Reference Model of RTS

Reference Model of Real Time System

The model that focuses on the relevant characteristic such as timing properties , resource requirement of the system components and the way in which the operating system allocates the available system resources among them is called reference model of RTS. Its main goal is to abstract away from the functional characteristics and focus on the living properties and resource requirement. According to this model, each system is characterized by three elements.

1. A workload model that describes the application supported by the system.
2. A resource model that describes the system resources available to the application.
3. Algorithms that define how the application system uses the resources at all the times.

Each job is characterized by

1. Temporal Parameters
2. Interconnection Parameters
3. Resource Parameters
4. Functional Parameters

Temporal Parameters

The temporal parameter of a job is the parameter which describes its timing constraints and behavior such as release time (r_i), absolute deadline (d_i), relative deadline (D_i), execution time (e_i) and feasible interval [r_i, d_i].

Fixed, Jitter and Sporadic Release Time

Release time may exactly may not be known, only that ri is in a range [r_i^-, r_i⁺]. ri can be as earliest as ri- and as late as latest release time ri+. When only the range of ri is known, then this range is called jitter in ri or the release time Jitter. When the jitter is negligible compared to other temporal parameters, then the actual release time of each job is given by its earliest or latest release time then we can call it a fixed release time.

Every real time system is required to respond to unexpected external events which occurs at random instance of time. During such events the system executes a set of jobs in response. The release times of these jobs are known until the events triggering them occur. These jobs are called sporadic jobs or aperiodic jobs because they are released at random instant of time. Though both of these jobs occur at random instant, the sporadic jobs have hard timing constraints and aperiodic jobs have soft timing constraints.

Periodic Task Model

It is a deterministic work load model used to describe hard real time application such as digital control and real time monitoring system. There are two methods and tools to support the design analysis and validation of real time systems supported by basic model.

The period, execution time and the phases of the task play important role in this model. When each computation or data transmission is executed repeatedly at regular intervals or semi-regular intervals in order to provide a function of the system on a continuous basis, then the task is called a Periodic Task. The periodic task T_i is a sequence of jobs j_i. When there are more than one task in the system then hyperbolic is considered as one of the important parameter. The hyperbolic of a set of periodic task is the L.C.M. of their periods Pi for i = 1, 2, 3, . . . . . . . , n. The maximum number N of jobs in each hyper period is equal to

Where H = hyper period

For example, the length of hyper period of the periodic task with periods with 3, 4 and 10 is 60 and total number of jobs in the hype period is

[60/3 + 60/4 + 60/10] = 20 + 15 + 6 = 41

The period Pi of periodic task is the minimum length of all time intervals between released times of consecutive jobs in a task and execution time is the maximum execution time of all the jobs. The released time r_i,1 of the first job J_i,1, in each task ti is called phase of the task (φ). Therefore, φ = r_i,1.

The ratio of the execution time to the period of the job is called utilization i.e. ui = ei/p_i.

It is equal to the fraction of time, a truly periodic task with period Pi and execution time ei. It keeps the processor busy. It gives an upper bound to the utilization of any task ti. The total utilization of all the tasks in the system is the sum of the utilizations of the individual tasks in it. Therefore, U_i =∑ u_i, (n , i = 1).

References

Liu, Jane W. S. Real Time System. Integre Technical Publishing Co., Inc, Jan 12, 2000. print.