IT-606 Embedded Systems (Software)

1© Krithi Ramamritham / Kavi Arya

IT-606Embedded Systems

(Software)

Krithi RamamrithamS. RameshKavi Arya

KReSIT/ IIT Bombay

Real-Time Support


Functional Design & Mapping

HW1 HW2 HW3 HW4Hardware Interface

RTOS/Drivers

Thr

eadArchitectural

Design

F1F2

F3

F4

F5Functional

Design

(F3) (F4)

(F5)

(F2)

Source:Source:Ian Phillips, ARMIan Phillips, ARM

VSIA 2001

Source:Source:Ian Phillips, ARMIan Phillips, ARM

VSIA 2001


What is “real” about real-time?computer world

e.g., PC average response for

user Interactive

occasionally longer reaction: user annoyed computer controls

speed of user

“computer time”

real world

Industrial system, airplane

environment has own speed

reaction too slow: deadline miss

reaction: damage, pot. loss of human life

computer must follow speed of environment

“real-time”


A real-time system is a system that reacts to events in the environment by performing predefined actions

I/O - data

I/O - data

Real-Time Systems

Real-timecomputing system

event

action

within specified time intervals.

time


CLIENT SERVER

Flight Avionics

Constraints on responses to pilot inputs, aircraft state updates


Constraints:–Keep plastic at proper temperature (liquid, but not boiling)–Control injector solenoid (make sure that the motion of the piston reaches the end of its travel)


Real-Time Systems: Properties of Interest

• Safety: Nothing bad will happen.

• Liveness: Something good will happen.

• Timeliness: Things will happen on time -- by their deadlines, periodically, ....


Performance Metrics in Real-Time Systems

• Beyond minimizing response times and increasing the throughput:

achieve timeliness.• More precisely, how well can we

predict that deadlines will be met?


Types of RT Systems

Dimensions along which real-time activities can be categorized:

• how tight are the deadlines?

--deadlines are tight when

laxity (deadline -- computation time) is small.

• how strict are the deadlines?

what is the value of executing an activity after its deadline?

• what are the characteristics of environment? how static or dynamic must the system be?


deadline (dl)

+

Hard, soft, firm

• Hard -- result useless or dangerousif deadline exceeded

value

time-

hardsoft

• Soft -- result of some - lower value if deadline exceeded

• Firm -- If value drops to zero at deadline


Examples

• Hard real time systems– Aircraft– Airport landing

services– Nuclear Power

Stations– Chemical Plants– Life support

systems

• Soft real time systems– Mutlimedia– Interactive video

games


Real-Time: Items and Terms

Task– program, perform service, functionality– requires resources, e.g., execution time

Deadline– specified time for completion of, e.g., task– time interval or absolute point in time– value of result may depend on completion

time


Plan

• Special Characteristics of Real-Time Systems

• Real-Time Constraints

• Canonical Real-Time Applications

• Scheduling in Real-time systems

• Operating System Approaches


Timing Constraints

Real-time means to be in time --- how do we know something is “in time”?how do we express that?

• Timing constraints are used to specify temporal correctness “finish assignment by 2pm”,

“be at station before train departs”.• A system is said to be (temporally) feasible, if it

meets all specified timing constraints.• Timing constraints do not come out of thin air:

design process identifies events, derives models, and finally specifies timing constraints


Overall Picture…

physical properties of environment

timing constraints

model - design

analysis, testing

run-time dispatching

in field use

functional

temporal


• Periodic– activity occurs repeatedly– e.g., to monitor environment values, temperature, etc.

time

period

periodic


• Aperiodic– can occur any time– no arrival pattern given

time

aperiodicaperiodic


• Sporadic– can occur any time, but– minimum time between arrivals

time

mint

sporadic


Who initiates (triggers) actions?

