characteristics time constraints shdlischeduling...scheduling the heart of a rtos is the short-term...

RTOSRTOS

CharacteristicsCharacteristicsTime constraintsTime constraints

S h d liS h d liSchedulingScheduling

C. Brandolese, W. FornaciariC. Brandolese, W. Fornaciari

RealReal--Time SystemsTime Systems

Definition of real-time system“ l h h h f “A real-time system is one in which the correctness of the computation not only depends on the logical correctness of the computation, but also on the time pat which the result is computed. If the timing constraints of the system are not met, system failure is said to have occurred”s sa d to ave occu ed

Tasks or processes attempt to control or react to external events occuring in “real time”RT processes must be able to keep up events

Hard Real Time: timing violation produces a disasterS ft R l Ti ti i g i l ti d i bl b t Soft Real Time: timing violations are undesirable but not catastrophic

C. Brandolese, W. FornaciariC. Brandolese, W. Fornaciari -- 22 --

RTOSRTOS

A Real-Time Operating System (RTOS)Is just one element of a complete real-time systemMust provide sufficient functionality to enable the

ll l ti t t t it i toverall real-time system to meet its requirementsShould ensure that all time critical events are handled as quickly and efficiently as possibleas quickly and efficiently as possible

RTOS are characterized by these main featuresDeterministic behaviourDeterministic behaviourResponsivenessUser controlReliability and fail-soft operation

RTOS: DeterminismRTOS: Determinism

Operations are performed at fixed, predetermined times or within predetermined time intervalsConcerned with

How long the operating system delays before acknowledging an interruptWhether the system has sufficient capacity to handle Whether the system has sufficient capacity to handle all requests within required time

RTOS should show minimum interrupt latencyRTOS should show minimum interrupt latencyTypical measure:

Max delay to start ISR with highest priorityRT: 1µs - 1msNon-RT: 10ms - 100 ms

RTOS: ResponsivenessRTOS: Responsiveness

How long, after acknowledgment, the operating system takes to service the interruptIncludes the time to begin execution of the ISR

If a context switch is necessary, the delay is longer than an ISR executed within the context of the current processcurrent process

Includes the amount of time to perform the interrupt, depending also on the specific hardware p , p g pDepends on the possible ISR nesting and if ISRs can be interrupted

RTOS: User controlRTOS: User control

Much broader in RTOS than in ordinary O.S.User should be able to

Specify paging or process swappingDecide which processes must reside in main memoryEstablish the rights of processes

d l kFine-grained control over task prioritiesSelect algorithms for disks scheduling...

RTOS: ReliabilityRTOS: Reliability

In the application domain of RTOS, degradation of performances may have catastrophic consequencesperformances may have catastrophic consequences

Attempt either to fix the problem or minimize its effects while continuing to rungGraceful degradation can be not enough

Fail-softAbility to fail in such a way to preserve as much data and capability as possible

E g multiple attempts to improve data consistencyE.g., multiple attempts to improve data consistency

StabilityThe system adopts policies such that the most critical y p phigh priority tasks execute first, even if some needs of less important tasks can be not always met

RTOS: Typical featuresRTOS: Typical features

Low cost, small sizeManagement of at least 20 tasksHave minimum interrupt latencyD t i i ti ti ti f ll k l iDeterministic execution time for all kernel servicesProvide at least the following services

Allow tasks to be dynamically created and deletedAllow tasks to be dynamically created and deletedProvide semaphore management services and IPCAllow time delays and timeouts on kernel servicesyManagement of special alarmsUse of special file to accumulate data at a fast ratePreemptive priority-based scheduling with fast context switch

Time constraintsTime constraints

Computational activities in a real-time system generally have timing constraints

computation time

ready scheduled completed deadline

Time constraintsTime constraints

The "ready" event may bePeriodic

Tasks: Overall activityJ b I t ti ti i di id ll h d l d Jobs: Instantiations or individually scheduled computations of the task. A task is a stream of jobs

Aperiodic but predictablepe od c but p ed ctableUnpredictable

The "computation time" may beFixed in durationVariableUnpredictable

DeadlinesDeadlines

DeadlinesHard

The computation must be completed by the deadline time or a fatal error will occurtime or a fatal error will occur

SoftThe deadline may just be a recommendation or The deadline may just be a recommendation or preference for completion of the computation

A "Value Function"Indicates the criticality of a deadlineIndicates the value that the completion of a task

t ib t t th ll l f th tcontributes to the overall value of the system

DeadlinesDeadlines

In a non real-time system the time by which a task is completed does not influence the overall value –or quality – of the systemTh l f ti i th f ll iThe value function is the following

DeadlinesDeadlines

Catastrophic effect Hard Deadline

-∞ tR0

Ramped Hard Deadline Soft Deadline

tR D tR D

PreemptionPreemption

A computation or task is preemptable if it can be interrupted (suspended) when another more critical task needs to be executedA t ti bA computation can be

PreemptablePreemptable with one or more non preemptible Preemptable with one or more non-preemptible critical regionsNon-preemptableNon preemptable

