Systems that require results within deadlines. Focuses on deadlines, latency, and periodic tasks. Scheduler must support preemptive, priority-based scheduling.
Safety-Critical System
A real-time system with catastrophic results in case of failure.
Periodic Real-Time Task
Each task has:
- Processing time
- Deadline
- Period
- Must satisfy:
- Rate =
Types
Hard Real-Time
Where deadlines must never be missed. Single missed deadline implies system failure. Scheduler must have the ability to meet deadlines.
Soft Real-Time
Where deadlines are important but not critical. Occasional misses are acceptable.
Algorithms
Rate Monotonic Scheduling
Aka. RMS. Priority = inverse of period.
Works only if CPU load is below schedulable limit. Can still miss deadlines.
Earliest Deadline First
Aka. EDF. Earlier deadline means higher priority.
Proportional-Share Scheduling
System has total shares. Each task gets shares. Each task is ensured to get fraction of CPU time.