Real-Time Scheduling: RM vs EDF

Real-Time Scheduling: RM vs EDF

Periodic tasks on one CPU. Each task releases a job every period T; the job needs C units of CPU time and must finish within its relative deadline D. The timeline shows the CPU row and one row per task, with releases, deadlines, preemptions and deadline misses.

How to use

• Choose RM (or DM when some D < T) or EDF, then play or step through scheduling events.
• Edit C, T and D of each task in the Tasks panel (up to 6 tasks; + adds, × removes) or pick a Preset.
• Horizon sets the simulated length (1–2000); = LCM uses the hyperperiod.
• Each task row shows U = C/T and the current job: progress, absolute deadline, preemptions and misses. The bar under the timeline compares ΣU with the RM bound and the EDF limit.
• Compare runs both policies on the same task set side by side.

Rules

RM

Rate Monotonic: fixed priority, shorter period T first.

DM

Deadline Monotonic: fixed priority by D; used automatically when some D < T.

EDF

Earliest Deadline First: the nearest absolute deadline runs.

U

Utilization C/T; ΣU > 1 means no scheduler can keep up.

RM bound

ΣU ≤ n(2^1/n − 1) guarantees RM (sufficient, not necessary; D = T only).

EDF test

D = T: schedulable iff ΣU ≤ 1. D < T: density ΣC/D ≤ 1 is sufficient.

LCM

Hyperperiod: the schedule repeats after the LCM of the periods.

Model

One CPU, integer time, all tasks released at 0, a job still unfinished at its deadline is aborted, ties go to the lower task number.

Legend

Keys