Response-time analysis
For arbitrarily fixed priorities, including RM, DM and others, the Response Time Analysis is an exact test (i.e., a necessary and sufficient condition) in the following conditions:
full preemption, synchronous release, independent tasks and D ≤ T.
Worst-case response time (WCRT, Rwc, R,...) = maximum time interval between arrival and finish instants.
Schedulability test based on the WCRT
The WCRT of a given task occurs when the task is activated at the same time as all other high-priority tasks (critical instant).
Ii – interference caused by the execution of higher priority tasks.
Computing Rwci
The equation is solved iteratively. Stop conditions are:
A deadline is violated.
Rwci(m) > Di
Convergence (two successive iterations yield the same result).
Rwc i (m + 1) = Rwc i (m)
Algorithm
Example
Restrictions to the schedulability tests previously presented
The previous schedulability tests must be modified in the following cases:
Non-preemption.
Tasks not independent.
Share mutually exclusive resources.
Have precedence constrains.
It is also necessary to take into account the overhead of the kernel, because the scheduler, dispatcher and interrupts consume CPU time.
Impact of non-preemption
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