Example: Chemical process – controlled so that temperature stays below

danger level– warning is triggered before danger point …… so that cooling can still occurTwo possibilities:– action whenever temp raises above warn

-- event triggered– look every fixed time interval; action taken when temp above warn -- time triggered


TT

ET

time

t


TT

ET

time

t


ET vs TT

• Time triggered– Stable number of invocations

• Event triggered– Only invoked when needed– High number of invocation and

computation demands if value changes frequently


Other Issues to worry about• Meet requirements -- some activities may run

only:– after others have completed - precedence constraints– while others are not running - mutual exclusion– within certain times - temporal constraints

• Scheduling– planning of activities, such that required timing is kept

• Allocation– where should a task execute?


Plan


• Real-Time Constraints• Canonical Real-Time Application

Coding• Scheduling in Real-time

systems • Operating System Approaches


A Typical Real time system

Temperature sensor

CPU

Memory

Input port

Output portHeater


Code for exampleWhile true do

{

read temperature sensor

if temperature too high

then turn off heater

else if temperature too low

then turn on heater

else nothing

}


Comment on code

• Code is by Polling device (temperature sensor)

• Code is in form of infinite loop

• No other tasks can be executed

• Suitable for dedicated system or sub-system only


Extended polling example

Computer

Temperature Sensor 1

Task 1

Task 2

Task 3

Task 4

Conceptual link




Heater 1

Heater 2

Heater 3

Heater 4


Polling

• Problems– Arranging task priorities– Round robin is usual within a priority level– Urgent tasks are delayed


Interrupt driven systems

• Advantages– Fast– Little delay for high priority tasks

• Disadvantages– Programming– Code difficult to debug– Code difficult to maintain


How can we monitor a sensor every 100 ms

Initiate a task T1 to handle the sensor

T1: Loop

{Do sensor task T2

Schedule T2 for +100 ms

}

Note that the time could be relative (as here) or could be an actual time - there would be slight differences between the methods, due to the additional time to execute the code.


An alternative…

Initiate a task to handle the sensor T1

T1:

Do sensor task T2

Repeat

{Schedule T2 for n * 100 ms

n:=n+1}

There are some subtleties here...


Clock, interrupts, tasks

Clock ProcessorInterrupts

Task 1 Task 2 Task 3 Task 4

Job/Task queue

Examines

Tasks schedule events using the clock...


Plan


• Real-Time Constraints• Canonical Real-Time Applications• Scheduling in Real-time systems • Operating System Approaches


Why is scheduling important?

Definition:

A real-time system is a system that reacts to events in the environment by performing predefined actions within specified time intervals.

Real-timecomputing system

time

I/O - data

I/O - data

event

action


Schedulability analysis

a.k.a. feasibility checking:

check whether tasks will meet their

timing constraints.


Scheduling Paradigms

Four scheduling paradigms emerge, depending on

• whether a system performs schedulability analysis

• if it does, – whether it is done statically or dynamically

– whether the result of the analysis itself produces a schedule or plan according to which tasks are dispatched at run-time.


1. Static Table-Driven Approaches

• Perform static schedulability analysis by checking if a schedule is derivable.

• The resulting schedule (table) identifies the start times of each task.

• Applicable to tasks that are periodic (or have been transformed into periodic tasks by well known techniques).

• This is highly predictable but, highly inflexible. • Any change to the tasks and their characteristics may

require a complete overhaul of the table.


2.Static Priority Driven Preemptive approaches

• Tasks have -- systematically assigned -- static priorities.• Priorities take timing constraints into account:

– e.g. rate-monotonic the lower the period, the higher the priority.

• Perform static schedulability analysis but no explicit schedule is constructed– RMA - Sum of task Utilizations <= ln 2.– Task utilization = computation-time / Period

• At run-time, tasks are executed highest-priority-first, with preemptive-resume policy.

• When resources are used, need to compute worst-case blocking times.


Static Priorities: Rate Monotonic Analysis

presented by Liu and Layland in 1973

Assumptions• Tasks are periodic with deadline equal to

period.Release time of tasks is the period start time.

