Page 38 - DCAP608_REAL TIME SYSTEMS

Unit 3: Hard Versus Soft Real-time System

It is commonly accepted that a controlled process can sporadically tolerate a missed deadline, if Notes
not by much. This notion presupposes a controller not tuned to meet hard deadlines but some
other kind of time limit.
However, the characterisation of a deadline is by itself a relatively unexplored problem in the
real-time community. Most of the literature seems to consider that deadlines are somehow
provided by others, possibly by control system engineers. Moreover, techniques to calculate
systems’ deadlines are very seldom presented.

Nevertheless, soft and hard deadlines are universally used, and many suggestions appear in the
literature reasoning about the existence of other kinds of deadlines, besides these classical ones.
Moreover, there is a growing tendency to classify real-time services according to their associated
benefit/cost functions, and to establish on them a set of pertinent points of time concerning
application goals. This means that there is a growing feeling that traditional definitions and
interpretations of deadlines are poor since they cannot describe reality in a satisfactory way nor
can be explicitly employed for a best-effort scheduling.

Task Discuss in groups, different techniques to calculate systems.

3.4 Hard and Soft Timing Constraint

Common definitions are based on:
 functional criticality:
 soft: meeting the constraints (deadlines) is desirable, but a few misses do no serious
harm,
 hard: missing a deadline is a fatal fault;
 usefulness of late results:
 soft: usefulness decreases gradually with tardiness,

 hard: usefulness drops to zero or becomes negative when tardiness is larger than
zero;
 deterministic or probabilistic nature of constraints:

 soft: deadlines can be missed occasionally, with low probability,
 hard: deadlines must never be missed.

3.4.1 Hard vs. Soft Deadlines

 Hard Deadline: Late result may be a fatal flaw, useless, or cause disastrous consequences
 Soft Deadline: Timely completion desirable.

Late results useful to some degree

 Quantitative Measure: Overall system performance as function of tardiness of jobs.

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