what is real time system? how they are scheduled? what is EDF? what is RMA? EDF vs RMA?

2. A Real Time System is any information
processing system which has to respond to
externally generated input stimuli within a
finite & specified period.
In real time computing , the correctness of
the computation depends not only on the
logical results but also on the time at
which the results are produced.

3. In RTS a substantial
fraction of the design
effort goes into
making sure that the
task deadlines are
met.
A Deadline is a
given time after a
triggering event , by
which a response has
to be completed.

4. 1. Hard real
time systems 2. Soft real time
systems

5. Hard Real Time Systems
System should meets the timings constraints.
 Response time overrun leads to disastrous
damage
Example : Flight Control System
SOFT REAL TIME SYSTEMS
 Deadline overruns are tolerable, but not
desired.
 There are no catastrophic consequences of
missing one or more deadlines.
 Example : Multimedia System

6. Operating System Issues
Architectural Issues
Other Issues

7.  By task scheduling , we means that a computer’s
resources must be shared out equitably among the
users.
 In general-purpose system , Round robin scheduling
is used , which ensures that user does not get a
disproportionate share of the computer’s services.
 But Round robin scheduling approach does not
work in Real Time applications because priority
scheduling makes the difference in the two.

8. Computation Priority Scheduling
Offline Static Preemptive
(precomputed)
Non
Online
Dynamic Preemptive
(Dynamically)

9. A1. No task has any non- preempt able section & cost of
preemption is negligible.
A2. Only processing requirements are significant;
memory, I/O & other resource requirements are
negligible.
A3. All tasks are independent.
A4. The deadline of a task is equal to its period.
A5. All the tasks in the task set are periodic.

10.  Optimal uniprocessor algorithm
 Also known as Deadline Monotonic
Scheduling algorithm
 Tasks are preemptable
 Dynamic priorities
 Scheduler picks task , whose deadline is
due next
 A set of tasks is schedulable ( in hard
real time sense ) , if the total processor
utilization is not greater than 1.

11. NECESSARY & SUFFICIENT
CONDITION
In EDF , for a task set is said to be
schedulable is if it satisfy the following
necessary and sufficient condition :
Σ (i=1 to n) e/p ≤ 1
i i

12. Consider 3 periodic processes scheduled
using EDF, the following acceptance test shows
that all deadlines will be met.
Execution Time = ei Period = pi
Process
P1 1 8
P2 2 5
P3 4 10

13. The utilization will be:
U = { ( 1/8 ) + ( 2/5 ) + ( 4/10 ) }
= 0.925 ( or 92.5 % )
The theoretical limit for any number of
processes is 100% ( or less than 1 ) and
so the system is schedulable .

14. Different possible scenarios
Sufficient
but NOT
necessary
If di ≥ pi If di < pi
Necessary
and Necessary
sufficient and
sufficient
∑ ei/pi ≤ 1 ∑ ei/di ≤ 1 Fail
Pass
Schedulable Processor demand
Based test

15. •Optimality
ADVANTAGES •Reduces number of task
switches
•Deteriorates badly under
overload
DRAWBACKS •Scheduling is more
expensive
•Needs smarter scheduler

16.  Static priority based algorithm : tasks
scheduled according to priorities
 Classical uniprocessor scheduling
algorithm
 Also called Liu-Layland algorithm or Clock
driven uniprocessor algorithm
 Tasks are pre-emptable

17.  It says that if the total utilization of the task
is not greater than n(21/n -1) where n stands
for number of tasks to be scheduled then
RM algorithm will schedule all the task to
meet their respective deadlines.
 The RMA assigns priorities to the tasks
based on their rate of occurrence, i.e. the
lower the occurrence rate of the task the
lower the priority assigned to it.

18.  In RMA the priority of the task is directly
proportional to its rate of occurrence or it is
inversely proportional to the period

19.  A set of periodic real time tasks would not be
RMA schedulable unless they satisfy the
following necessary condition:-
Σ (i=1 to n) U =Σ
i (i=1 to n) e /p ≤ 1
i i

20.  Also referred to as Liu and Layland’s condition.
 It implies that a set of ‘n’ real time periodic tasks
are schedulable under RMA if :
Σ (i=1 to n) U ≤ n
i (21/n -1)

21.  If the set of tasks satisfies this sufficient condition
then it is guaranteed that the set of tasks would be
RMA schedulable.
CASES :
Consider the case when
• there is only 1 task in the system, i.e., n=1
U1 = 1
• there is 2 tasks in the system, i.e., n=2
U2 = 0.824
• n=∞, applying L’Hospitals Rule
ln 2 = 0.69
CONCLUSION:
From above computation it is clear that maximum
CPU utilization that can be achieved under RMA is 1.

22. Consider 3 tasks to be scheduled using RMA, the
following acceptance test determines whether that
all deadlines will be met or not.
TASK EXECUTION PERIOD
TIME
T1 20 100
T2 30 150
T3 60 200

23. Checking for the NECESSARY CONDITION :
Σ(i=1 to n) U =Σ
i (i=1 to n) e /p
i i ≤ 1
U1 0.2
U2 0.2
U3 0.3
Σ(i=1 to 3) U 0.7

24. Checking for the SUFFICIENT CONDITION :
Σ(i=1 to n) Ui ≤ n(21/n -1)
For n=3,
n(21/n -1) = 3(21/3 -1)
= 0.78 < 1
Ui = 0.7 < 0.78
Hence, sufficient condition is satisfied.
Therefore, all the given three tasks are
schedulable under RMA.

25. • possesses good transient
Advantages overload handling
• difficult to support
aperiodic and sporadic
tasks
Disadvantages
• not optimal when task
periods and deadlines
differ

26. EDF RMA
 EDF is also called Deadline-  RMA is also called Liu-
Monotonic Scheduling Layland algorithm.
algorithm
 Static priority algorithm
 Dynamic priority algorithm
 All tasks are assumed to be
 Tasks do not have to be
periodic
periodic
 Full processor can be  It lacks flexibility. For e.g.
feasibly scheduled, if RMA assumes that tasks are
utilization of task set is no infinitely preempt able.
greater than 1
 Simpler implementation, even
 It requires a run time in systems without explicit
overhead. Deteriorates during support for timing constraints
overload conditions (periods, deadlines)