• Tasks do not suspend themselves• Tasks have bounded execution time• Tasks are independent• Scheduling overhead negligible


RMA: Design Time vs. Run Time

At Design Time:Tasks priorities are assigned according

to their periods; shorter period means higher priority


RMA: Design Time vs. Run TimeSchedulability testTask set is schedulable if

Very simple test, easy to implement.

Run-time The ready task with the highest priority is

executed.

C i

T ii1

n

n(21/ n 1)


RMA: Exampletaskset: t1, t2, t3, t4 t1 = (3, 1) t2 = (6, 1) t3 = (5, 1) t4 = (10, 2)

The schedulability test:

1/3 + 1/6 + 1/5 + 2/10 ≤ 4 (2(1/4) - 1) ?

0.9 < 0.75 ?

…. not schedulable


RMA…A schedulability test is • Sufficient: there may exist tasksets that fail the test, but are

schedulable• Necessary: tasksets that fail are (definitely) not schedulableThe RMA schedulability test is sufficient, but not

necessary.When periods are harmonic, i.e., multiples of each other, utilization can

be 1.


Exact RMA

by Joseph and Pandya, based on critical instance analysis

(longest response time of task, when it is released at same time as all higher priority tasks)


What is happening at the critical instance?

• Let T1 be the highest priority task. Its response

time

R1 = C1 since it cannot be preempted

• What about T2 ?

R2 = C2 + delays due to interruptions by T1.

Since T1 has higher priority, it has shorter period.

That means it will interrupt T2 at least once,

probably more often.

Assume T1 has half the period of T2,

R2 = C2 + 2 x C1


Exact RMA….

In general:

Rni denotes the nth iteration of the response time of task i

hp(i) is the set of tasks with higher priority as task i

R CR

TCi

ni

in

jj hp i

j

1

( )


Example - Exact AnalysisLet us look at our example, that failed the pure rate monotonic test, although we can schedule it Exact analysis says so.

• R1 = 1; easy• R3, second highest priority task

hp(t3) = T1

R3 = 2R C C

R C C

R R

t

t

t t

t t

t t

31

1 1 2

32

1 1 2

33

32

3 1

3 1

1

3

2

3


• R2, third highest priority taskhp(t2) = {T1 ,T3 }

R2 = 3

R C C C

R C C C

R R

t

t

t t

t t t

t t t

21

1 1 1 3

22

1 1 1 3

23

22

2 1 3

2 1 3

1

3

1

5

3

3

3

5


• R4, third lowest priority taskhp(t4) = {T1 ,T3 ,T2 }

R4 = 9 Response times of first instances of all tasks < their periods => taskset feasible under RM scheduling

R C C C C

R C C C C

R C C C C

t

t

t

t t t t

t t t t

t t t t

41

2 1 1 1 5

42

2 2 1 1 6

43

4 1 2 3

4 1 2 3

4 1 2 3

2

3

2

6

2

5

5

3

5

6

5

5

6

3

6

6

6

5

2 2 1 2 7

44

2 3 2 2 9

45

2 3 2 2 9

45

44

4 1 2 3

4 1 2 3

7

3

7

6

7

5

9

3

9

6

9

5

R C C C C

R C C C C

R R

t

t

t t

t t t t

t t t t


3. Dynamic Planning based Approaches

• Feasibility is checked at run-time -- a dynamically arriving task is accepted only if it is feasible to meet its deadline. – Such a task is said to be guaranteed to meet its

time constraints• One of the results of the feasibility analysis can be

a schedule or plan that determines start times

• Has the flexibility of dynamic approaches with some of the predictability of static approaches

• If feasibility check is done sufficiently ahead of the deadline, time is available to take alternative actions.


4. Dynamic Best-effort Approaches

• The system tries to do its best to meet deadlines.

• But since no guarantees are provided, a task may be aborted during its execution.

• Until the deadline arrives, or until the task finishes, whichever comes first, one does not know whether a timing constraint will be met.

