Overheads in Schedulability Analysis

Despite best efforts there are always some overheads in the system and these must be taken for systems in industrial context:

  1. Context switches
  2. Interrupts
  3. RT clock overheads

These overheads vary depending on the priority scheduling model. Here I will show how these could be taken into consideration in Fixed Priority.

Context switches are nothing more than extra cost which needs to be accounted for every time a task is being switched. Hence the response time equation is:

R_i = CS^1 + C_i + B_i + \sum_{k \in hp(i)}\left\lceil\frac{R_i}{T_j}\right\rceil (CS^1 + CS^2 + C_j)

Interrupts are used to handle sporadic tasks. Hence in order to incorporate them into the equation we have to add the time it tasks for the event handler to deal with the request:

\sum_{k \in S}\left\lceli\frac{R_i}{T_k}\right\rceil IH

S is the set of sporadic tasks and IH is a cost of a single interrupt (assuming they occur at maximum priority level)

There are also some Clocks overheads to consider. Clock itself has interrupts e.g:

  1. Cost for moving one task from delay to run queue
  2. Cost of moving groups of tasks

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