Question

1. What are 3 differences between periodic and aperiodic real-time tasks?

2. What is the primary difference between a Real-Time OS and a general operating system.

Answer 1

1.

Periodic Tasks: In periodic task, jobs are released at regular intervals. A periodic task is one which repeats itself after a fixed time interval. A periodic task is denoted by five tuples: T_i = < Φ_i, P_i, e_i, D_i >
Where,

• Φ_i – is the phase of the task. Phase is release time of the first job in the task. If the phase is not mentioned then release time of first job is assumed to be zero.
• P_i – is the period of the task i.e. the time interval between the release times of two consecutive jobs.
• e_i – is the execution time of the task.
• D_i – is the relative deadline of the task.

For example: Consider the task T_i with period = 5 and execution time = 3
Phase is not given so, assume the release time of the first job as zero. So the job of this task is first released at t = 0 then it executes for 3s and then next job is released at t = 5 which executes for 3s and then next job is released at t = 10. So jobs are released at t = 5k where k = 0, 1, . . ., n

Aperiodic Tasks: It is a sequential program that is invoked by the occurrence of an event. An event may be generated by the processes external to the system or by processes internal to the system. Dynamically arriving tasks can be categorized on their criticality and knowledge about their occurrence times. In this type of task, jobs are released at arbitrary time intervals i.e. randomly. Aperiodic tasks have soft deadlines or no deadlines.

NOTE: I could not think of 3 differences between them, as I have never come across any difference than the one mentioned here.

2)

The basic difference of using a GPOS or an RTOS lies in the nature of the system – i.e whether the system is “time critical” or not!

A GPOS is used for systems/applications that are not time critical.  Example:- Windows, Linux, Unix etc.

An RTOS is used for time critical systems. Example:- VxWorks, uCos etc.