• Permits any reasonable scheduling approach, EDF, Highest-priority,…


Cyclic scheduling

• Ubiquitous in large-scale dynamic real-time systems

e.g., space shuttle, LCA

• Combination of both table-driven scheduling and priority scheduling.

• Tasks are assigned one of a set of harmonic periods.

• Within each period, tasks are dispatched according to a table that just lists the order in which the tasks execute.


• Slightly more flexible than the table-driven approach

• no start times are specified

• In many actual applications, rather than making worse-case assumptions, confidence in a cyclic schedule is obtained by very elaborate and extensive simulations of typical scenarios.


Plan







Real-Time Operating SystemsSupport process management and synchronization, memory

management, interprocess communication, and I/O. Three categories of real-time operating systems:

– small, proprietary kernels.

e.g. VRTX32, pSOS, VxWorks– real-time extensions to commercial timesharing operatin

systems.• e.g. RT-Linux, RT-NT

– research kernels

e.g. MARS, ARTS, Spring, Polis


Real-Time Applications Spectrum

Hard

Soft

Real-Time Operating System

General-PurposeOperatingSystem

VxWorks, Lynx, QNX, ...

Linux, NT

Windows CE

Intime, HyperKernel, RTX


Real-Time Applications Spectrum

Hard

Soft

Real-Time Operating System

General-PurposeOperatingSystem

VxWorks, Lynx, QNX, ...Intime, HyperKernel, RTX

Linux, NT

Windows CE


Embedded (Commercial) Kernels

Stripped down and optimized versions of timesharing operating systems.

• Intended to be fast– a fast context switch,– external interrupts recognized quickly– the ability to lock code and data in memory– special sequential files that can accumulate data

at a fast rate


• To deal with timing requirements– a real-time clock with special alarms and

timeouts

– bounded execution time for most primitives

– real-time queuing disciplines such as earliest deadline first,

– primitives to delay/suspend/resume execution

– priority-driven best-effort scheduling mechanism or a table-driven mechanism.

• Communication and synchronization via mailboxes, events, signals, and semaphores.


Real-Time Extensions to General Purpose Operating

SystemsE.g., extending LINUX to RT-LINUX, NT to RT-NT• Advantage:

– based on a set of familiar interfaces (standards) that speed development and facilitate portability.

• Disadvantages

– Too many basic and inappropriate underlying assumptions still exist.


Windows NT -- for RT applications?

• Scheduling and priorities– Preemptive, priority-based scheduling

non-degradable priorities priority adjustment

– No priority inheritance– No priority tracking – Limited number of priorities– No explicit support for guaranteeing timing constraints


NT Thread Priority = Process class + level

Real-timeclass

2625242322

16 Idle

Above NormalNormalBelow NormalLowest

Highest31 Time-critical

Dynamicclasses

15 Time-critical

14131211

15

High class

1 Idle

987

11

Normal class10

5432

6

Idle class

ThreadLevel


Typical Scheduling

• Threads scheduled by executive.

• Priority based preemptive scheduling.

Interrupts

Deferred Procedure Calls (DPC)

System anduser-level threads


Windows NT -- for RT applications? (contd.)

• Quick recognition of external events– Priority inversion due to Deferred Procedure Calls

(DPC)• I/O management• Timers granularity and accuracy

– High resolution counter with resolution of 0.8 sec. – Periodic and one shot timers with resolution of 1 msec.

• Rich set of synchronization objects and communication mechanisms. – Object queues are FIFO


Linux for RT apps?

• Linux (and its Real Time versions) are free!!

• Linux (and its Real Time versions) are Open Source!!

• Easy for developing RT applications


Linux for real-time?

