Claude-Joachim Hamann

DROPS: OS support for distributed multimedia applications

The characterising new requirement for distributed multimedia applications is the coexistence of dynamic real-time and non-real-time applications on hosts and networks. While some networks (e.g., ATM) in principle have the capability to reserve bandwidth on shared links, host systems usually do not. DROPS (Dresden Real-time OPerating System) is being built to remedy that situation by providing resource managers that allow the reservation of resources in advance and enforce that reservations. It allows the coexistence of timesharing applications (with no reservations) and real-time applications (with reservations). By outlining the principle architecture, some design decisions, and first results, the paper demonstrates how these objectives can be met using straightforward OS technology. It argues that middleware for diverse platforms cannot meet these objectives efficiently without proper core operating system support.

Quality-assuring scheduling-using stochastic behavior to improve resource utilization

We present a unified model for admission and scheduling, applicable for various active resources such as CPU or disk to assure a requested quality in situations of temporary overload. The model allows us to predict and control the behavior of applications based on given quality requirements. It uses the variations in the execution time, i.e., the time any active resource is needed We split resource requirements into a mandatory part which must be available and an optional part which should be available as often as possible but at least with a certain percentage. In combination with a given distribution for the execution time we can move away from worst-case reservations and drastically reduce the amount of reserved resources for applications which can tolerate occasional deadline misses. This increases the number of admittable applications. For example, with negligible loss of quality our system can admit more than two times the disk bandwidth than a system based on the worst-case. Finally, we validated the predictions of our model by measurements using a prototype real-time system and observed a high accuracy between predicted and measured values.

Integrated resource management for data stream systems

Data stream systems have to deal with massive data volumes. To perform several queries in parallel or to perform even a single query, resources must be planned carefully and the resulting quality-of-service (QoS) is lower than the best one. Typical QoS measures are the output delay and the amount of data in the stream used for the processing. In this paper, we introduce a model which allows to describe stream operators and the streams between the operators of an operator graph belonging to a stream query. The model allows us to calculate the resources consumed by a query graph given a certain result quality. Furthermore, it can be used to determine in advance if the quality-of-service requirement of a given query can be met with the actual available system resources. This model is the basis for building QoS-guaranteeing systems.

Avoiding timing channels in fixed-priority schedulers

A practically feasible modification to fixed-priority schedulers allows to avoid timing channels despite threads having access to precise clocks. This modification is rather simple: we compute at admission time a static predicate that states whether a thread may possibly leak information; if such a thread blocks we switch to the idle thread instead. We describe the modified scheduler, provide a mechanical PVS-based proof of noninterference and show how common admission algorithms can be reused to give real-time guarantees for this modified scheduler. While providing similar isolation guarantees, our approach outperforms time-partitioning schedulers in terms of achieved real-time guarantees.

Probabilistic Admission Control to Govern Real-Time Systems under Overload

Existing real-time research focuses on how to formulate. model and enforce timeliness guarantees for task sets whose correctness has a temporal aspect. However; the resulting systems often exhibit poor resource utilization due to the resource scheduler reserving more resources than required in order to ensure that admitted schedules can be satisfied under worst case conditions. Weakening the guarantees leads to the known concepts of firm and soft real-time tasks, butt we think the paradigm needs to be shifted further,: reifying efficient utilization. With Quality-Assuring Scheduling (QAS) we presented such an algorithm. However: its practical applicability is restricted to uniform and harmonic periods, due to its complexity for arbitrary periods. To overcome this limitation, we introduce Quality-Rate-Monotonic Scheduling (QRMS), which, although slightly more pessimistic, is less complex compared to QAS. Thee admission control is again based on a probabilistic model to ensure that a requested fraction of jobs is successfully executed. Thus the amount of missed deadlines can be externally controlled, even in sustained overload situations.

On confidentiality-preserving real-time locking protocols

Coordinating access to shared resources is a challenging task, in particular if real-time and security aspects have to be integrated into the same system. However, rather than exacerbating the problem, we found that considering real-time guarantees actually simplifies the security problem of preventing information leakage over shared-resource covert channels. We introduce a transformation for standard real-time resource locking protocols and show that protocols transformed in this way preserve the confidentiality guarantees of the schedulers on which they are based. Through this transformation, we were able to prove that four out of the seven investigated protocols are information-flow secure.

Scheduling Real-Time Components Using Jitter-Constrained Streams

Component-based applications require good middleware support. In particular, business logic should be separated from management code for guaranteeing nonfunctional properties of a system. We present an approach called Container-Managed Quality Assurance, in which a component container uses nonfunctional specifications of components to determine how to use these components, and which system resources to allocate, to provide certain services with guaranteed nonfunctional properties. As an example, we show how this technique can be applied to automatically allocating CPU and memory resources for components with real-time constraints. To this end, we use a mathematical model based on jitterconstrained streams, a mathematical abstraction of event streams.