1. Enforcing Real-Time Behavior I
Chris Gill, James Orr, and Son Dinh
CSE 522S - Advanced Operating Systems
Washington University in St. Louis
St. Louis, MO 63130

2. Uniprocessor Real-Time Systems
Each task τi is modeled as cyclic process, with period Ti, deadline Di, and computation time Ci
Ex: Task τ1 T1=D1=5, C1=4
Goal: meet all tasks’ deadlines
Time τ1 5 10 15
C. Liu, and J. Layland, “Scheduling Algorithms for Multiprogramming in a
Hard Real-Time Environment,” Journal of the ACM 20(1), January 1973.
CSE 522S – Advanced Operating Systems

3. Uniprocessor Real-Time Systems
Task τi utilization Ui=Ci/Ti is % of processor to schedule a task
Can schedule task sets with total utilization up to (e.g., earliest-deadline first, or with rate monotonic if harmonic)
Taskset 1
Taskset 2
Task Ci
Ti=Di Ui τ1 2 10
0.20 τ2 6 20
0.30 τ3 7 14
0.50
Task Ci
Ti=Di Ui τ1 3 12
0.25 τ2 7 20
0.35 τ3 40
0.50
1.00
1.10
C. Liu, and J. Layland, “Scheduling Algorithms for Multiprogramming in a
Hard Real-Time Environment,” Journal of the ACM 20(1), January 1973.
CSE 522S – Advanced Operating Systems

4. Real-Time Terminology
Release time is when a job of a task is ready
Start time is when a job is first scheduled
Response time is completion minus release
Sometimes also called “response latency”
Execution time is sum of scheduled intervals
Jitter is the variation in any one of those
Higher priority job may preempt running one
Τ1,1
Τ2,1
Τ1,1
Time
released
resumed
preempted
completed
CSE 522S – Advanced Operating Systems

5. CSE 522S – Advanced Operating Systems
Scheduling Classes
Linux schedulers are implemented as different scheduling classes
Normal:
SCHED_IDLE: lowest priority scheduler
SCHED_BATCH: low priority, non-interactive CFS tasks
SCHED_NORMAL: regular, interactive CFS tasks
Real-time:
SCHED_RR: round-robin
SCHED_FIFO: first-in, first-out
SCHED_DEADLINE: earliest deadline first (rt-preempt)
CSE 522S – Advanced Operating Systems

6. CSE 522S – Advanced Operating Systems
SCHED_RR
Round-robin scheduling
Among tasks of equal priority:
Rotate through all tasks
Each task gets a fixed time slice
Cannot run if higher priority tasks are runnable
Task 1
Task 2
Task 3
Time
CSE 522S – Advanced Operating Systems

7. CSE 522S – Advanced Operating Systems
SCHED_FIFO
First-in, First-out scheduling
The first enqued task of highest priority executes to completion
A task will only relinquish a processor when it completes, yields, or blocks
Task 1
Task 2
Task 3
Time
CSE 522S – Advanced Operating Systems

8. CSE 522S – Advanced Operating Systems
SCHED_DEADLINE
Earliest Deadline First (EDF) scheduling
Whichever task has next deadline gets to run
Theory exists to analyze such systems
Linux implements bandwidth reservation to prevent deadline abuse
Task 3
Deadline: 8 Exec time: 2
Task 2
Deadline: 12 Exec time: 3
Task 1
Deadline: 5 Exec time: 4
Time 0
Task 1
Task 2
Task 3 5 8 12
CSE 522S – Advanced Operating Systems

9. Periodic Real-Time Tasks on Linux
Threads wake up, run task code, suspend
Thread per priority level may work for FIFO
Thread per task may be needed for RR, DEADLINE
Timers expire, wake threads, reschedule
Number of timers needed depends on granularity of differences in task release times
A single timer can keep a “wheel” of release times
Task 1
Task 2
Task 3 5 10 15
CSE 522S – Advanced Operating Systems