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Real Time Operating System

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Comparison with conventional

OSGPOSGeneral purpose OS

Too costly for embedded

applications

Increased demand onRT functionality

Windows NT, 2K,

XP,

Solaris, IBM AIX, HP-

UX Linux

Etc

RTOSRealtime OS

Embedded applicationsIndustrial robots, spacecraft,industrial control, flight control,and scientific research equipmentHigh degree of configurability andextensibility required

Linux? RT Linux VxWorks Windows CE QNX LynxOS RTEMS OS-9

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Comparison with conventional

OSRT systems require specific support from OSConventional OS kernels are inadequate w.r.t. RTrequirements

Multitasking/scheduling provided through system calls does not take time into account (introduce unboundeddelays)

Interrupt management achieved by setting interrupt priority > than process priority increase system reactivity but may cause unboundeddelays on process execution even due to unimportant interrupts

Basic IPC and synchronization primitives may causepriority inversion (high priority task blocked by alow priority task)

No concept of RT clock/deadline

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Goal: Minimal Response Time

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Real-Time Operating Systems

Definition Systems whose correctness depends on theirtemporal aspects as well as their functionalaspects

A system is said to be Real Time if it isrequired to complete its work & deliver itsservices on time.

Performance measure

Timeliness on timing constraints (deadlines) Speed/average case performance are lesssignificant.

Key property Predictability on timing constraints

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Features of Real-Time Operating

Systems

Timeliness OS has to provide mechanisms for

time management handling tasks with explicit time constraints

Achieved through properscheduling

algorithms Core of an RTOS! Predictability

to guarantee in advance the deadlinesatisfaction

to notify when deadline cannot be guaranteed Affected by several issues such as

characteristics of the processor (pipelinig,cache, DMA, ...), I/O & interrupts, Synchronization &IPC, Architecture, Memory management, Applicationsand Scheduling!

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Features of RTOS: Services

SchedulingResource AllocationInterrupt handling: Interrupt Latency should be verysmall

Kernel has to respond to real time events Interrupts should be disabled for minimum possible

timeFor embedded applications Kernel Size should besmall

Should fit in ROMSophisticated features can be removed

No Virtual Memory No Protection

Fault tolerance HW/SW failures must not cause a crash Design for peak load

All scenarios must be considered

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Characteristics of Real Time

Operating Systems

Large and complex OR small and embedded Vary from a few hundred lines of assembler or Cto millions of lines of lines of high-level language code Concurrent control of separate system

components Devices operate in parallel in the real-world, hence,better to model this parallelism by concurrent entities in theprogram

Facilities to interact with special purpose

hardware Need to be able to program devices in a reliableand abstract way

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Characteristics of Real Time

Operating SystemsExtreme reliability and safety

Embedded systems typically control the environment inwhich they operate

Failure to control can result in loss of life, damage toenvironment or economic loss

Guaranteed response times We need to be able to predict with confidence the worst caseresponse times for systems

Efficiency is important but predictability is essential In RTS, performance guarantees are:

Task- and/or class centric Often ensured a priori

In conventional systems, performance is: System oriented and often throughput oriented Post-processing ( wait and see )

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Structure of Real time systems

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Tasks in RTOS (2)

Task : a sequence of similar jobs Periodic task (p,e)

Its jobs repeat regularly

Periodp = inter-release time (0

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Performance measures, estimating

program runtimes

Completion ratio / miss ration

Maximize total usefulness value

(weighted sum)

Maximize value of a taskMinimize lateness

Minimize error (imprecise tasks)

Feasibility (all tasks meet their deadlines)

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Performance metric: Response

Time

Response time Duration from released time to finish time

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(4,1)

(5,2)

(10,2)

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5

10

10 15

15

T1

T2

T3

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Performance metric: Response

Time

Response time Duration from released time to finish time

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(4,1)

(5,2)

(10,2)

Response Time of T3

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5

10

10 15

15

T1

T2

T3

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Task assignment

Cyclic executive schedulingCooperative scheduling

scheduler relies on the current process to give upthe CPU before it can start the execution of anotherprocess

A static priority-driven schedulercan preemptthe current process to start a new process.Priorities are set pre-execution

E.g., Rate-monotonic scheduling (RMS), DeadlineMonotonic scheduling (DM)

A dynamic priority-driven scheduler can assign,and possibly also redefine, process priorities atrun-time.

E.g., Earliest Deadline First (EDF), Least LaxityFirst (LLF)

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Scheduling in RTOS : Deadlines

Hard deadline Disastrous or very serious consequences may occur ifthe deadline is missed Validation is essential : can all the deadlines be met,even under worst-case scenario?

Deterministic guarantees

Soft deadline Ideally, the deadline should be met for maximumperformance. The performance degrades in case ofdeadline misses.

Best effort approaches Statistical guarantees

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Schedulability

Property indicating whether a real-time

system (a set of real-time tasks) can meet

their deadlines

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(4,1)

(5,2)

(7,2)

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Real-Time Scheduling

Determines the order of real-time task

executions

Static-priority scheduling

Dynamic-priority scheduling

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(4,1)

(5,2)

(7,2)

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5

10

10 15

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RM (Rate Monotonic)

Optimal static-priority scheduling

It assigns priority according to period

A task with a shorter period has a higher priority

Executes a job with the shortest period

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(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3

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RM (Rate Monotonic)

Executes a job with the shortest period

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(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3

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RM (Rate Monotonic)

Executes a job with the shortest period

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(4,1)

(5,2)

(7,2)

Deadline Miss !