PreemptionPreemption

IRQpreemption completed

Task 1

Task 2

IRQno preemption completedno preemption completed

Task 1

Task 2

SchedulingScheduling

The heart of a RTOS is the short-term schedulerNot crucial

Fairness, Minimum average response time, …

C i lCrucialAll hard-RT tasks must complete by their deadlineAs many soft-RT tasks as possible should also complete As many soft-RT tasks as possible should also complete meeting their deadlines

Most RTOSs are unable to deal with deadlinesDesigned to be as responsive as possible to RT tasks

When deadline approaches, RT tasks can be quickly h d l dscheduled

This approach requires deterministic response time sometimes below milliseconds

sometimes below milliseconds

In a Round Robin non-preemptive schedulerThe RT task is appended at the end of the ready queue

Wait for its next timeslice

Th d l b t bl f RT li tiThe delay can be unacceptable for RT applications

Request for RT Task

Task 1 Task 2 Task 3 Task N RT Task Task 1

Scheduling time

Priority driven non-preemptive schedulerRT tasks have higher priorityA RT task is scheduled when the current task T

Is blocked, orRuns to completion (even if with low priority)

The RT is added at the head of the ready queueThe RT is added at the head of the ready queuePossible delay of seconds, unacceptable for real-time

Request for RT Task

Task T RT Task Task N

Scheduling time

Priority driven preemptive on preemption pointsPreemption can only take place at the end of some regular intervalsI th i tIn those points

The highest priority task is scheduledIncluding kernel tasksIncluding kernel tasks

Delays in the order of some ms, adequate for some applications not for more demanding ones

Request for RT Task

RT Task Task T (resumed)

Scheduling time

Task T

preemptionpoints

Scheduling timep

Immediate preemptive schedulingThe service to the interrupt is almost immediate

Apart from the case OS is executing a critical region

S h d li d l f ll d t 100 lScheduling delays fall down to 100 µs, or lessA minimum amount of time is nevertheless required to save the context and to switch to the new contextsave the context and to switch to the new context

Good for critical systems

Request for RT Task

RT Task Task T (resumed)

Scheduling time

Task T

preemptionpoints

Scheduling timep

Factors influencing schedulingFactors influencing scheduling

Schedulability analysisYes/NoStatic/dynamic

The result of the analysis can beA clear scheduling

l f ll f k d hA "plan" to follow at run-time for task dispatching

Classes of algorithmsS i bl d iStatic table-drivenStatic priority-driven preemptiveDynamic planning basedDynamic planning-basedDynamic best-effort

Static tableStatic table--drivendriven

Through a static feasibility analysis of schedule, determines at run time when a task must begin determines, at run-time, when a task must begin execution

Applicable to periodic tasksApplicable to periodic tasksAnalysis inputs

Periodic arrival timeExecution timePeriodic ending deadlineR l ti i it f h t kRelative priority of each task

Predictable but inflexible approachAny change in the requirements of tasks imply the Any change in the requirements of tasks imply the computation of a new schedule

Example: Earliest-deadline first

Static priorityStatic priority--driven preemptivedriven preemptive

Uses standard priority-driven preemptive schedulerStatic analysis is performed

No schedule is drawn-upAssigns priorities to tasksIn RT systems depends on time constraints associated to tasksto tasks

Example: Rate Monotonic

Dynamic planningDynamic planning--basedbased

Feasibility is determined at run-time Rather then offline

A task is accepted for execution If and only if it is feasible to meet its time constraints

Before its execution An attempt is made to create a schedule including previous tasks and the new oneThe deadline of the extended task set must be metThe deadline of the extended task set must be met

Dynamic bestDynamic best--efforteffort

Used in many commercial RT systems lEasy to implement

No feasibility analysis is performedTypically the tasks are aperiodic and no static Typically, the tasks are aperiodic and no static scheduling analysis is possible

When a task arrivesThe system assign a priority based on its characteristics

E.g., based on earliest deadlinesE.g., based on earliest deadlinesThe system tries to meet deadlines and aborts any started process whose deadline is missed

il h k i l d ( d dli i ) i i Until the task is completed (or deadline arrives), it is unknown if time constraints will be met

Deadline schedulingDeadline scheduling

Real-time applications are not concerned with speed but with completing tasksPriorities

Provide a crude tool Do not capture the requirement of completion or initiation at the most valuable timeinitiation at the most valuable time

Other deadline related information should be taken into accountinto account

Deadline scheduling: Deadline scheduling: Other informationOther informationOther informationOther information

Ready time h h k d f Time at which task is ready for execution

For periodic task it is a sequence of times Known in advanceKnown in advance

Starting/completion deadlineTypical RT application have either deadlines, but not yp pp ,both

Processing timeI it i li dIn some cases it is suppliedIn others cases the operating system measures an exponential averagep g

Resource requirementsIn addition to microprocessor

Deadline scheduling: Deadline scheduling: Other informationOther informationOther informationOther information

PriorityMeasures relative importance of tasksHard-RT tasks have “absolute” priority

