Scheduling and Resource Access Protocols: Basic Aspects

CS5270 Lecture 3 1

Scheduling and Resource Access Protocols: Basic Aspects

CS 5270 Lecture 3

CS5270 Lecture 3 2

A Classic Policy

• Rate Monotonic Scheduling.– Task set : {J1, J2, …, Jn}– Each task is periodic. T1, T2,.., Tn

– i = 0 for each i.– Di = Ti for each i.– Pre-emption allowed.– Only one processor– No precedence constraints– No shared resources.

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RMS

• The RMS algorithm:– Assign a static priority to the tasks according

to their periods. Priority of a task does not change during

execution.

– Tasks with shorter periods have higher priorities.

– Preemption policy: If Ti is executing and Tj arrives which has higher

priority (shorter period), then preempt Ti and start executing Tj.

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RMS Example

(3, 2)

(5, 1)

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RMS Example

(3, 1)

(5, 2)

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RMS Results

• RMS is optimal.– If a set of of periodic tasks (satisfying the

assumptions set out previously) is not schedulable under RMS then no static priority algorithm can schedule this set of tasks.

• RMS requires very little run time processing.

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Process Utilization Factor

• Task set = {T1, T2, …, Tn}

• Process Utilization Factor– Ci / Ti

– C1 / T1 + C2 / T2 + … Cn / Tn

• If this factor is GREATER than 1 then the task set can not be scheduled.– Why?

• If UF ≤ 1 it may be RMS-schedulable.

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RMS Schedulability

• Task set = {T1, T2, …, Tn}

• If UF Ulub then it is guaranteed to be schedulable.

• Ulub - The least upper bound of processor

utility.

• For RMS, Ulub = n( 21/n – 1)

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Process Utilization Factor

• Task set = {T1, T2, …, Tn}

• If UF Ulub then it is guaranteed to be schedulable.

• But if UF is greater than Ulub and not greater than 1, we must check explicitly whether the task set is RMS-schedulable.

10

RMS Schedulability

n Ulub

1

1.000

2 0.828

3 0.780

4 0.757

5 0.743

6 0.735

7 0.729

0.690

This is only a sufficient criterion!

This criterion may fail and yet an RMS may exist.

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RMS Example

(3, 2)

(5, 1)

UF = 0.66 + 0.20 = 0.86

Ulub = 0. 828

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RMS Example

(3, 1)

(5, 2)

UF = 0.33 + 0.40 = 0.73

Ulub = 0. 828

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EDF

• Earliest Deadline First.– Tasks with earlier deadlines will have higher

priorities.– Applies to both periodic and aperiodic tasks.– EDF is optimal for dynamic priority algorithms.– A set of periodic tasks is schedulable with

EDF iff the utilization factor is not greater

than 1.

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An Example

• {T1, T2}

• T1 = ( 5, 2)

• T2 = (7, 4)

• UF = 0.4 + 0.57 = 0.97

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An RMS Schedule?

Time-Overflow

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The Example

• UF = 0.4 + 0.57 = 0.97

• Guaranteed to be schedulable under EDF!

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An EDF Schedule

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Shared Resources

• Inter-process communication– Shared resources, critical sections, priority

inversion, access protocols.

• Material taken from:– Buttazzo’s Book: parts of chapter 7.

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The Host Computer

DSP Processor

ASIC

Timer

Memory

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Tasks

DATA SETS

TASK1

TASK2

TASK3

TASK4

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Resource Constraints

• resource:– software structure used by a task during its execution.– A data structure, variables, an area of main memory,

a file, a piece of code, a set of registers of a peripheral device.

• Shared resource:– Used by more than one task.

• Exclusive resource:– No simultaneous access.– Require mutual exclusion.– Operating must provide a synchronization mechanism

to ensure sequential access..

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Critical Section

• Critical section:– A piece of code belonging to task executed

under mutual exclusion constraints.

• Mutual exclusion enforced by semaphores.– wait(s)

Blocked if s = 0.

– signal(s) s is set to 1 when signal(s) executes.

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Structure of Critical Sections.

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Wait State

• A task waiting for an exclusive resource is blocked on that resource.

• Tasks blocked on the same resource are kept in a wait queue associated with the semaphore protecting the resource.

• A task in the running state executing wait(s) on a locked semaphore (s = 0) enters the waiting state.

• When a task currently using the resource executes signal(s), the semaphore is released.

• When a task leaves its waiting state (because the semaphore has been released) it goes into the ready state:

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Waiting State

26

Blocking via Exclusive Resource

J1 has higher priority than J2.

Preemption is in play.

Only one processor available.

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Resource Access Protocols

• Multiple tasks.

• Uniprocessor

• Shared resources.– Need proper protocols for accessing shared

resources.– Resource access protocols.

• Avoid priority inversion!

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Priority Inversion.

