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School of Computer Science & Engineering
University of New South Wales
 Advanced Operating Systems 
 COMP9242 2002/S2 

Simplified Real-Time Process Model

• 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.

Example

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!

More flexible alternative: Use priorities

• 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.