1. Realtime System Fundamentals : Scheduling and Priority-based scheduling
B. Ramamurthy
Amrita-UB-MSES
5/11/2013

2. Realtime scheduling We will 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
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3. Task State Diagram New Ready Blocked Run 5/11/2013 Task admitted
Resources allocated
Dispatched; cpu allocated
Event occurred
Task exit
Blocked
Run
Waiting for event
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4. Deadline driven scheduling
Parameters: ready time, starting deadline, completion deadline, processing time, resource requirement, priority, preemptive or non-preemptive
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5. Deadline Scheduling
Process Arrival Time Execution Time Ending Deadline
A(1)
A(2)
A(3)
A(4)
A(5)
• • • •
B(1)
B(2)
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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
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7. Aperiodic Task set
Arrival Time Execution Time Starting Deadline A B C D E
Use earliest deadline with unforced idle time
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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)
Amrita-UB-MSES
5/11/2013

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”
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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
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11. Semaphores
See semaphore.h of xinu
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5/11/2013

12. Semaphore: wait() ppcb->sem = sem; /* record semaphore id in pcb */
enqueue(currpid, psem->queue);
resched(); /* place in wait queue and reschedule */ }
restore(ps); /* restore interrupts */
return OK;
Amrita-UB-MSES
5/11/2013

13. Semaphore: signal()
/*signal - signal a semaphore, releasing one waiting process, and block
sem id of semaphore to signal
OK on success, SYSERR on failure */
syscall signal(semaphore sem) {
irqmask ps;
register struct sentry *psem;
ps = disable(); /* disable interrupts */
if ( isbadsem(sem) ) /* safety check */
restore(ps);
return SYSERR; }
psem = &semtab[sem]; /* retrieve semaphore entry */
if ( (psem->count++) < 0 ) /* release one process from wait queue */
{ ready(dequeue(psem->queue), RESCHED_YES); }
restore(ps); /* restore interrupts */
return OK;
Amrita-UB-MSES
5/11/2013

14. Semaphore: usage Problem 1: Problem 2:
Create 3 tasks that each sleep for a random time and update a counter.
Counter is the critical resources shared among the processes.
Only one task can update the counter at a time so that counter value is correct.
Problem 2:
Create 3 tasks; task 1 updates the counter by 1 and then signal task 2 that updates the counter by 2 and then signals task 3 to update the counter by 3.
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15. Problem 1
#include <..> //declare semaphore semaphore mutex1 = newsem(1); int counter = 0; //declare functions: proc1,proc1, proc3 ready(create((void *)proc1, INITSTK, INITPRIO, “PROC1",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc2, INITSTK, INITPRIO, “PROC2",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc3, INITSTK, INITPRIO, “PROC3",, 2, 0, NULL), RESCHED_NO);
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16. Problem 1: multi-tasks
void proc1() { while (1) { sleep (rand()%10); wait(mutex1); counter++; signal(mutex1); } } void proc2() //similarly proc3
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17. Problem 1 Task 1 Task 2 Counter1 Task 3 5/11/2013
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18. Problem 2
semaphore synch12 = newsem(0); semaphore synch23 = newsem(0); semaphore synch31 = newsem(0); ready(create((void *)proc1, INITSTK, INITPRIO, “PROC1",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc2, INITSTK, INITPRIO, “PROC2",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc3, INITSTK, INITPRIO, “PROC3",, 2, 0, NULL), RESCHED_NO); signal(synch31);
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19. Task flow void proc1() void proc2() void proc3() 5/11/2013 {
while (1) {
sleep (rand()%10);
wait(synch31);
counter++;
signal(synch12);
} }
void proc2()
wait(synch12);
signal(synch23);
void proc3()
sleep(rand()%10);
wait(synch23);
signal(synch31); } }
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5/11/2013

20. 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?
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21. Priority inversion (Priority: t1>t2>t3)
Critical
section
task1
time
blocked
task2
task3
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22. Problem: Priority inversion Solution1: Priority Inheritance
task1
blocked
Task 2 delayed
task2
Priority of t1
inherited
Critical
section
Priority reverted
To t3
task3
time
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5/11/2013

23. 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
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5/11/2013