Real-Time Operating System
Chapter – 8
Embedded System: An integrated
approach
Real Time Tasks
• What is Real Time?
– Simply, the time measured by physical clock.
– Anything is ‘real time’ means it has direct relation
with actual time.
• Real time task:
– Performance is judged on basis of time.
– Correct result = correct output + correct time
• Incorrect timing of result leads to :
– System failure OR
– Reduced QoS
Real Time Operating System(Chapter 8)
• Real Time Tasks
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Process control in industrial plants
Robotics
Air Traffic control
Telecommunications
Weapon guidance system e.g. Guided missiles
Medical diagnostic and life support system
Automatic engine control system
Real time data base
Mars Rovers
• Curiosity : OS – VxWorks, Processor BEA’s RAD 750
Real time speech or video processing
• A speech or moving picture sample of 1
second, if processed in 1 second or in less,
makes it real time processing
RTOS
• Question: Is real time systems and embedded
systems are same?
Terms and definition
• Release time (or ready time): This is the time instant at which a
task(process) is ready or eligible for execution
• Schedule Time: This is the time instant when a task gets its chance
to execute
• Completion time: This is the time instant when task completes its
execution
• Deadline: This is the instant of time by which the execution of task
should get completed.
• Runtime: The time taken without interruption to complete the task,
after the task is released
Terms and Definitions
• Tardiness: Specifies the amount of time by
which a task misses its deadline. Its is equal to
the difference between completion time and
deadline
• Laxity: Is defined as deadline minus remaining
computation time. The laxity of task is the
maximum amount of time it can wait and still
meets its deadline
Soft and Hard Real Time task
• Hard Real time task:
– Task must complete before or at deadline.
– Value of completing the task after deadline is zero.
• Software Real Time
– Missing deadline is penalty
– Penalty increases as tardiness increase
Real Time Operating System(Chapter 8)
• Types of Real Time Tasks
– Hard Real-time task
• Air traffic control
• Vehicle subsystems control
• Nuclear power plant control
– Soft Real-time Task
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Multimedia transmission and reception
Networking, telecom (cellular) networks
Web sites and services
Computer games.
Firm Real Task
Periodic Task
Aperiodic Task
Sporadic Task
Preemptible/Non-Preemptible Tasks
RTOS
• Do an embedded system need an operating
system?
Real Time Operating System(Chapter 8)
• Real time in operating systems:
– The ability of the operating system to provide a
required level of service in a bounded response
time.
– It responds to inputs immediately(Real-Time).
– Here the task is completed within a specified time
delay.
– In real life situations like controlling traffic signal
or a nuclear reactor or an aircraft,
– The operating system has to respond quickly.
Real Time Operating System(Chapter 8)
• Characteristic of Real time in operating
systems:
– Consistency
– Reliability
– Scalability
– Predictability
– Performance
Real Time Operating System(Chapter 8)
• Functions of Real time in operating systems:
– Task management
– Scheduling.
– Resource Allocation.
– Interrupt Handling.
Real Time Operating System(Chapter 8)
• Scheduling in Real time in operating systems:
– No of tasks
– Resource Requirements
– Release Time
– Execution time
– Deadlines
• Rate Monotonic Algorithm
• Earliest Deadline First Algorithm
Real Time Scheduling Algorithms
• General Purpose Operating System’s
Scheduling Algorithms:
• FCFS, SJF, SRTF, Priority Scheduling, Round
Robin
• Some of them are applicable for RTOS.
• There more specific RTOS algorithms
RTOS Scheduling Classification
Real Time
Scheduling
Off-line
On-line
Static
Priority
Preemptive
Non Preemptive
Dynamic
Priority
Planning
Based
Best Effort
Off Line Scheduling (Pre Run time
scheduling)
• They generate scheduling information prior to
system execution (Deterministic System
Model)
• This scheduling is based on :
– Release time
– Deadlines
– Execution
• Disadvantage: Inflexibility, If any parameter
changes, the policy will have to be redone
On-Line Scheduling
• Number and types of tasks, associated
parameters are not known in advance.
• Scheduling must accommodate dynamic
changes
• Online Scheduling are of two types:
– Static Priority
– Dynamic Priority
Static Priority
• Tasks with highest priority gets the chance to
execute first
• This can be preemptive or Non-preemptive
• Non preemptive: task with highest priority
runs till it completes.
• Preemptive: The execution of task can be preempted when higher priority task appears in
ready queue.
Dynamic Priority
• Priority can be allowed to change at run time
• Scheduling needs more computation
• Pre-emption may or may not be used
• Flexibility is quite high in such system
• Two subsets:
1. Planning Based:
– Guarantees deadline for all accepted task.
2. Best effort algorithm does its best to maximize
performance.
– Guarantees meeting deadline for hard time task.
– Optimizes the performance of soft time task
Problem: Static Priority
Tasks
Priority
Period
CPU Burst
T1
1
7
2
T2
2
17
4
T3
3
24
8
• Draw the Gantt-chart for scheduling these tasks.
• Assume the all jobs have same release time
• Schedule the process to meet their deadline
– Without Pre-emption
– With Pre-emption
• Note: Deadline of task is the time when its next burst arrive
Rate Monotonic Algorithm
• Introduced by Liu and Layland in 1973
• Assigning priorities as a monotonic function of the rate of a
process.
• (Monotonic means either increasing or decreasing)
• Priorities are assigned according to increased period of a
process.
• As period increases the priority decreases.
• The process with lowest period will have highest priority
RM Algorithm
• RM provides simple inequality, to verify the
sufficient condition for RM algorithm
• Where C is CPU Burst , P or T is Period
• If the condition satisfied, RM will schedule tasks
within their respective deadline
• This is sufficient, but not necessary condition
Example
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Tasks
Priority
Period (T)
CPU Burst(C)
T1
1
7
2
T2
2
17
4
T3
3
24
8
CPU utilization for the set of task:
LHS : 2/7 + 4/17 + 8/24 = 0.812
RHS : = 3 X (2 1/3 -1)= 3 x .26 = .78
LHS> RHS :RM does not get satisfied
But the task still might get schedulable by RM
technique
Problem 1
Tasks
Period (P or T)
CPU Burst(C)
T1
15
4
T2
12
2
T3
20
5
Problem 2
Tasks
Period
CPU Burst
Release Time
T1
3
1
0
T2
10
3
1
T3
15
4
3