Analysis: CPU utilization bound
Last updated
Sistemas Operativos E De Tempo-real
Last updated
Since the schedule is built online it is important to determine a priori if a given task set meets or not its temporal requirements.
There are three types of schedulability tests:
Based on CPU utilization.
Based on CPU load (processor demand).
Based on response time.
EDF tests based on CPU utilization (n independent tasks, with full preemption).
D = T
<=> Task set is schedulable.
Allows using 100% of CPU with timeliness guarantees.
D < T
<=> Task set is schedulable.
Sufficient condition, only.
Pessimistic test (as for RM, inflates the utilization).
In fact τ3 fails a deadline!
Be fair on the attribution of resources (e.g. CPU).
EDF is intrinsically fairer than RM, in the sense that tasks see its relative deadline increased as the absolute deadline approaches, independently of its period or any other static parameter.
Consequences:
Deadlines are easier to met.
As the deadline approaches preemptions suffered by a given task are reduced.
The slack of tasks that are quick but have large deadlines can be used by other task (higher jitter on tasks with shorter periods).
-> 1 activation per period NOT guaranteed.
<=> one activation per period IS guaranteed.
