Enhancing the Performance of Distributed Real-Time Systems
Doctoral thesis, 2007
Advanced embedded systems can consist of many sensors, actuators and processors that are
deployed on one or several boards, while having a demand of interacting with each other and
sharing resources. Communication between different components usually has strict timing
constraints. There is thus a strong need to provide solutions for time critical communication.
This thesis focuses on both the support of real-time services over standard switched Ethernet
networks and the improvement of systems' real-time characteristics, such as reducing delay and
jitter in processors and on communication links.
Switched Ethernet has been chosen in this work because of its major advantages in
industry; it supports higher bit-rates than most other current LAN (Local Area Network)
technologies, including field buses, still at a low cost. We propose using a star network topology
with a single Ethernet switch. Each node is connected to a separate port of the switch via a fullduplex
link, thereby eliminating collisions. A solid real-time communication protocol for
switched Ethernet networks is proposed in the thesis, including a real-time layer between the
Ethernet layer and the TCP/IP suite. The network has the capability of supporting both real-time
and non real-time traffic and assuring adaptation to the surrounding protocol standards.
Most embedded systems work in a dynamic environment, where the precise behavior of
the network traffic can usually not be predicted. To support real-time services, we have chosen
the Earliest Deadline First scheduling algorithm (EDF) because of its optimality, high efficiency
and suitability for being used in adaptive schemes. To be able to increase the amount of
guaranteed real-time traffic, the notion of Asymmetric Deadline Partitioning Scheme (ADPS) is
introduced. ADPS allows distribution of the end-to-end deadline of a message, sent from any
source node in the network to any destination node via the switch, into two sub-deadlines, one
for each hop according to the load of the physical link that it must traverse.
For the EDF scheduling algorithm, the feasibility test is one of the most important
techniques that provides us with information about whether or not the real-time traffic can be
guaranteed by the network. With the same computational complexity as the feasibility test, a
method has been developed to compute the minimum EDF-feasible deadline for a real-time
task. The importance of this method in real-time applications lies in that it can be effectively
used to reduce the response times of specific control activities or limit their input-output jitter.
To allow more flexibility in the control of delay and jitter in real-time systems, a general
approach for reducing task deadlines according to the requirements of individual tasks has been
developed. The method allows the user to specify a deadline reduction factor for each task in
order to better exploit the available slack according to the tasks' actual requirements.
reduction of delay