The instant at which a job completes execution is called completion time. The deadline of a job is the instant of time by which its execution is required to be completed. Deadline time is divided into the two categories.
Relative deadline – the maximum allowable job response time
Absolute deadline – the instant of time by which a job is required to be completed. It often called simply as deadline. The absolute deadline is the sum of the relative deadlines and release time.
I.e. absolute deadline = release time + relative deadline
From figure, absolute deadline is represented by the interval for a job Ji is the interval ( ri, di ].
A constraint imposed on the timing behavior of a job a timing constraint. Deadlines are the example of timing constraints i.e. a timing constraint of a job can be specified in terms of its release time and relative or absolute deadlines.
For example:
A system to monitor and control a heating furnace and requires 20ms to initialize when turned on. After initialization, every 100 ms, and the system:
- Samples and reads the temperature sensor
- Computes the control-law for the furnace to process temperature readings, determine the correct flow rates of fuel, air and coolant
- Adjusts flow rates to match computed values
The periodic computations can be stated in terms of release times of the jobs computing the control-law: J0, J1, …, Jk, … as shown in fig below
The release time of Jk is computed as below.
Release time = 20 + (k × 100) ms
Suppose each job must complete before the release of the next job then
- relative deadline of Jk’ s is 100 ms
- absolute deadline of Jk’ s is 20 + ((k + 1) × 100) ms
Alternatively, each control-law computation may be required to finish sooner i.e. the relative deadline is smaller than the time between jobs, allowing some slack time for other jobs. The difference between the completion time and the earliest possible completion time is called the slack time.
