1. Realtime System Fundamentals : Scheduling and Priority-based scheduling
B. Ramamurthy
cse321-fall2013
11/27/2018

2. Realtime scheduling Realtime system scheduling as in:
Earliest deadline scheduling (EDS)
Starting deadline
Completion deadline
Dynamic priority scheduling
Rate monotonic scheduling (RMS)
Periodic tasks are prioritized by the frequency of repetition (high priority to tasks with shorter periods)
Preemptive scheduling
Fixed priority scheduling
Schedulability according to RMS
Σ(Ci/Ti) <= n(21/n-1)
Cyclic executives (pre-scheduled)
Concepts of cycle, slot and frame
Repeated execution times
cse321-fall2013
11/27/2018

3. Task State Diagram New Ready Blocked Run 11/27/2018 Task admitted
Resources allocated
Dispatched; cpu allocated
Event occurred
Task exit
Blocked
Run
Waiting for event
cse321-fall2013
11/27/2018

4. Deadline driven scheduling
Parameters: ready time, starting deadline, completion deadline, processing time, resource requirement, priority, preemptive or non-preemptive
cse321-fall2013
11/27/2018

5. Deadline Scheduling
Process Arrival Time Execution Time Ending Deadline
A(1)
A(2)
A(3)
A(4)
A(5)
• • • •
B(1)
B(2)
cse321-fall2013
11/27/2018

6. Fixed-priority scheduling; A has priority B has priority
deadline
A1 B1 A2 B1 A3 B2 A4 B2 A5 B2
A1 A2 B1 A3 A4 A5, B2
(missed) A1
A2 A3 A4
A5, B2
B1 A2 A3 B2 A5
A1 B1 A2 B1 A3 B2 A4 B2 A5
Fixed-priority scheduling;
A has priority
B has priority
Earliest-deadline scheduling
using completion deadlines B1
cse321-fall2013
11/27/2018

7. Aperiodic Task set
Arrival Time Execution Time Starting Deadline A B C D E
Use earliest deadline with unforced idle time
cse321-fall2013
11/27/2018

8. Rate-monotonic scheduling
First proposed by Liu.
For RMS, the highest-priority task is the one with the shortest period, the
second highest-priority task is the one with the second shortest period, and so on.
Schedulability according to RMS
Σ(Ci/Ti) <= n(21/n-1)
cse321-fall2013
11/27/2018

9. Resources & Critical Resources
Shared resources: need mutual exclusion
Tasks cooperating to complete a job
Tasks contending to access a resource
Tasks synchronizing
Critical resources and critical region
A important synchronization and mutual exclusion primitive / resource is “semaphore”
cse321-fall2013
11/27/2018

10. Critical sections and Semaphores
When multiples tasks are executing there may be sections where only one task could execute at a given time: critical region or critical section
There may be resources which can be accessed only be one of the processes: critical resource
Semaphores can be used to ensure mutual exclusion to critical sections and critical resources
cse321-fall2013
11/27/2018

11. Semaphore Implementation
Define a semaphore as a class:
class Semaphore
{ int value; // semaphore value
processQueue L; // process queue
//operations
wait()
signal() }
In addition, two simple utility operations:
block() suspends the process that invokes it.
wakeup() resumes the execution of a blocked process P.
11/27/2018

12. Semantics of wait and signal
Semaphore operations now defined as
S.wait(): S.value--;
if (S.value < 0) {
add this process to S.L; block(); // block a process }
S.signal(): S.value++;
if (S.value <= 0) {
remove a process P from S.L; wakeup(); // wake a process
11/27/2018

13. Semaphores for CS
Semaphore is initialized to 1. The first process that executes a wait() will be able to immediately enter the critical section (CS).
Now other processes wanting to enter the CS will each execute the wait() thus decrementing the value of S, and will get blocked on S. (If at any time value of S is negative, its absolute value gives the number of processes waiting blocked. )
When a process in CS departs, it executes S.signal() which increments the value of S, and will wake up any one of the processes blocked. The queue could be FIFO or priority queue.
11/27/2018

14. Semaphores Semaphore init() Semaphore wait() Semaphore signal()
cse321-fall2013
11/27/2018

15. Priority Inversion
When we allow concurrent task to execute and with semaphore and mailboxes and other synchronization primitives, it is possible that a low priority task may come to block a high priority task. This situation is known as priority inversion.
What happened on Mars?
cse321-fall2013
11/27/2018

16. Priority inversion (Priority: t1>t2>t3)
Critical
section
task1
time
blocked
task2
task3
cse321-fall2013
11/27/2018

17. Problem: Priority inversion Solution1: Priority Inheritance
task1
blocked
Task 2 delayed
task2
Priority of t1
inherited
Critical
section
Priority reverted
To t3
task3
time
cse321-fall2013
11/27/2018

18. Solution2:Priority Ceiling ProtocolCS
Used by
Priority Ceiling S1
t1,t2
P(t1) S2
t1,t2,t3 S3 t3
P(t3)
Solution2:Priority Ceiling Protocol
Acquire S1
Release S1
task1
Attempt to
Acquire S1
Acquire S1
Acquire S2
No way
task2
Acquire S2
Release S2
Critical
section
task3
time
cse321-fall2013
11/27/2018