Glossary · Automation software engineering and architecture
Rate-monotonic scheduling
Also known as: RMS, Rate-monotonic analysis
German: Ratenmonotone Ablaufplanung
In real-time systems, rate-monotonic scheduling is a fixed-priority preemptive scheduling method in which periodic tasks with shorter periods receive higher priorities; it comes with schedulability tests that show whether all deadlines are met.
- Software engineering
In one sentence
Rate-monotonic scheduling gives periodic tasks with shorter periods higher fixed priorities and allows deadlines to be checked analytically.
Example
The controller's 1 ms current loop gets the highest priority, the 4 ms position loop the next, and the 100 ms communication task the lowest.
How it applies
- Engineering: Rate-monotonic priority assignment is optimal among fixed-priority schemes for independent periodic tasks with deadlines equal to their periods. Real systems with shared resources and interrupts require extended analysis.
- Verification: Schedulability analysis uses the Worst-case execution time (WCET) of each task. Measured execution times alone are not a proof.
- Documentation: The timing analysis, its assumptions (periods, WCET, blocking) and results belong in the software design documentation. When tasks change, the analysis must be updated.
Rate-monotonic vs. earliest-deadline-first
Rate-monotonic scheduling uses fixed priorities derived from periods. Earliest-deadline-first (EDF) assigns priorities dynamically by the nearest deadline and can use the processor more fully, but its behavior under overload is harder to predict.