• Could increase priority for “real-time” tasks and assume they get scheduled

• Problem – Linux optimizes average case whereas an RTOS should work under worst case assumptions


Linux – A Simplified View


Linux – conflicts with RT constraints

• Coarse grained synchronization – long intervals when a task has exclusive use of data ( as fine grained – leads to lot of overhead reducing the average case performance)

• Linux batches many operations for efficient use of H/W– e.g freeing a list of pages when memory is

full reducing the worst case performance


• Linux doesn't preempt low-priority task during system call

• Linux will make high priority tasks wait for low priority to release resources


Real Time Linux approaches

• Modify the current Linux kernel to guarantee RT constraints– Used by KURT

• Make the standard Linux kernel run as a task of the real-time kernel– Used by RT-Linux, RTAI

Slides thanks to: Swaminathan Sivasubramanian, Iowa State University


Modifying Linux kernel

• Advantages– Most problems, such as interrupt handling,

already solved– Less initial labour

• Disadvantages– No guaranteed performance– RT tasks don’t always have precedence

over non-RT tasks.


Running Linux as a process of a second RT kernel

• Advantages– Can make hard real time guarantees– Easy to implement a new scheduler

• Disadvantages– Initial port difficult, must know underlying

hardware– Running a small real-time executive is not

a substitute for a full fledged RTOS


KURT Overview• Developed at University of Kansas

• Soft real-time system

• Refines the temporal granularity of Linux– Motivation: RT tasks may need a time

resolution on the order of microseconds, while non-RT tasks may need a resolution of only milliseconds


• Result:

Timer interrupts are programmed to service earliest scheduled event (results in aperiodic timer interrupts)


KURT Overview (continued)

• Not suitable for hard real-time systems

• KURT can’t guarantee priority of RT tasks over non-RT tasks

• An RT task can be blocked by a non-RT task (eg: during disk I/O) leading to priority inversion

• Suitable for soft RT systems


RT-Linux Overview

• Open source Linux project

• Supports x86, PowerPC, Alpha

• Patch of the regular Linux kernel (simply install the patch and recompile the kernel)

• Provides an RT API for developers

• Runs Linux kernel as lowest priority process


RT-Linux Task Structure


RT Linux interrupt

handler

RT - Int

dispatcher

Linux Interrupt

dispatcher

RT-Linux Interrupt Dispatcher

RT int

Non RT int


RT-Linux Overview (continued)

• RT tasks are coded as modules

• Modules are inserted and removed at users discretion

• Extremely good at handling periodic tasks

• Communicates with non-RT kernel and other RT tasks via fifo queues

• Tools are provided for graphical analysis of RT execution


Problems with RT-Linux

• Currently no support for aperiodic tasks

• Not very useful for complex RT systems

• Currently limited to simple problems


CLIENT SERVER

Flight Simulator with RTLinux Components


Reader

Equation

Solver

Writer

Flight Dynamics Module

Network

Fifo1

Fifo 2


Example (Contd …)

• In the FD Module, there are three threads the reader, the equation solver and the writer.

• Reader reads the data from network and writes it in the fifo1.

• Equation solver reads data from fifo1, computes output based on this input and writes output in the fifo2.


Example (Contd …)

• Writer thread reads data from fifo2 and sends it over the network.

• If the three threads were in different modules, then we could not have used fifo for passing data between them.


Modules

• Module is a program that can be loaded or removed from the kernel at runtime without having to stop or compile the kernel.

• Each module requires two basic functions, init_module() and cleanup_module().


Init_module()

• This function is called when the module is loaded.

• Any thread that is to be used in the module has to be created here using pthread_create() call.

• The thread has to be passed a function name (defined in the module) which defines the code that the thread will run.


Init_module() (Contd…)

• Any fifo that is to be used in the module has to be created here by rtf_create().

• It is a good practice to destroy the fifo before creating it, as an already created fifo might have some data in it or some other module might be using it.

• Any other variables can also be initialized in this function.


Cleanup_module()

• All the fifos and threads created in the init_module() function should be destroyed here.

• Any other action that is to be taken when the module is removed, should be implemented here.


Threads

• We can create different modules for each thread or implement them in the same module, its just a programming decision.

