14 Other Schduling Models

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2008Chapter-8 L22: "Embedded Systems - Architecture,

Programming and Design" , Raj Kamal, Publs.: McGraw-Hill,Inc.

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REAL TIME OPERATING SYSTEMS

Lesson-22:

Other Scheduling MethodsOther Scheduling Methods


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1. Common scheduling models1. Common scheduling models


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Common scheduling modelsCommon scheduling models

Cooperative Scheduling of ready tasks in acircular queue. It closely relates to functionqueue scheduling.

Cooperative Scheduling with PrecedenceConstraints

Cyclic scheduling of periodic tasks and RoundRobin Time Slicing Scheduling of equal prioritytasks

Preemptive Scheduling

Scheduling using 'Earliest Deadline First' (EDF)precedence.


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Common scheduling modelsCommon scheduling models

Rate Monotonic Scheduling using higher rate of

events occurrence First precedence

Fixed Times Scheduling

Scheduling of Periodic, sporadic and aperiodicTasks

Advanced scheduling algorithms using the

probabilistic Timed Petri nets (Stochastic) or

Multi Thread Graph for the multiprocessors andcomplex distributed systems.


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2. Earliest Deadline First' (EDF)2. Earliest Deadline First' (EDF)

precedenceprecedence


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EDFEDF

When a task becomes ready, its will be considered

at a scheduling point.

The scheduler does not assign any priority.

It computes the deadline left at a scheduling point. Scheduling point is an instance at which the

scheduler blocks the running task and re-computes

the deadlines and runs the EDF algorithm and

finds the task which is to be run


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EDFEDF

An EDF algorithm can also maintain a

priority queue based on the computation

when the new task inserts


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Programming and Design" , Raj Kamal, Publs.: McGraw-Hill,

Inc.

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3. Rate Monotonic Scheduler3. Rate Monotonic Scheduler


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Rate Monotonic SchedulerRate Monotonic Scheduler

Rate monotonic scheduler computes the

priorities,p, from the rate of occurrence of

the tasks.

The i-th task priority,pi is proportional to (

1/ti) where ti is the period of the occurrence

of the task event.


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Rate Monotonic SchedulerRate Monotonic Scheduler

RMA gives an advantage over EDF because

most RTOSes have provisions for priority

assignment. Higher priority tasks always get

executed


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4. Precedence Assignment in the4. Precedence Assignment in the

Scheduling AlgorithmsScheduling Algorithms


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Precedence Assignment in the SchedulingPrecedence Assignment in the Scheduling

AlgorithmsAlgorithms

Best strategy is one, which is based on EDF

(Earliest Deadline First) precedence.

Precedence is made the highest for a task

that corresponds to an interrupt source,

which occurs at the earliest a succeeding

times. and which deadline will finish

earliest at the earliest


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Precedence Assignment in the SchedulingPrecedence Assignment in the Scheduling

AlgorithmsAlgorithms

We assign precedence by appropriate

strategy in the case of the variable CPU

loads for the different tasks and variable

EDFs .


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Dynamic Precedence AssignmentDynamic Precedence Assignment

Firstly, there is deterministic or static assignmentof the precedence

It means firstly there is rate monotonic scheduling

(RMS).

Later on the scheduler dynamically assigns and

fixes the timeout delays afresh, and assigns the

precedence as per EDF.

The need for of the dynamic assignment arises due

to the sporadic tasks and the distributed or

multiprocessor indeterminate environment.


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5. Fixed (Static) Real Time Scheduling5. Fixed (Static) Real Time Scheduling

of the Tasksof the Tasks


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Fixed (Static) Real Time Scheduling of theFixed (Static) Real Time Scheduling of the

TasksTasks

A scheduler is said to be using a fixed time

scheduling method when the schedule is

static and deterministic


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Methods for Fixed (Static) Real TimeMethods for Fixed (Static) Real Time

SchedulingScheduling

(i) Simulated annealing method different

schedules fixed and the performance is

simulated. Now, schedules for the tasks are

gradually incremented by changing theinterrupt timer settings (using a

corresponding OS function) till the

simulation result shows that none is missing

its deadline.


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(ii)Heuristic method. Here, the reasoning

or past experience lets us helps to define

and fixing the fixed schedules.


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(iii)Dynamic programming model. This is

as follows: Certain specific running

program first determines the schedules for

each task and then the timer interrupt sloads the timer settings from the outputs

from that program


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SummarySummary


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We learntWe learnt

EDF

Rate Monotonic Scheduling using higher

rate of events occurrence First precedence

Fixed Times Scheduling: Simulatedannealing, Heuristic and Dynamic

programming


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End of Lesson 22 of Chapter 8End of Lesson 22 of Chapter 8

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14 Other Schduling Models