Lecture 11 Realtime OS

8/6/2019 Lecture 11 Realtime OS

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Introduction toReal-time Operating Systems (RTOS) andScheduling Algorithms

Lecturer: Prof. Yifeng Zhu

Fall, 2010


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What is a Real-Time System?

Real-time systems have been defined as: "thosesystems in which the correctness of the system

depends not only on the logical result of the

computation, but also on the time at which the resultsare produced"; J. Stankovic, "Misconceptions About Real-Time Computing,"

IEEE Computer, 21(10), October 1988.


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Real-Time Characteristics

Real-time systems often are comprised of acontrolling system, controlled system and

environment.

Controlling system: acquires information about environmentusing sensors and controls the environment with actuators.

Timing constraintsderived from physicalimpact ofcontrolling systems activities. Hard and soft

constraints. Periodic Tasks: Time-driven recurring at regular intervals. Aperiodic: Event-driven.


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Typical Real-Time Systems

Controller(Controlling System) Controlled

System

Environment

Sensors

Actuators


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Hard versus Soft Hard: failure to meet constraint is a fatal fault. Validated

system always meets timing constraints. Deterministic constraints Probabilistic constraints Constraints in terms of some usefulness function.

Soft: late completion is undesirable but generally notfatal. No validation or only demonstration job meets some statistical constraint. Occasional missed deadlines or aborted execution is usually considered

tolerable.

Often specified in probabilistic terms


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Characteristics of Real-Time Operating Systems

Unique requirements in five general areas:

1. Determinism2. Responsiveness3. User control4. Reliability5. Fail-soft operation


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1. Determinism No OS is fully deterministic due to resource competition

among multiple processes

Metrics: the maximum delay before acknowledging a highpriority device interrupt

In real-time OS, usually upper-bounded In non-real-time OS, usually unbounded In non-RT this delay may be in the order of 10s and 100s of

millisecs, whereas in an RT it may have an upper-bound offew microsec to 1 millisec.


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1. Determinism2. Responsiveness

Metrics: the time for servicing the interrupt once it has beenacknowledged.

Comprises: Time to transfer control, (and context switch) and execute the

ISR

Time to handle nested interrupts, the interrupts that should beserviced when executing this ISR. Higher priority Interrupts.

response time = f(responsiveness, determinism)


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1. Determinism2. Responsiveness3. User control

User has a much broader control in RT-OS than in regular OS Priority Hard or soft deadlines Deadlines Memory Management: paging or swapping Name the processes to be resident in memory Scheduling policies


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1. Determinism2. Responsiveness3. User control4. Reliability

Failure in RT-OS may be catastrophic: life and death, financialloss, equipment damage.

In the event of a failure, immediate detection and correction isimportant.

Notify user processes to rollback. Apply compensation.


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RTOS Characteristics

Reliability

Number of 9s Downtime per yr Typical applications3 Nines (99.9%) ~ 9 hrs Desktop

4 Nines (99.99%) ~ 1 hr Enterprise Servers

5 Nines (99.999%) ~ 5 min Carrier-Class Servers

6 Nines (99.9999%) ~ 31 sec Carrier Switch Equipment

Reliability / Availability ChartProviding Open Architecture High Availability Solutions (HA Forum)


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1. Determinism2. Responsiveness3. User control4. Reliability5. Fail-soft operation

the ability of a system to fail in such a way as to preserve as muchcapability and data as possible

A real-time system is stable/fail-soft if, in cases where it isimpossible to meet all task deadlines, the system will meet thedeadlines of its most critical, highest-priority tasks, even if someless critical task deadlines are not always met.


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To meet these requirements, real-time OS typically include the followingfeatures:

Fast process or thread switch Small size (with corresponding minimal functionality) Ability to respond to external interrupts quickly Multitasking with interprocess communication tools such as semaphores,

signals, and events

Use of sequential files that can accumulate data at a fast rate Preemptive scheduling based on priority Minimization of intervals during which interrupts are disabled Primitives to delay tasks for a fixed amount of time and to pause/resume

tasks

Special alarms and time-outs


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Preemptive vs Non-preemptive scheduling

Preemptive Scheduling Task execution is preempted and resumed later Preemption occurs to execute higher priority task. Offers higher schedulability Involves higher scheduling overhead due to context switching

Non-preemptive Scheduling Once a task starts executing, it completes its full execution Offers lower schedulability Less overhead due to less context switching


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Some Definitions

Timing constraint: constraint imposed on timing behaviorof a job: hard or soft.

Release Time: Instant of time job becomes available forexecution. If all jobs are released when the system begins

execution, then there is said to be no release time

Deadline: Instant of time a job's execution is required tobe completed. If deadline is infinity, then job has nodeadline. Absolute deadline is equal to release time plus

relative deadline

Response time: Length of time from release time to instantjob completes.


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Real-Time Workload

Job (unit of work) a computation, a file read, a message transmission, etc

Attributes Resources required to make progress Timing parameters

Released

Absolute

deadline

Relative deadline

Execution time


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Real-Time Task

Task : a sequence of similar jobs Periodic task (p,e)

Its jobs repeat regularly Period p = inter-release time (0 < p) Execution time e = maximum execution time (0 < e < p) Utilization U = e/p

5 10 150


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Schedulability

Property indicating whether a real-time system (a set ofreal-time tasks) can meet their deadlines

(4,1)

(5,2)

(7,2)


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Real-Time Scheduling

Determines the order of real-time task executions Static-priority scheduling Dynamic-priority scheduling

(4,1)

(5,2)

(7,2)

5

5

10

10 15

15


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RM (Rate Monotonic)

Optimal static-priority scheduling It assigns priority according to period A task with a shorter period has a higher priority Executes a job with the shortest period

(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3


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RM (Rate Monotonic)

Executes a job with the shortest period

(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3


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RM (Rate Monotonic)

Executes a job with the shortest period

(4,1)

(5,2)

(7,2)

Deadline Miss !

5

5

10

10 15

15

T1

T2

T3


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Response Time

Response time Duration from released time to finish time

(4,1)

(5,2)

(10,2)

5

5

10

10 15

15

T1

T2

T3


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EDF (Earliest Deadline First)

Optimal dynamic priority scheduling A task with a shorter deadline has a higher

priority

Executes a job with the earliest deadline(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3


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Executes a job with the earliest deadline

(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3


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Executes a job with the earliest deadline

(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3


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Optimal scheduling algorithm if there is a schedule for a set of real-time tasks,

EDF can schedule it.

(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3


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EDF & RM

Real Time Scheduling algorithms RMS EDF


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EDF & RM

Another example of real-time scheduling with RMS and EDF


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Real-time Scheduling

Dynamic planning-based scheduling After the task arrives before execution begins, a

schedule is prepared that includes the new as well

as the existing tasks. If the new one can go without affecting the existing

schedules than nothing is revised.

Else schedules are revised to accommodate thenew task.

Remember that sometimes new tasks may berejected if deadlines cannot be met.


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Real-Time Scheduling

Dynamic best-effort scheduling: used in most commercial RTs of today tasks are aperiodic, no static scheduling is possible some short-term scheduling such as shortest deadline first

is used.

Until the task completes we do not know whether it hasmet the deadline.


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Acknowledgement

These slides are modified from the lecture slides ofInsup Lee @ University of Pennsylvania

http://www.cis.upenn.edu/~lee/home/home.html

Lecture 11 Realtime OS

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