• Normally real time threads are implemented as periodic threads, because otherwise they would not allow any non-real-time task to run.


Behaviour of Threads

• They are used as normal C programming threads and their scheduling depends on the priority assigned to them.

• The time taken by a thread to execute is independent of the module in which it was created.


Fifos

• Fifos are used for passing data between real-time and non-real-time tasks.

• Non-real-time tasks treat fifos as ordinary files.

• A fifo should be created by a real-time task, before any non-real-time task can use it.


Fifos (Contd…)

• A fifo cannot be used simultaneously by two modules.

• Fifo can be used to pass data between real-time threads by implementing both threads in the same module.

• To send data rtf_put() call is used and to receive rtf_get() call is used.


RTLinux as the OS contd…

• Provides scheduling latency of the order of ms for real-time threads

• Provides interrupt latency of the order of ms for real-time interrupts

• Provides co-existence of real-time and non real-time interrupt handling for the same device

• Provides fifo based communication mechanism between real-time and non-real time threads


Research Operating Systems

• MARS – static scheduling

• ARTS – static priority scheduling

• Spring –dynamic guarantees

• Polis – synthesizing OSs


MARS -- TU, Vienna (Kopetz)

Offers support for controlling a distributed application based entirely on time events (rather than asynchronous events) from the environment.

• A priori static analysis to demonstrate that all the timing requirements are met.


• Uses flow control on the maximum number of events that the system handles.

• Based on the time driven model -- assume everything is periodic.

• Static table-driven scheduling approach

• A hardware based clock synchronization algorithm

• A TDMA-like protocol to guarantee timely message delivery


ARTS -- CMU (Tokuda, et al)

• The ARTS kernel provides a distributed real-time computing environment.

• Works in conjunction with the static priority driven preemptive scheduling paradigm.

• Kernel is tied to various tools that a priori analyze schedulability.


• The kernel supports the notion of real-time objects and real-time threads.

• Each real-time object is time encapsulated -- a time fence mechanism: The time fence provides a run time check that ensures that the slack time is greater than the worst case execution time for an object invocation


SPRING – Umass. (Ramamritham & Stankovic)

• Real-time support for multiprocessors and distributed systems

• Strives for a more flexible combination of off-line and on-line techniques– Safety-critical tasks are dealt with via static table-

driven scheduling. – Dynamic planning based scheduling of tasks that

arrive dynamically.


• Takes tasks' time and resource constraints into account and avoids the need to a priori compute worst case blocking times

• Reflective kernel retains a significant amount of application semantics at run time – provides flexibility and graceful degradation.


Polis: Synthesizing OSs

• Given a FSM description of a RT application

• Each FSM becomes a task

• Signals, Interrupts, and polling

• Tasks with waiting inputs handled in FIFS order (priority order – to be done)


• Some interrupts can be made to directly execute the corresponding task

• Needed OS execute synthesized based on just what is needed


Distributed RT

RT-node

RT-node

RT-node

RT-nodegateway

RT-nodegateway

real-time bus

fieldbusessensorsactuatorsbackbone NW


Environment

• Input from the environment via data collected by sensors

• Output to environment via actuators

• Real-time systems “sees” and interacts with environment via sensors and actuators


Real-time buses• Central communication medium for (distributed) real-

time system• Data protocol specific• Temporally predictable, at least time bounded,

transmission• Synchronized (sec)• Membership info (who is alive)• Fault tolerant

– more than one physical network– no single point of failure


Fieldbus

• connects sensors (actuators) with RTS (possibly via dedicated gateway nodes)

• needs to collect data and time of observation

• latency: time passed since event occurrence


CAN

control area networkvery popular, used, e.g., by automotive industry (Volvo)– sender intends to send - puts bit on

channel– collisions resolved by priorities


TDMA - TTP

time division multiple access - time triggered protocol– network time divided into slots– static assignment of slots to nodes– implicit flow control


Summary






Documents

IT-606 Embedded Systems (Software)