Real-Time Operating Systems

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Real-Time Operating Systems

Praveen Varma

www.carrertime.in

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Contents

– Introduction– Characteristic of RTOS– Real-Time task scheduling

• Clock-driven• Event-driven

– Scheduling of real-time task on a uniprocessor• Rate Monotonic Analysis (RMA)• Earliest Deadline First (EDF)• Scheduling with limited priority levels

– Features of RTOS– Commercial real-time operating systems

• RT Linux, PSOS, VRTX, WinCE

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Real Time System

• A system is said to be Real Time if it is required to complete it’s work & deliver it’s services on time.

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Types of RTS:

• Hard real-time System: – Breaking the limit is always seen as a fundamental

failure– Nuclear Power Plant Controller– Flight Control System

• Soft real-time System:– Breaking the time limit is unwanted, but is not

immediately critical– web sites and services– Satellite-based applications

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• Firm Real-Time Systems– If a deadline is missed occasionally, the system does

not fail– The results produced by a task after the deadline are

rejected– Video Conferencing

– Satellite Based Tracking

of enemy movementUtility

D Time

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Real Time OS

Hardware

Standard OS

RT Applications

RT extension

Applications

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Characteristic of RTOS

• Time constraints• Correctness criterion (not only logical but also

time)• Safety-Critically (safety + reliability)

• Task Criticality (cost of failure of task)

• Custom Hardware• Reactive• Stability• Exception Handling

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OS Real-Time Extensions

• Extension of an OS by real-time components

• Cooperation between RT- and non-RT parts

• Advantages: rich functionality

• Disadvantage: – Computing and memory resources

• Example: RT-Linux, Solaris, Windows NT

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Components of a RTOS

• Process (task) management– Scheduler– Synchronization mechanism

• Interprocess communication (IPC)• Semaphores

• Memory management• Interrupt service mechanism• I/O management• Hardware abstraction layer• Development Environments• Board Support Packages (BSP)

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Real-Time Task Scheduling

• Scheduling of tasks is the primary means by which the operating system meets task deadlines.

• So, scheduling is an important problem.• Lot of work has been done in developing

appropriate schedulers for real-time tasks– Scheduling on uniprocessors– Scheduling on multiprocessors and distributed

systems

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Real Time Task Scheduling

1.Clock Driven:- Table-driven & Cyclic

2. Event Driven:- Simple Priority Based

- Rate Monotonic Analysis (RMA)

- Earliest Deadline First (EDF)

3. Hybrid:- Round-robin

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Clock-driven Scheduling

• Table-driven scheduling• Decision regarding which job to run next is made

at specific time instants– Timers are used to trigger the decision point– The job-list along with information regarding

which task to be run for how long are stored in a table

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Clock-driven Scheduling

Task Start time Stop timeT1 100T2 101 150T3 151 225T4 226 300

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Clock-Driven Scheduling

• The scheduler develops a permanent schedule for a period (P1,P2,...,Pn) and stores in a table.

• Round robin scheduling is an example of clock-driven scheduling

• Clock-driven schedulers are – Simple: used in low cost applications

– Efficient: very little runtime overhead

– Inflexible: Very difficult to accommodate dynamically changing task set or task characteristics.

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Priority-based Schedulers

• These are also called event-driven schedulers– Scheduling decisions are made when certain events

occur• Tasks becoming ready• Tasks completing execution

• These are called preemptive schedulers– When a higher priority task becomes ready it

preempts the executing lower priority task

• These are greedy schedulers: – They never keep the processor idle if a task is ready.

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Priority-based Schedulers

• Static priority schedulers:– The task priorities once assigned by the

programmers, do not change during runtime– RMA (Rate Monotonic Algorithm) is the optimal static

priority scheduling algorithm

• Dynamic priority– The task priorities can change during runtime based

on the relative urgency of completion of tasks

– EDF (Earliest Deadline First) is the optimal uniprocessor scheduling algorithm

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Priority-based Schedulers

• First let us consider the simplest scenario:– Uni-processor– Independent tasks

• Tasks do not share resources • There is no precedence ordering among the tasks

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Priority-based Scheduling

• Independent tasks executed on a uniprocessor – Two algorithms pretty much summarise the

important results in this scenario

• EDF (Earliest Deadline First)• RMA (Rate Monotonic Analysis )

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EDF

• EDF is the optimal uniprocessor scheduling algorithm– If EDF cannot feasibly schedule a set of tasks, there

exists no other scheduling algorithms to do that.

