BASIC CONCEPTS - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 26

J 1

t

J 2

t

J 3

t

σ (t)

3
2
1

t

Figure 2.3 Example of a preemptive schedule.

2.2.1 TIMING CONSTRAINTS

Real-time systems are characterized by computational activities with stringent timing
constraints that must be met in order to achieve the desired behavior. A typical timing
constraint on a task is the deadline , which represents the time before which a process
should complete its execution without causing any damage to the system. If a deadline
is specified with respect to the task arrival time, it is called a relative deadline , whereas
if it is specified with respect to time zero, it is called an absolute deadline . Depending
on the consequences of a missed deadline, real-time tasks are usually distinguished in
three categories:

Hard : A real-time task is said to be hard if missing its deadline may cause catas-
trophic consequences on the system under control.

Firm : A real-time task is said to be firm if missing its deadline does not cause
any damage to the system, but the output has no value.

Soft : A real-time task is said to be soft if missing its deadline has still some utility
for the system, although causing a performance degradation.

In general, a real-time task τ i can be characterized by the following parameters: