Page 47 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes 4.2.2 Execution Time

Another temporal parameter of a job, J , is its execution time, e . It the time required to complete
i i
the execution of J when it executes alone and has all the resources it requires. The actual amount
i
of time to complete its execution may vary for reasons like execution depending on the input
data. The actual amount of time to transmit each frame of MPEG compressed video is different
from frame to frame, because of the number of bits used to encode different frames. What can in
fact be determined a priori are the maximum and minimum amounts of time required to
complete each job (analysis and measurement and testing) We know that the execution time e of
i
the job J is in the range [e +, e +].
i i i
We usually assume that we know e + and e - of every hard real-time job J but the actual execution
i i i
time may be unknown.
Self Assessment

Fill in the blanks:

1. A ……………… model of real-time systems is needed to focus on aspects of the system
relevant to its real-time timing and resource properties.
2. A system may be characterized by ……………… models.

3. A ……………… model describes the applications.
4. A ……………… model describes the system resources.
5. The scheduling and ……………… algorithms define how the application uses the resources.
6. Resources always means a ……………… resource.

4.3 Periodical Task Model

The concept of periodical task model is discussed below:

4.3.1 Periods, Execution Times and Phases of Periodic Tasks

Each computation or data transmission that is executed repeatedly at regular or semi-regular
time intervals in order to provide a function of the system is modelled as a period task. Simply
a periodic task occurs at regular intervals. Each task T is characterized by the following parameters.
i
The release time (r): the time when the task first released. The period (T): the constant interval
between jobs. The relative deadline (D): the maximum acceptable delay for task processing. The
computation time (C): the worst case execution time (WCET) of any job. The tasks in the system
can be denoted by T , T ,…,T . Individual jobs in a task Ti can be denoted as T , T ….T The
1 2 n i,1 i,2 i,n
release time r of the first job J in each task T is called the phase of T . Different tasks may have
i,1 i,1 i i
the same phase
Example:

42 LOVELY PROFESSIONAL UNIVERSITY