Page 141 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes 14.2 Practical Factors

We have assumed that:
 Jobs are preemptible at any time

 Jobs never suspend themselves
 Each job has distinct priority
 The scheduler is event driven and acts immediately
 These assumptions are often not valid and how does this affect the system

14.2.1 Blocking and Priority Inversion

A ready job is blocked when it is prevented from executing by a lower-priority job; a priority
inversion is when a lower-priority job executes while a higher-priority job is blocked.
These occur because some jobs cannot be pre-empted:
 Many reasons why a job may have non-preemptible sections

 Critical section over a resource
 Some system calls are non-preemptible
 Disk scheduling

 If a job becomes non-preemptible, priority inversions may occur, these may cause a higher
priority task to miss its deadline
 When attempting to determine if a task meets all of its deadlines, must consider not only
all the tasks that have higher priorities, but also non-preemptible regions of lower-priority
tasks.
Add the blocking time when calculating if a task is schedulable

14.2.2 Self-Suspension and Context Switches

Self-Suspension

 A job may invoke an external operation (e.g. request an I/O operation), during which
time it is suspended.
 This means the task is no longer strictly periodic and again need to take into account self-
suspension time when calculating a schedule.
Context Switches

 Assume maximum number of context switches K for a job in T is known; each takes t
i i CS
time units.
 Compensate by setting execution time of each job, e = e + 2t (more if jobs self-
actual CS
suspend, since additional context switches).

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