Real-Time Scheduling

operating-systems scheduling

Definition

Real-Time Scheduling

Real-time scheduling is used in systems where correctness depends not only on the logical outcome but also on the time at which the result is produced.

Types

Hard Real-Time

Missing a deadline causes total system failure (medical devices, flight control).

Soft Real-Time

Missing a deadline is undesirable but acceptable; performance degrades but system survives (video streaming).

Earliest Deadline First

EDF

Earliest Deadline First (EDF) is a dynamic strategy prioritising the task with the nearest deadline.

• Selection: Task with earliest deadline
• Mode: Preemptive
• Optimal: Guaranteed optimal for uniprocessor systems

Schedulability Test for periodic tasks (execution $C_i$, period $T_i$ = deadline):
${\sum }_{i=1}^n\frac{C_i}{T_i}\le 1$

Priority Types

Static Priorities

Assigned before runtime based on task characteristics (e.g., Rate Monotonic Scheduling: shorter periods = higher priority).

Dynamic Priorities

Determined during execution based on factors like deadlines (e.g., EDF).