Page 12 - DCAP608_REAL TIME SYSTEMS

Unit 1: Concept of Real-time System

Let us now analyse the impact of the failure of a soft real-time task to meet its deadline, by Notes
taking the example of the railway reservation task. If the ticket is printed in about 20 seconds, we
feel that the system is working fine and get a feel of having obtained instant results. As already
stated, missed deadlines of soft real-time tasks do not result in system failures. However, the
utility of the results produced by a soft real-time task falls continuously with time after the
expiry of the deadline as shown in Fig. 1.1. In Fig. 1.1, the utility of the results produced are 100%
if produced before the deadline, and after the deadline is passed the utility of the results slowly
falls off with time. For soft real-time tasks that typically occur in practical applications, the time
bounds usually range from a fraction of a second to a few seconds.

1.4 Hard Versus Soft Real-time System

An OS that can absolutely guarantee a maximum time for the operations it performs is referred
to as hard real-time. In contrast, an OS that can usually perform operations in a certain time is
referred to as soft real-time.

Lab Exercise Go to Visit http://www.dauniv.ac.in/downloads/EmbsysRevEd_PPTs/
Chap_8Lesson13EmsysNewHardSoftRTConsideration.pdf and collect more information
on Hard and soft real-time systems.
Hard real-time systems are designed to absolutely guarantee that a task will execute within a
certain worst-case timeframe. Therefore, for projects involving safety or systems that could
result in a large investment in the event of failure, hard real-time is often a requirement. On the
other hand, soft real-time systems are designed to satisfy your timing requirements most of the
time but without absolute certainty. This can be acceptable for operations like video processing,
where a lost data frame is not good but may not necessarily be a critical problem.

Figure 1.2: Hard vs Soft Real-time Applications

Hard real-time systems guarantee (when programmed correctly) that a deadline will be
consistently met, while soft real-time systems may periodically exceed the deadline.

Self Assessment

State whether the following statements are True or False:
7. Hard real-time systems are usually employed when there are multiple, connected systems
that must be maintained despite shifting events and circumstances.

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