Page 11 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes 2. An operating system must have a known maximum ……………… for each of the critical
operations that it performs.
3. A ……………… system is one that must process information and produce a response
within a specified time, else risk severe consequences, including failure.
4. Real-time systems are often expected to involve ………………
5. The separation of an activity into several distinct, semi-autonomous tasks leads to the
question of task ………………
6. Hard real-time systems are often used in ……………… systems.

1.3 Soft Real-time Systems

Soft real-time systems also have time bounds associated with them. However, unlike hard and
firm real-time tasks, the timing constraints on soft real-time tasks are not expressed as absolute
values. Instead, the constraints are expressed either in terms of the average response times
required.
Soft real-time systems are usually employed when there are multiple, connected systems that
must be maintained despite shifting events and circumstances. These systems are also used
when concurrent access requirements are present. For example, the software used to maintain
travel schedules for major transportation companies are often soft real-time. It is necessary for
such software to update schedules with little delay. However, a delay of a few seconds is not
likely to cause mayhem.
An example of a soft real-time task is web browsing. Normally, after an URL (Uniform Resource
Locater) is clicked, the corresponding web page is fetched and displayed within a couple of
seconds on the average. However, when it takes several minutes to display a requested page, we
still do not consider the system to have failed, but merely express that the performance of the
system has degraded.
Another example of a soft real-time task is a task handling a request for a seat reservation in a
railway reservation application. Once a request for reservation is made, the response should
occur within 20 seconds on the average. The response may either be in the form of a printed
ticket or an apology message on account of unavailability of seats. Alternatively, we might state
the constraint on the ticketing task as: At least in case of 95% of reservation requests, the ticket
should be processed and printed in less than 20 seconds.

Figure 1.1: Utility of the Results Produced by a Soft Real-Time Task as a Function of Time

Utility
Deadline
100%

0
Response Time
Source: http://www.nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Embedded%
20systems/Pdf/Lesson-28.pdf

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