Page 37 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes 3.3.1 Deadline

A deadline is a timing milestone. If a deadline is missed by a computer-controller, the controlled
system may transit to an undesirable state.
In hard real-time systems, according to the usual definition, a deadline that is not met can lead
to a catastrophic failure. This means that the criteria used to establish deadlines are safety based.
Control system engineers, on the other hand, use performance criteria to establish the desired
response time of a controlling computer.
The deadlines suggested by these scientific communities are not mutually exclusive but only
different entities perceived in particular and equally important contexts. Moreover, they show
the disassociation of controllers’ timing constraints into those related to safety – hard deadlines
– and those related to performance – performance deadlines.
Performance deadlines are usually more confining than hard deadlines. Therefore, a computer-
controller, designed to meet performance deadlines, does not drive the controlled system to an
unsafe state as soon as one of them is missed, but only later, when a hard deadline is disrespected.
Performance and hard deadlines are thus separated by a grace-time. This notion can help in the
design of low-cost, yet highly reliable control systems.

Lab Exercise Go to URL http://paginas.fe.up.pt/saic/Membros/apmag/Ficheiros/drts.pdf
and collect more information on unified view of deadlines.

3.3.2 Time Constraints

Real-time systems are usually classified into soft and hard. Classically, in a soft real-time system,
missing a deadline is inconvenient but not damaging to the environment; in hard real-time
system, missing a deadline can be catastrophic, and thus unacceptable.
The traditional view of the temporal merit of a hard real-time computation (i.e., the relationship
between the computation completion time and the resulting temporal merit of that computation)
is usually modelled by a step time-value function: if a controller service is completed before a
given deadline it yields a constant positive value while completing it any time later may incur
in a catastrophic failure. From this point of view, hard deadlines are established in a safety-
based context.
This means that when a computer is part of a hard real-time system, all the software running on
it has to be tuned to satisfy all controlled system deadlines.

Notes Computations

Computations often present non-binary time constraints, even when a large merit penalty
is incurred for completing it after a deadline. Also, there are many cases in real-time
applications where some diminished merit is attained for completing a computation
within an allowable period after a deadline. Moreover, the acceptability of the completion
times of a set of computations must consider their collective merit instead of the individual
ones presents and schedules some smooth time-value functions illustrating Jensen’s point
of view.

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