The timing constraint are classified as hard and soft based on the functional criticality of jobs, usefulness of late results, and deterministic or probabilistic nature of the constraints.
A timing constraint or deadline is hard if the failure to meet it is considered to be a fatal fault. A hard deadline is imposed on a job because a late result produced by the job after the deadline may have disastrous consequences. E.g.
- a late command to stop a train may cause a collision
- a bomb dropped too late may hit a civilian population instead of the intended military
A timing constraint or deadline is soft if a few misses of deadlines do no serious harm but only the system’s overall performance becomes poorer. The system performance becomes poorer when more and more jobs with soft deadlines complete late so that late completion of a job is undesirable.
Hard Timing Constraints and Temporal Quality-of-Service Guarantees:
The timing constraint of a job is hard then it is called as hard real-time job. The validation is required that meets the system timing constraints to demonstrate a real time system by using a provably correct, efficient procedure or exhaustive simulation and testing.
In case of soft timing constraint is impose on the job then it is called soft job and there is no system validation is required to demonstrate the real time system but timing constraint must meet the some statistical constraints i.e., a timing constraint specified in terms of statistical averages.
The temporal quality of service measures the systems in terms of different parameters like response time, jitter etc. when the system requires the validation of these parameter to guarantee and satisfaction of real time system over the define timing constraint then these timing constraint are called hard.
Some real time system are operated to give the best quality of service but there is no care of violation of timing constraint slightly and no needs of validation then these timing constrains are called soft.
Hence the validation of temporal quality of system must be needed in case of hard real time system and there is no guarantee of best quality service whereas quality of service must garneted in soft real time system. Timing constraints can be expressed in many ways:
- Deterministic: it a constraint expressed in terms of numeric
- g. the relative deadline of every control-law computation is 50 ms; the response time of at most 1 out of 5 consecutive control-law computations exceeds 50ms
- Probabilistic: it is a constraint expressed in terms of
- g. the probability of the response time exceeding 50 ms is less than 0.2
- In terms of some usefulness function
- g. the usefulness of every control-law computation is at least 0.8