Sadegh Davari

Guaranteeing synchronous message deadlines with the timed token medium access control protocol

We study the problem of guaranteeing synchronous message deadlines in token ring networks where the timed token medium access control protocol is employed. Synchronous bandwidth, defined as the maximum time for which a node can transmit its synchronous messages every time it receives the token, is a key parameter in the control of synchronous message transmission. To ensure the transmission of synchronous messages before their deadlines, synchronous capacities must be properly allocated to individual nodes. We address the issue of appropriate allocation of the synchronous capacities. Several synchronous bandwidth allocation schemes are analyzed in terms of their ability to satisfy deadline constraints of synchronous messages. We show that an inappropriate allocation of the synchronous capacities could cause message deadlines to be missed, even if the synchronous traffic is extremely low. We propose a scheme, called the normalized proportional allocation scheme, which can guarantee the synchronous message deadlines for synchronous traffic of up to 33% of available utilization. >

Guaranteeing synchronous message deadlines with the timed token protocol

The problem of guaranteeing synchronous message deadlines in token ring networks in which the timed token medium access control protocol is used is discussed. Synchronous capacity, defined as the maximum time for which a node can transmit its synchronous messages every time it receives the token, is a key parameter in the control of synchronous message transmission. To ensure the transmission of synchronous messages before their deadlines, synchronous capacities must be properly allocated to individual nodes. Several synchronous capacity allocation schemes are analyzed in terms of their ability to satisfy deadline constraints of synchronous messages. It is shown that an inappropriate allocation of the synchronous capacities could cause message deadlines to be missed, even if the synchronous traffic is extremely low. The normalized proportional allocation scheme, which can guarantee the synchronous message deadlines for synchronous traffic of up to 33% of available utilization is proposed.<<ETX>>

Sources of unbounded priority inversions in real-time systems and a comparative study of possible solutions

In the design of real-time systems, tasks are often assigned priorities. Preemptive priority driven schedulers are used to schedule tasks to meet the timing requirements. Priority inversion is the term used to describe the situation when a higher priority tasks execution is delayed by lower priority tasks. Priority inversion can occur when there is contention for resources among tasks of different priorities. The duration of priority inversion could be long enough to cause tasks to miss their deadlines. Priority inversion cannot be completely eliminated. However, it is important to identify sources of priority inversion and minimize the duration of priority inversion. IN the paper we present a comprehensive review of the problem of and solutions to unbounded priority inversion.

Priority ceiling protocol in Ada

The priority ceiling protocol (PCP) is an effective protocol for minimizing priority inversions in real-time scheduling. Priority inversion occw-s when a high priority task is blocked by a low priority task, such as at a shared semaphore or a protected operation. PCP guarantees the absence of chained priority inversion or deadlock. The ceiling locking (CL) priority on protected objects of Ada-95 also has these properties but has some limitations. For example, tasks cannot suspend themselves inside a protected operation. PCP has no such restrictions. Thus, PCP is more appropriate for some real-time applications. PCP has not been implemented in any language, · including Ada. A guideline for emulating PCP using Ada-83 exists but it lacks generality and flexibility. This paper discusses an implementation of PCP in Ada-95, the Ada-95 features that enable the implementation, the design of the implementation and some related issues.

The Need for a New Generation of Integrated Systems Software for Real-Time Applications

There is an increasing need for mission and safety critical (mask) computing applications. Some of the future mask applications will be large, long lived, complex, non-stop, remotely distributed, real-time, fault tolerant and expensive. Examples include the faa’s advanced automation system, nasa’s proposed manned missions to Mars, and dod’s needs for improved automated support for command, communications, control and intelligence. Such applications cannot be safely and affordably mapped to concepts and components of systems software that were not designed to support the life cycle of mask applications. Operating systems, database management systems, data communications systems, user interface systems and applications cannot be constructed with different paradigms and technology and then somehow be certified for collective use in hard real-time, safety critical environments. Issues such as distribution, fault tolerance and survivability, and non-stop operation further complicate these challenges.