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Subsections
- Fixed set of processes,
- all processes periodic with known periods ,
- processes independent (note: no IPC!),
- ignore system overheads,
- deadline, , equal to period,
- fixed (and known) worst-time execution time ,
- , , are multiples of minor cycle time
.
Allows static analysis and a static schedule.
|
Process |
A |
B |
C |
D |
E |
|
25 |
25 |
50 |
50 |
100 |
|
10 |
8 |
5 |
4 |
2 |
- Worst case if all come at once: critical instant.
- Fixed schedule covers one major cycle time
.
- Schedule is just a list of executions (
):
A, B, C, A, B, D, E, A, B, C, A, B, D.
Note: No preemption ==> no concurrency
control necessary!
- Priorities based on timeliness requirements, not
``importance''.
- Higher-priority processes preempt lower-priority ones.
Frequently used scheme is rate-monotonic priority assignment
(RMPA):
- Priority is based on period:
.
- Is optimal in a sense:
- Everything that can be scheduled can be done
statically by RMPA.
everything doable statically is doable with RMPA.
Next: Schedulability
Up: 10-smp
Previous: Real-time OS Issues
Gernot Heiser
2002-10-11
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