9 task scheduling- co operative models

8/9/2019 9 Task Scheduling- Co Operative Models

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2008Chapter-8 L16: "Embedded Systems - Architecture,

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

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REAL TIME OPERATING SYSTEMS

Lesson-16:Task Scheduling Cooperative ModelsTask Scheduling Cooperative Models


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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. Cooperative Scheduling in the cyclic2. Cooperative Scheduling in the cyclic

orderorder


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Cooperative Scheduling in the cyclic orderCooperative Scheduling in the cyclic order

Each task cooperate to let the running taskfinish

Cooperative means that each task

cooperates to let the a running one finish.

None of the tasks does block in-between

anywhere during the ready to finish states.

The service is in the cyclic order


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WorstWorst--case latencycase latency

Same for every task

Tworst = {(sti + eti )1 + (sti + eti )2 +...+ (sti + eti)N-1 + (sti + eti )N} + tISR.

tISR is the sum of all execution times for the ISRs

For an i-th task, switching time from one task to

another be is sti and task execution time be is eti

i = 1, 2, , N 1 , N, when number of tasks = N


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

Inc.

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Program counter assignments (switch) at different

times, when the on the scheduler calls the o tasks

from the list one by one in the circular queue fromthe list.


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Cyclic scheduling model in tasks scheduling


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First three tasks among N- tasks in washing machine

tasks scheduling


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Messages from the scheduler and task programs

contexts at various instances in washing machine

tasks scheduling


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2. Cooperative Scheduling of Ready Tasks2. Cooperative Scheduling of Ready Tasks

in Listin List


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Cooperative Scheduling in the order inCooperative Scheduling in the order in

which a task is initiated on interruptwhich a task is initiated on interrupt

None of the tasks does block in-between

anywhere during the ready to finish states.

The service is in the order in which a task is

initiated on interrupt.


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Same for every task in the ready list

Tworst = {(dti + sti + eti )1 + (dti + sti + eti )2+...+ (dti + sti + eti )n-1 + (dti + sti + eti )n} + tISR.


For an i-th task, let the event detection time with

when an event is brought into a list be is dti ,

switching time from one task to another be is stiand task execution time be is eti

i = 1, 2, , n 1 , n


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Scheduler in which the scheduler inserts into a list

the ready tasks for a sequential execution in a

cooperative mode


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3. Cooperative Scheduling of Ready Tasks3. Cooperative Scheduling of Ready Tasks

Using an Ordered List as per precedenceUsing an Ordered List as per precedence

ConstraintsConstraints


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Cooperative Scheduling in the order ofCooperative Scheduling in the order of

Ready Tasks using an Ordered List as perReady Tasks using an Ordered List as per

priority precedencepriority precedence

Scheduler using a priority parameter,taskPriority does the ordering of list of thetasksordering according to the precedenceof the interrupt sources and tasks.

The scheduler first executes only the firsttask at the ordered list, and the total, equalsthe to period taken by the first task on at the

list. It is deleted from the list after the firsttask is executed and the next task becomesthe first.


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Cooperative Scheduling in the order ofCooperative Scheduling in the order of

Ready Tasks using an Ordered List asReady Tasks using an Ordered List as

per priority precedenceper priority precedence

The insertions and deletions for forming theordered list are made only at the beginning

of the cycle for each list.


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Not Same for every task. Varies from (dti +sti + eti ) p(m)} + tISRto {(dti + sti + eti )p1 + (dti + sti + eti ) p2 +...+ (dti+ st

i+ et

i)

p(m-1)+ (dt

i+ st

i+ et

i)

p(m)} + t

ISR.


For an i-th task, let the event detection time withwhen an event is brought into a list be is dti ,switching time from one task to another be is stiand task execution time be is eti

i = 1, 2, , m 1 , m; m is number of ISRs andtasks in the list


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Cooperative Priority based scheduling of the ISRs

executing in the first layer and Priority based ready

tasks at an ordered list executing in the second layer


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Program counter assignments at different times onProgram counter assignments at different times on

the scheduler calls to the ISRs and the correspondingthe scheduler calls to the ISRs and the corresponding

taskstasks


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Example of ACVMExample of ACVM

First the coins inserted by the user are read,

then the chocolate delivers, and then display

task displays thank you, visit again

message.

Each task cooperates with other to finish.

The precedence of tasks in the ready list

reading coins is highest, then of chocolatedelivery and display for the ordered list of

the ready tasks.


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SummarySummary


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We learntWe learnt

Each task cooperate to let the running taskfinish in cooperative scheduling model

Cyclic order

Cyclic in list of initiated tasks into a readylist

Cyclic in ordered list of initiated tasks into a

ready list and list arranged in order of ISR

and task priorities


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End of Lesson 16 of Chapter 8End of Lesson 16 of Chapter 8

9 task scheduling- co operative models

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