benefits and problems observed when using reservations in these realworld scenarios. In both cases, we found that a real-time scheduler can provide the needed predictability for the application in the presence of competing applications, while also providing other benefits, such as minimizing the soft modem's impact on the scheduling predictability of other computations in the system. We also describe the methodologies we used to analyze the real-time behavior of the operating system and applications during these studies, including the use of instrumented kernels to produce execution traces

Model driven engineering method for SAIA architecture design

National audienceABSTRACT. SAIA is an architectural style for the development of systems dedicated to process control. Designing an architecture that conforms to a style implies the manipulation of a lot of entities and the respect of numerous constraints. The approaches based on models and models transformations are well adapted to manage the complexity and to enforce the separation of concerns. This paper presents a model driven engineering method for the development and the validation of systems that conform to SAIA. Moreover, a tool supporting the method allows a systematic use of models and transformations.RÉSUMÉ. SAIA est un style architectural destiné au développement de systèmes dédiés au contrôle de procédés. Construire une architecture conforme à un style architectural donné nécessite la manipulation de nombreuses entités et le respect de nombreuses contraintes. Les approches basées sur les modèles et les transformations de modèles permettent de gérer cette complexité et d'imposer une séparation des préoccupations. Notre objectif est alors de fournir une méthode basée sur les concepts définis par l'ingénierie dirigée par les modèles pour le développement et la validation de systèmes conformes à SAIA. Enfin, un outil implémente la méthode proposée afin d'aider le concepteur à respecter les modèles et leurs transformations

Operating system profiling via latency analysis

Operating systems are complex and their behavior depends on many factors. Source code, if available, does not directly help one to understand the OS’s behavior, as the behavior depends on actual workloads and external inputs. Runtime profiling is a key technique to prove new concepts, debug problems, and optimize performance. Unfortunately, existing profiling methods are lacking in important areas—they do not provide enough information about the OS’s behavior, they require OS modification and therefore are not portable, or they incur high overheads thus perturbing the profiled OS. We developed OSprof: a versatile, portable, and efficient OS profiling method based on latency distributions analysis. OSprof automatically selects important profiles for subsequent visual analysis. We have demonstrated that a suitable workload can be used to profile virtually any OS component. OSprof is portable because it can intercept operations and measure OS behavior from user-level or from inside the kernel without requiring source code. OSprof has typical CPU time overheads below 4%. In this paper we describe our techniques and demonstrate their usefulness through a series of profiles conducted on Linux, FreeBSD, and Windows, including client/server scenarios. We discovered and investigated a number of interesting interactions, including scheduler behavior, multi-modal I/O distributions, and a previously unknown lock contention, which we fixed.

Using hierarchical scheduling to support soft real-time applications in general-purpose operating systems

Journal ArticleThe CPU schedulers in general-purpose operating systems are designed to provide fast response time for interactive applications and high throughput for batch applications. The heuristics used to achieve these goals do not lend themselves to scheduling real-time applications, nor do they meet other scheduling requirements such as coordinating scheduling across several processors or machines, or enforcing isolation between applications, users, and administrative domains. Extending the scheduling subsystems of general-purpose operating systems in an ad hoc manner is time consuming and requires considerable expertise as well as source code to the operating system. Furthermore, once extended, the new scheduler may be as inflexible as the original. The thesis of this dissertation is that extending a general-purpose operating system with a general, heterogeneous scheduling hierarchy is feasible and useful. A hierarchy of schedulers generalizes the role of CPU schedulers by allowing them to schedule other schedulers in addition to scheduling threads. A general, heterogeneous scheduling hierarchy is one that allows arbitrary (or nearly arbitrary) scheduling algorithms throughout the hierarchy. In contrast, most of the previous work on hierarchical scheduling has imposed restrictions on the schedulers used in part or all of the hierarchy. This dissertation describes the Hierarchical Loadable Scheduler (HLS) architecture, which permits schedulers to be dynamically composed in the kernel of a general-purpose operating system. The most important characteristics of HLS, and the ones that distinguish it from previous work, are that it has demonstrated that a hierarchy of nearly arbitrary schedulers can be efficiently implemented in a general-purpose operating system, and that the behavior of a hierarchy of soft real-time schedulers can be reasoned about in order to provide guaranteed scheduling behavior to application threads. The flexibility afforded by HLS permits scheduling behavior to be tailored to meet complex requirements without encumbering users who have modest requirements with the performance and administrative costs of a complex scheduler. Contributions of this dissertation include the following. (1) The design, prototype implementation, and performance evaluation of HLS in Windows 2000. (2) A system of guarantees for scheduler composition that permits reasoning about the scheduling behavior of a hierarchy of soft real-time schedulers. Guarantees assure users that application requirements can be met throughout the lifetime of the application, and also provide application developers with a model of CPU allocation to which they can program. (3) The design, implementation, and evaluation of two augmented CPU reservation schedulers, which provide increase scheduling predictability when low-level operating system activity steals time from applications