Subtask structureTask possibly decomposed in

M d b k h l i h h d RT d dliMandatory subtasks: the only with hard RT deadlinesOptional subtasks: all other subtasks

Design issuesDesign issues

Which task to schedule next?For a given preemption strategy, scheduling tasks with the earliest deadline minimizes the fraction of tasks that miss their deadlines tasks that miss their deadlines

True for single and multiprocessor configurations

What sort of preemption is allowed?What sort of preemption is allowed?When starting deadlines are given, non-preemptive scheduler makes senseThe RT task has the responsibility to block itself afterexecuting the critical portion, allowing starting other RT d dliRT deadlines

Example: Periodic tasksExample: Periodic tasks

A system collects and processes data from two sensors, A and BDeadlines:

A must be read every 20 msB must be read every 50 ms

P i ti (i l di OS h d) Processing time (including OS overhead): Processing data from A requires 10 msP i g d t f B i 25 Processing data from B requires 25 ms

The system makes scheduling decision every 10 ms

Process Arrival time Execution time DeadlineA(1) 0 10 20A(2) 20 10 40A(2) 20 10 40A(3) 40 10 60A(4) 60 10 80… … … …… … … …

B(1) 0 25 50B(2) 50 25 100… … … …

B(1) deadline

A(1) deadline

A(2) deadline

A(3) deadline

A(4) deadline

A(5) deadline

A(6) deadline

Task A

Task B

0 10 20 30 40 50 60 70 80 90 100 110 120

A(2) A(3) A(4) A(5) A(6) A(7)

B(2) B(3)

time0 10 20 30 40 50 60 70 80 90 100 110 120

Fixed priority schedulingTask A has priority Task B may miss some deadlines

A(1) B(1) A(2) A(3) A(4) A(5) A(6) A(7)B(2) B(3)B(1) B(2) B(2)B(3)

time0 10 20 30 40 50 60 70 80 90 100 110 120

B(1) is not rescheduled sinceits deadline has been reached

d ll h b l d

and will thus be violated

Fixed priority schedulingTask B has priority Task A may miss some deadlines

B(1) A(2) A(3) A(5)B(2) B(3)

time0 10 20 30 40 50 60 70 80 90 100 110 120

A(1) missed deadline A(4) missed deadline

Earliest deadline first with preemptionCompletion deadlines are consideredAt any preemption point the scheduler gives priority t th t k ith th li t d dlito the task with the earliest deadlineAll task can meet their deadlines

A(1) B(1) A(2) A(3) A(4) A(5) A(6) B(3)B(1) B(2)B(2)

time0 10 20 30 40 50 60 70 80 90 100 110 120

All task can be scheduled within their completion deadlines

Example: Aperiodic tasksExample: Aperiodic tasks

Five tasks All tasks have an execution time of 20 msArrival times are unknown Starting deadlines are considered

A B C D EA B C D E

AB C DE

time0 10 20 30 40 50 60 70 80 90 100 110 120

Arrival timesArrival times

Starting deadlines

Earliest deadline firstStarting deadlines are unknown before the task becomes ready for schedulingS h d l th d t k ith th li t d dli Schedule the ready task with the earliest deadline and let the task run to completion

A B C D E

AB C DE

A E DC

time0 10 20 30 40 50 60 70 80 90 100 110 120

B missed deadline Tasks D and E are readyTask E has earliest deadline

Earliest deadline with unforced idle timesPossible if deadlines are know before the task is ready

Schedule the eligible task with the earliest deadline L t th t k t l tiLet the task run to completion

Elegible tasks may not be ready Processor remains idle even though ready tasks existProcessor remains idle even though ready tasks exist

A B C D E

AB C DE

B E DC A

time0 10 20 30 40 50 60 70 80 90 100 110 120

Tasks A, D and E are readyTask E has earliest deadline

First-Come, First-Serve (FCFS)Tasks are ordered based on their arrival timesTasks are served if their starting deadline is metTasks B and E do not meet their deadlines

A B C D EE missed deadline

AB C DE

time0 10 20 30 40 50 60 70 80 90 100 110 120

Tasks D and E are ready, with D coming firstTask D is served

B missed deadline

Rate Monotonic SchedulingRate Monotonic Scheduling

Each task has A period T An execution time C for each occurrence

In uniprocessor systems C < T

lProcessor Utilization is U = C/T (<100%)

Period

Task P P P Ptime

ExecutionTime

IdleTime

Assigns priorities to tasks On the basis of their periodsShortest period task has highest priority

High Highest priority

Highest rate

Highest priority

Rate (Hz)

A measure of a the effectiveness of a periodic scheduling algorithm is its capability to meet deadlinesC id N t k ith Consider N tasks with

A fixed period Ti

A fixed execution time CA fixed execution time Ci

To meet all deadlines, the processor utilizations of individual tasks must not exceeds the available individual tasks must not exceeds the available computational power

N C∑=

characteristics time constraints shdlischeduling...scheduling the heart of a rtos is the short-term...

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