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5

10

10 15

15

T1

T2

T3

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EDF (Earliest Deadline First)

Optimal dynamic priority scheduling

A task with a shorter deadline has a higher

priority

Executes a job with the earliest deadline

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(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3

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EDF (Earliest Deadline First)

Executes a job with the earliest deadline

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(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3

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EDF (Earliest Deadline First)

Executes a job with the earliest deadline

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(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3

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EDF (Earliest Deadline First)

Executes a job with the earliest deadline

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(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3

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EDF (Earliest Deadline First)

Optimal scheduling algorithm if there is a schedule for a set of real-time

tasks,

EDF can schedule it.

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(4,1)

(5,2)

(7,2)

5

5

10

10 15

15

T1

T2

T3

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EDF Overload Conditions

Domino effect during overload conditions Example: T1(4,3), T2(5,3), T3(6,3), T4(7,3)

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T1

50 7

T2 T3 T4

3 6

Deadline Miss !

T1

50 7

T3

3 6

Better schedules :

T1

50 7

T4

3 6

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RM vs. EDF

Rate Monotonic Simpler implementation, even in systemswithout explicit support for timing constraints(periods, deadlines)

Predictability for the highest priority tasks

EDF Full processor utilization

Misbehavior during overload conditions

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Priority(P1) > Priority (P2) P1, P2 share a critical section (CS) P1 must wait until P2 exits CS even if P(P1) > P(P2) Maximum blocking time equals the time needed by P2 to execute its CS

It is a direct consequence of mutual exclusionIn general the blocking time cannot be bounded by CS of the lower priorityprocess

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Priority inversion (2)

Typical characterization of priority inversion A medium-priority task preempts a lower-priority task which

is using a shared resource on which a higher priority task is

blocked

If the higher-priority task would be otherwise ready to run,

but a medium-priority task is currently running instead, a priority

inversion is said to occur

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Priority Inheritance

Basic protocol [Sha 1990]1.A job Juses its assigned priority, unless it is in its CS and blockshigher priority jobs In which case, Jinherits PH, the highest priorityof the jobs blocked by JWhen Jexits the CS, it resumes thepriority it had at the point of entry into the CS2.Priority inheritance is transitive

AdvantageTransparent to schedulerDisadvantageDeadlock possible in the case of bad use of semaphoresChained blocking: if P accesses n resources locked by processeswith lower priorities, P must wait for n CS

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Priority Inheritance (2)

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Priority Inheritance (3)

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Deadlocks

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Priority Inheritance (4): ChainedBlocking

A weakness of the priority inheritance protocol is that it does not preventchained blocking.Suppose a medium priority thread attempts to take a mutex owned by alow priority thread, but while the low priority thread's priority is elevated tomedium by priority inheritance, a high priority thread becomes runnable andattempts to take another mutex already owned by the medium prioritythread. The medium priority thread's priority is increased to high, but the

high priority thread now must wait for both the low priority thread and themedium priority thread to complete before it can run again.The chain of blocking critical sections can extend to include the criticalsections of any threads that might access the same mutex. Not only doesthis make it much more difficult for the system designer to computeoverhead, but since the system designer must compute the worst caseoverhead, the chained blocking phenomenon may result in a much less

efficient system.These blocking factors are added into the computation time for tasks in theRMA analysis, potentially rendering the system unschedulable.

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Priority Ceiling

In priority ceiling protocol, each resource is assigned apriority ceiling, which is a priority equal to the highestpriority of any task which may lock the resource.A task T is allowed to enter a critical section only if itsassigned priority is higher than the priority ceilings of all

semaphores currently locked by tasks other than T.Task T runs at its assigned priority unless it is in acritical section and blocks higher priority tasks.When a task exits the critical section it resumes thepriority it had at the point of entry into the critical section.Prevents Deadlocks and Chained Blocking

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Priority Ceiling (2)

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p0>p1>p2

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Intertask Communication &

Synchronization

These services makes it possible topass information from one task to

another without information ever beingdamaged.

Makes it possible for tasks tocoordinate & productively cooperatewith each other.

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Inter-Task communication &

Synchronization

The most important communication b/w tasks in an OS isthe passing of data from one task to another.

If messages are sent more quickly than they can behandled, the OS provides message queues for holding themessages until they can be processed.

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Message

Producer Task

Message

Receiver Task

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Message passing in RTOS

In RTOS, the OS copies a pointer to the message,delivers the pointer to the message-receiver task,and then deletes the copy of the pointer withmessage-sender task.

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Message Sender

Task

RAM

Message Receiver

TaskRTOS

msg_ptr

msg_ptr

Message Message

Classification (based on

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Classification (based onarchitectures) and applications of

various RTOSRTOS for small footprint, mobile and connecteddevices The real-time features ofWindows CE 3.0, a high lymodular real-time embedded OS for small footprint, mobile32-bit intelligent connected devices.

RTOS for complex, hard real-time applications LynxOS , a POSIX-compatible, multiprocess,multithreaded OS designed for complex real-timeapplications that require fast and deterministic response.

General purpose RTOS in the embedded industry VxWorks, the most widely adopted RTOS in the

embedded industry. VxWorks is a flexible, scalable RTOSwith over 1800 APIs and is available on all popular CPUplatforms.

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Classification (based on

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Classification (based onarchitectures) and applications of

various RTOSRTOS for the Java Platform The Jbed RTOS package is a real-time capable virtualmachine developed for embedded systems and Internetapplications under the Java platform.

Objected-oriented RTOS

pSOSystem is a modular object-oriented operatingsystem. The objects in pSOS include tasks, memoryregions, message queues, and semaphores.

Real-time features in general purpose OS Adding real-time features to common general purposeoperating systems viz., Windows NT and UNIX.

Research real-time kernels: Real-time kernels such as Spring and Arx are used forongoing research on RTOS