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 task's 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 dead lines. Priority inversion cannot be completely eliminated. However, it is important to identify sources of priority inversion and minimize the duration of priority inversion. In this paper, a comprehensive review of the problem of and solutions to unbounded priority inversion is presented

Architectural impact of FDDI network on scheduling hard real-time traffic

The architectural impact on guaranteeing synchronous message deadlines in FDDI (Fiber Distributed Data Interface) token ring networks is examined. The FDDI network does not have facility to support (global) priority arbitration which is a useful facility for scheduling hard real time activities. As a result, it was found that the worst case utilization of synchronous traffic in an FDDI network can be far less than that in a centralized single processor system. Nevertheless, it is proposed and analyzed that a scheduling method can guarantee deadlines of synchronous messages having traffic utilization up to 33 pct., the highest to date

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 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. We address the issue of appropriate allocation of the synchronous capacities. Several synchronous capacity 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 percent of available utilization. To date, no other synchronous capacity allocation scheme has been reported to achieve such substantial performance. Another major contribution of this paper is an extension to the previous work on the bounded token rotation time. We prove that the time elapsed between any consecutive visits to a particular node is bounded by upsilon TTRT, where TTRT is the target token rotation time set up at system initialization time. The previous result by Johnson and Sevcik is a special case where upsilon = 2. We use this result in the analysis of various synchronous allocation schemes. It can also be applied in other similar studies

Data Management Systems (DMS): Complex data types study. Volume 1: Appendices A-B. Volume 2: Appendices C1-C5. Volume 3: Appendices D1-D3 and E

Two categories were chosen for study: the issue of using a preprocessor on Ada code of Application Programs which would interface with the Run-Time Object Data Base Standard Services (RODB STSV), the intent was to catch and correct any mis-registration errors of the program coder between the user declared Objects, their types, their addresses, and the corresponding RODB definitions; and RODB STSV Performance Issues and Identification of Problems with the planned methods for accessing Primitive Object Attributes, this included the study of an alternate storage scheme to the 'store objects by attribute' scheme in the current design of the RODB. The study resulted in essentially three separate documents, an interpretation of the system requirements, an assessment of the preliminary design, and a detailing of the components of a detailed design

Scheduling periodic-time-critical tasks on multiprocessor computing systems /

The problem of allocating a set of periodic-time-critical tasks to processors in a multiprocessor system is considered. A periodic-time-critical task consists of a certain number of requests, arising periodically, each of which has a prescribed deadline. The allocation problem is to use a minimum number of processors subject to the condition that the tasks allocated to any processor must be feasibly schedulable according to some specified algorithm, i.e. the schedule provided by the algorithm must guarantee that the deadline of each request is honored. We first prove that this problem is NP-hard, and then present three heuristic algorithms and analyze their complexity and worst-case performance. One of the algorithms presented is an off-line algorithm and the other two are on-line. Two heuristic off-line algorithms for this problem are available in literature. The worst-case performance of our off-line algorithm is shown to be better than that of the two existing off-line algorithms. The on-line algorithms presented here are the only on-line algorithms presented for this problem to date. The time and space complexity of the presented on-line algorithms are shown to be better than those of the available off-line algorithms, and their worst-case performance are shown to be comparable to that of the available off-line algorithms. Finally, it is shown that if the set of tasks to be scheduled does not contain any task with utilization factor in the range (2(' 1/2)-1, 1/2 then the worst-case performance of one of the on-line algorithms will improve considerably

Normalized proportional synchronous bandwidth allocation in a token ring network by setting a maximum message transmission time

A method for real time message transmission in a token ring network is provided. The method initializes each node in the network, including setting a maximum message transmission time Hi such that ##EQU1## is the transmission time of a message in a message stream, Pi is the period length of the message stream, TTRT is the target token rotation time, and ? is the portion of TTRT that is unavailable to transmit messages. The messages are then transmitted when a token arrives at a node for the duration of Hi. Using this method with the timed token protocol, the synchronous messages are guaranteed to be transmitted before their deadlines if the utilization of synchronous messages is no more than 33% of available utilization.U