1. Marathwada Mitramandal’s
COLLEGE OF ENGINEERING
Karvenagar, Pune
Accredited with ‘A++’ Grade by NAAC
Presentation
On
Subject: 410241 :Design and Analysis of Algorithms
By
Dr. Swati Shekapure
Department of Computer Engineering
3. Embedded system scheduling algorithms
• From digital camera to mobile phones, embedded systems can be
found in every aspect of our life.
• In recent years, embedded systems have become more and more
resource intensive because of the increase demand for real-time and
quality-of-service(QoS) requirements.
• There is a urgent need to design a system that can meet the functional
and timing requirements.
4. Embedded system scheduling algorithms
Embedded systems - Computer systems that are designed to perform a
small or specific set of tasks, often in. At the bare minimum embedded
systems are single CPU systems with a relatively small and fixed amount of
memory but they can also include other IC’s or I/O devices which interface
with the main CPU.
Scheduling - A function of an operating system which manages which
process will get to run next on the CPU, and how long processes or tasks get
to run on the CPU.
5. Embedded system scheduling algorithms
• On embedded systems real-time processes must execute before a certain
time and sometimes they are performing atomic operations, which cannot
be disrupted once they start. These types of processes create the necessity
to use non-preemptive scheduling algorithms and the need to choose
processes in an order that will optimize system efficiency.
• Real time - Used to characterize a time constraint of a task of process.
Embedded systems sometimes have to be able to execute real time tasks
depending on the use of the embedded system.
• EDF - a dynamic scheduling algorithm used in real-time operating
systems. It assigns priority to process that has the earliest deadline.
7. Embedded system scheduling algorithms
There are several known real-time algorithms for embedded systems. They
are
• Earliest Deadline First (EDF),
• Least laxity first (LLF) and
• Rate Monotonic Scheduling (RMS).
8. Embedded system scheduling algorithms
• In EDF, tasks are scheduled by the order of deadline. In LLF, tasks are
scheduled by the order of laxity.
• EDF and LLF do not work very well in situations that soft real-time
applications compete with higher priority applications.
• In RMS, tasks are scheduled by static priority that is determined by
duration. The shorter the job duration is, the higher its priority is. It
guarantees time restraints up to 70% CPU load. When the load is above
70%, it does not support dynamic systems very well.
9. Embedded system scheduling algorithms
• In EDF, tasks are scheduled by the order of deadline.
• In LLF, tasks are scheduled by the order of laxity.
• EDF and LLF do not work very well in situations that soft real-time
applications compete with higher priority applications.
• In RMS, tasks are scheduled by static priority that is determined by
duration. The shorter the job duration is, the higher its priority is.
• It guarantees time restraints up to 70% CPU load. When the load is above
70%, it does not support dynamic systems very well.
10. Embedded system scheduling algorithms
Earliest deadline first (EDF) or least time to go is a dynamic priority
scheduling algorithm used in real-time operating systems to place
processes in a priority queue.
Whenever a scheduling event occurs (task finishes, new task released, etc.)
the queue will be searched for the process closest to its deadline. This
process is the next to be scheduled for execution.
11. Embedded system scheduling algorithms
• EDF is an optimal scheduling algorithm on preemptive uniprocessors, in
the following sense:
if a collection of independent jobs, each characterized by an arrival
time, an execution requirement and a deadline, can be scheduled (by any
algorithm) in a way that ensures all the jobs complete by their deadline,
the EDF will schedule this collection of jobs so they all complete by their
deadline.
12. Embedded system scheduling algorithms
U= 1/4 +2/6 +3/8 = 0.25 + 0.333 +0.375 = 0.95 = 95%
As processor utilization is less than 1 or 100% so task set is surely schedulable by EDF.
Task Release time(ri) Execution
Time(Ci
Deadline (Di) Time Period(Ti)
T1 0 1 4 4
T2 0 2 6 6
T3 0 3 8 8
13. Embedded system scheduling algorithms
1. At t=0 all the tasks are released, but priorities are decided according to their absolute deadlines
so T1 has higher priority as its deadline is 4 earlier than T2 whose deadline is 6 and T3 whose
deadline is 8, that’s why it executes first.
2. At t=1 again absolute deadlines are compared and T2 has shorter deadline so it executes and
after that T3 starts execution but at t=4 T1 comes in the system and deadlines are compared, at
this instant both T1 and T3 has same deadlines so ties are broken randomly so we continue to
execute T3.
14. Embedded system scheduling algorithm
3. At t=6 T2 is released, now deadline of T1 is earliest than T2 so it starts
execution and after that T2 begins to execute. At t=8 again T1 and T2 have same
deadlines i.e. t=16, so ties are broken randomly an T2 continues its execution and
then T1 completes. Now at t=12 T1 and T2 come in the system simultaneously so
by comparing absolute deadlines, T1 and T2 has same deadlines therefore ties
broken randomly and we continue to execute T3.
4. At t=13 T1 begins it execution and ends at t=14. Now T2 is the only task
in the system so it completes it execution.
15. Embedded system scheduling algorithm
5. At t=16 T1 and T2 are released together, priorities are decided according
to absolute deadlines so T1 execute first as its deadline is t=20 and T3’s deadline
is t=24.After T1 completion T3 starts and reaches at t=17 where T2 comes in the
system now by deadline comparison both have same deadline t=24 so ties broken
randomly ant we T continue to execute T3.
6. At t=20 both T1 and T2 are in the system and both have same deadline
t=24 so again ties broken randomly and T2 executes. After that T1 completes it
execution. In the same way system continue to run without any problem by
following EDF algorithm.
16. Wrap Up
● What do mean by Randomised and Approximate algorithms?
● What are the different Embedded system scheduling algorithms?
