Page 46 - DCAP608_REAL TIME SYSTEMS

Unit 4: A Reference Model of Real-time Systems

4.1 Processors and Resources Notes

All the resources can be divided into two parts.
 Processors
 Resources

Two processors are of the same type of they are functionally identical and can be used
interchangeably.
 Symmetric Multi-processors.

 Two transmission links connected with the same transmission rate between a pair of
sender and receiver.

A transmission link connecting an on-board flight management system to the ground controller
is different from the link connecting two air traffic control centres together as they cannot be
used interchangeably. By resources we specifically mean passive resources such as memory,
mutexes, database locks. A job needs resources as well as processor to execute.
Resources always mean a re-usable resource. A resource is plentiful if no job is ever prevented
from execution by the lack of this resource, i.e. a resource that can be shared by infinite number
of jobs for example a file readable by all. These resources need not to be explicitly modelled.
Memory is also essential, but many real-times system are designed in such a way that whenever
a job is scheduled to execute, it always has a sufficient amount of memory. We even omit
memory from our model whenever not required. Consider I/O bus, every computation job
must have the I/O bus in addition to the processor to execute. Often it is modelled as plentiful,
but what if we want to determine the real-time performance or effect of I/O bus arbitration.

4.2 Temporal Parameters of Real-time Workload

The number of tasks is one parameter. In most embedded systems, the number of tasks is fixed
in a particular operation mode. In some systems, however, the number of tasks may change as
tasks are added or deleted while the system executes. In an air traffic controller, each surveillance
task monitors a single aircraft. The number of tasks may change (added or deleted) when
aircrafts enter or leave the coverage area. Each job is characterized by temporal parameters,
functional parameters, resource parameters, and interconnecting parameters. Temporal
parameters tell us its timing constraints. Interconnection parameters describe how it depends
on other jobs and others depend on it.
Functional parameters specify the intrinsic properties of job. Resource parameters give us its
resource requirements.

4.2.1 Fixed Jittered and Sporadic Release Times

In many systems, we do not know the actual release time r of each job J , we are given a range [r -
i i i
,r +]. It can be as earliest as r - or as late as r +. This range is called release time jitter. Almost every
i i i
real-time system is required to respond to external events which occur at random instants of
time. On such events the system executes a set of jobs in response. The release time of these jobs
is not known until the event triggering them occurs.

These jobs are called sporadic or aperiodic jobs as these are released at random time. The pilot
may disengage the autopilot system at any time. When this occurs; the autopilot system changes
from cruise mode to standby mode. The jobs which executed for this mode change are sporadic.

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