0 1 2 3 4 5 6 7

J1 > J2 > J3

[3, 6] priority inversion period. J1 waits for the execution of J2 and the critical section of J3

J1

J2

J3

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Priority Inversion

• The Mars pathfinder Mission in 1997 ran into serious problem.

• The spacecraft began experiencing total system resets with loss of data each time.

• It turned out to be due to priority inversion.

• See the web page and the links there in the IVLE area!

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Avoiding Priority inversion

• Disallow preemption during the execution of a critical section.– Works only if critical sections are short.– Might unneccesarily block higher priority

processes that do not even use any shared resources!

• Resource access protocols:– Priority inheritance protocol.– Priority ceiling protocol.

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Priority Inheritance Protocol

• Tasks have nominal and active priorities.– Nominal priority:

assigned by the scheduling algorithm (RMS, EDF,..)

– Active priority assigned by the protocol –dynamically- to avoid

priority inversion.

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• Basic idea :– When Ji blocks higher-priority tasks, then its

active priority is set to the highest of the priorities of the tasks it blocks.

– Ji inherits -temporarily – the highest priority of the blocked tasks.

– This prevents medium priority tasks from preempting Ji and prolonging the blocking duration of the higher priority tasks.

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The Protocol:• Jobs are scheduled based on their active

priorities.• If Ji tries to enter a critical section and the

corresponding resource is being held by Jj then Ji is blocked; it is said to be blocked by J j.

• When a job is blocked on a semaphore, it transmits its active priority to the job that holds the semaphore; in general, a task inherits the highest priority of the jobs blocked by it.

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The Protocol:• When Jk exits a critical section, it unlocks

the semaphore; the job with the highest priority that is blocked on the semaphore, if any, is awakened. The priority of Jk is set to the highest priority of the job it is currently blocking. If none, its priority is set to its nominal one.

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Example

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Nested Critical Sections

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• Good news:– If there are m distinct semaphores that can

block a job J then J can be blocked for at most the duration of at most one critical section, one for each of the semaphores.

– It can never be as long as the WCET of a lower priority task.

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• Bad news:

• Chained Blocking:– J can get blocked on n critical sections held

by n distinct lower priority jobs.

• Deadlocks.

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Chained Blocking

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Deadlock

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Priority Ceiling Protocol

• Extension of the Priority Inheritance Protocol.• Avoids chained blocking and deadlocks.• Basic Idea:

– A task is not allowed to enter a critical section if there are already locked semaphores which could block it eventually (due to a sub-critical section nested within the entering critical section).

– Hence, once a task enters a critical section, it can not be blocked by lower priority tasks till its completion.

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• The Protocol:• Each semaphore S is assigned a priority ceiling C(S). It

is the priority of the highest priority task that can lock S. This is a static value.

• Suppose J is currently running and it wants to lock the semaphore S. J is allowed to lock S only if the priority of J is strictly higher than the priority ceiling C(S’) of the semaphore S’ where:– S’ is the semaphore with the highest priority ceiling among all

the semaphores which are currently locked by jobs other than J.

– In this case, J is said to blocked by the semaphore S’ (and the job currently holding S’).

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The Protocol:• When J gets blocked by S’ then the priority of J

is transmitted to the job that currently holds S’.• When J’ leaves a critical section guarded by S’

then it unlocks S’ and the highest priority job, if any, which is blocked by S’ is awakened.

• The priority of J’ is set to the highest priority of the job that is blocked by some semaphore that J’ is still holding. If none, the priority of J’ is set to be its nominal one.

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Example

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C (S0) = ? C(S1) = ? C(S2) = ?

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C (S0) = P0 C(S1) = P0 C(S2) = P1

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t2 : J1 can not lock S2. Currently J2 is holding S2 and C(S2) = P1 and the current priority of J1 is also P1.

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t5 : J0 can not lock S0. Currently J2 is holding S2 and S1 and C(S1) = P0 and the current priority of J0 is also P0. The (inherited) priority of J2 is now P0.

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t6 : J2 unlocks S1. It awakens J0. But J2’s (inherited) priority is now only P1 while P0 > C(S2) = P1. So J0 preempts J2 and runs to completion.

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t7 : J2 resumes execution with priority P1.

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t8 : J2 unlocks S2 and goes back to its nominal priority P2. So J1 preempts J0 and runs to completion.

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Two Key Properties

• Under priority ceiling protocol, a job can be blocked for at most the duration of one critical section.

• The priority ceiling protocol prevents deadlocks.

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Blocking Times

• Bi ----- The maximum blocking time of Ji.

• Dj,k ---- The duration the critical section in Jj whose access is regulated by the semaphore Sk.

– If Dj,k is 0 this means Jj does not use Sk.

• Then:– Bi = max{ Dj,k | Pj < Pi, C(Sk) Pi }

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Scheduling and Resource Access Protocols: Basic Aspects