7th Sem # ECE# EC8791-Embedded and Real Time Systems # Unit 4-Task Assignment and Scheduling Algorithms

1. RMK COLLEGE OF ENGINEERING AND
TECHNOLOGY
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
Ms. S. Rajalakshmi
EMBEDDED AND REAL TIME
SYSTEM

2. Unit- 4
REAL TIME SYSTEMS
Structure of a Real Time
System
01
Estimating Program Run
Time
02
Task Assignment and
Scheduling
03
Fault Tolerance
Techniques
04
Reliability , Evaluation05
Clock Synchronization06

3. • Infographic StylePrescribed Text Books & Reference Books
TEXT BOOK
Jane W.S.Liu,‖ Real Time Systems‖, Pearson
Education, Third Indian Reprint, 2003.(UNIT IV)
REFERENCE
C.M. Krishna, Kang G. Shin, ―Real-Time Systems‖,
International Editions, Mc Graw Hill 1997

4. S.No Topic Online
Source
Duration
1 Mastering RTOS: Hands on Free RTOS and STM32Fx
with Debugging
Udemy 15 hours
2 Embedded Hardware and Operating Systems Coursera 4 weeks

5. 3. TASK ASSIGNMENT AND SCHEDULING
Periodic Task: A task Ti is Periodic if it is released periodically for every Pi Seconds.
Constraint: Task to run exactly once every period.
Aperiodic/Sporadic Task: Can occur any time-no arrival pattern given
Release time(τ) - The time at which all the data that
are required to begin executing the task available.
Deadline(di) - The time by which task must
complete its execution.
Release time
(τ)
Relative
deadline
(di)
Absolute
deadline
(τ+di)
DeadlineArrival of a
Task
(τ)

6.  Task Consumes Resources -Processor Time, Memory and Input data.
 Task may have precedence constraints.
 if Task Ti output is needed as input by task Tj.
 Precedence Graph: Precedence constraint represented.
Task Precedence

7. Scheduling
Process by which the tasks that are to be executed are arranged in an order
Scheduler – Program responsible for scheduling process
Types of Scheduling
Off line Scheduling
On line Scheduling
Preemptive Task
Task that can be interrupted
Preemption
Process by the currently executing task is
paused or stopped in favour of a higher
priority Task

8. Scheduling Algorithms
1. UniProcessor Scheduling Algorithm
2. MultiProcessor Scheduling Algorithm
I. Traditional Rate Monotonic Scheduling Algorithm (RMS)
II. Rate Monotonic Defered Server (RMDS)
III. Earliest Deadline First (EDF)
IV. Precedence and Exclusion Condition
V. IRIS Tasks
I. Utilization balancing algorithm
II. Next-fit algorithm
III. Bin-packing algorithm
IV. Bin-packing algorithm
V. Myopic offline scheduling algorithm
VI. Myopic offline scheduling algorithm
VII.Assignment with Precedence Constrainsts

9. (i) Traditional rate-monotonic (RMS):
• Periodic, preemptible tasks
• Deadlines equal the task period.
• A task set of n tasks is schedulable under RM if its total
processor utilization is no greater than n(2/"-1).
• Task priorities are
• Some results are also available for the case where a task
deadline does not equal its period.
(ii) Rate-onotmonic deferred server
(DS):
• Similar to the RM algorithm, except
that it can handle both periodic
aperiodic tasks.
(iii)Earliest deadline first (EDF):
• Tasks are preemptible
• Task with the earliest deadline has the highest priority.
• If a task set is not schedulable on a single processor
by EDF, there is no other processor that can
successfully schedule that task set.

10. (iv)Precedence and exclusion conditions:
• Takes precedence conditions into account.
• Non preemptable task also considered
(v) Multiple task versions:
Suitable for system has primary and alternative
versions of some tasks.
If the system has enough time, it will execute the
primary; however, under conditions of overload, the
alternative may be picked.
(vi) IRIS tasks:
• Increased Reward with Increased Service.
• Tasks that be stopped early and still provide
useful output.

11. (i) Utilization balancing
algorithm:
Assigns tasks to processors one by one
in such a way that at the end of each
step, the utilization of the various
processors are as nearly balanced as
possible.
Tasks are assumed to be preemptible.
(iii) Bin-packing algorithm:
Assigns tasks to processors under the constraint at the
total processor utilization must not exceed a given
threshold. The threshold is set in such a way that the
uniprocessor scheduling algorithm is able to schedule the
tasks assigned to each processor. Tasks are assumed to be
preemptible.
(ii) Next-fit algorithm:
• The next-fit algorithm is designed to work in
conjunction with the rate-monotonic uniprocessor
scheduling algorithm.
• Divides the set of tasks into various classes.
• A set of processors is exclusively assigned to each task
class.
• Tasks are assumed to be preemptible.

12. (iv)Myopic offline scheduling algorithm:
• Deal with nonpre-emptible tasks.
• It builds up the schedule using a search.
(v)Focused addressing and bidding algorithm:
Tasks are assumed to arrive at the individual
processors.
A processor that finds itself unable to meet the
deadline or other constraints of all its tasks tries to
offload some of its workload onto other processors.
It does so by announcing which task(s) it would like
to offload and waiting for the other processors to
offer to take them up.
(vi) Buddy strategy: The buddy strategy takes roughly
the same approach as the focused addressing algorithm.
Processors are divided into three categories: under
loaded, fully loaded, and overloaded. Overloaded
processors ask the underloaded processor to offer to take
over some of their load.

13. (vii) Assignment
with precedence
constraints:
The last task
assignment algorithm
takes task
precedence
constraints into
account.
CLASSICAL UNIPROCESSOR
SCHEDULING ALGORITHMS
Assumptions made for both the RMS and
EDF algorithms.
A1. No task has any non pre-emptable section and the
cost of preemption is negligible.
A2. Only processing requirements are significant;
memory, I/O, and other re source requirements are
negligible.
A3. All tasks are independent; there are no precedence
constraints.

14. CLASSICAL UNIPROCESSOR SCHEDULING
ALGORITHMS
Assumptions made for both the RMS and
EDF algorithms.
A1. No task has any non pre-emptable section and the
cost of preemption is negligible.
A2. Only processing requirements are significant;
memory, I/O, and other re source requirements are
negligible.
A3. All tasks are independent; there are no precedence
constraints.

15. Task Scheduling Algorithms
Static Priority Algorithm: Task Priority does not change with in a mode.
Rate Monotonic Scheduling (RMS)
Dynamic Priority Algorithm: Priority can change with time.
Example: Earliest deadline First (EDF)
Preemptive Scheduling: A task can be
interrupted by other task.
Non Preemptive Scheduling: Once a task is
begun it must be run to complete or it gets locked by
resource.

16. THANK YOU