• Can schedule both periodic and aperiodic tasks• Schedulability check:

– Sum of utilization due to tasks is less than one.

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EDF

• EDF is – Simple– Optimal

• But, is rarely used– No commercially available operating system

directly supports EDF scheduling– Let us examine the disadvantages of EDF

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Disadvantages of EDF

• Transient overload handling– EDF has very poor, overload handling

capability– When a low-criticality task becomes delayed it

can make even the most critical task miss its deadline

– In fact, it is extremely difficult to predict which task would miss its deadline when a task takes more time

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Disadvantages of EDF

• Runtime efficiency– EDF is not very efficient– Inorder to implement EDF the tasks need to

be maintained sorted in a priority queue based on their deadline

– The complexity of maintaining a priority queue is (log n), where n is the number of tasks

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Rate Monotonic Algorithm

• The priority of a task is proportional to its rate of occurrence.

– The higher is the rate (or lower is the period) of a task, the higher is its priority.

Rate

Priority

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RMA

• RMA has been shown to be the optimal uniprocessor static priority scheduling algorithm

– If RMA cannot schedule a set of periodic tasks, no other scheduling algorithm can.

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Hybrid Schedulers

• Time-Sliced Round-Robin Scheduling:- Are very commonly used in traditional OS.

- Is a preemptive scheduling method.

- Here ready tasks are held in a circular queue.

- once a task is taken from queue then it runs for a time slice & if does not complete then it inserted back in to the queue.

- Here all tasks are equal with identical time slice.

It can be extended by putting larger time slice to the higher priority tasks.

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Resource sharing

• So far, the only resource that we considered is CPU.

• However, tasks may need to share resources such as files, memory, data structures.– These are nonpreemptable resources– Called critical sections in the operating

systems literature

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Critical Sections

• The traditional operating system solution to share critical sections– Is through the use of semaphores.

• However, in real-time systems this solution does not work well, it gives rise to:– Priority inversion– Unbounded priority inversion

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Features of RTOS

• Clock and Timer Support- most important issue- hard real time application support timer service with resolution of few microseconds.

• Real-Time Priority Levels- it must support static (or real-time) priority level.- where traditional OS dynamically changes the

priority levels of tasks to maximize the system throughput.

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Features of RTOS

• Fast Task Preemption– When a high priority task arrives, an low priority task

should preempt.– the waiting time of the high priority task to start

execution is expressed as task preemption time.

• Predictable and Fast Interrupt Latency- interrupt latency is the time delay between the occurrence of an interrupt and the running of the corresponding Interrupt Service Routing (ISR).- upper bound on interrupt latency must be bounded

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Features of RTOS

• Support for Resource sharing Among Real Time Tasks

- the resource to be shared between the real time task using Priority Ceiling Protocol (PCP)

• Requirements on Memory Management

- real time OS requires to provide virtual memory support for heavy real time tasks.

- embedded real time OS usually don't support virtual memory

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Features of RTOS

• Support for Asynchronous I/O- asynchronous I/O means non-blocking I/O.

- where as traditional read() or write() system call performs synchronous I/O.

- in asynchronous I/O, system call will return immediately once the I/O request has been passed down to the hardware or queued in the OS, typically before the physical I/O

operation has even begun.

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Commercial Real-Time Operating Systems

• Criteria for comparing RTOS:– Scheduling policy supported– Memory locking and other support– Timers– Interrupt handling– File system support– Device interfacing

• Embedded systems have small memories– The operating system size is important– Obviously, OS with large footprint cannot be used in

embedded applications– Power saving features are desirable

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Other RTOS issues

• Interrupt Latency should be very small– Kernel has to respond to real time events– Interrupts should be disabled for minimum

possible time

• For embedded applications Kernel Size should be small– Should fit in ROM

• Sophisticated features can be removed– No Virtual Memory

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Linux for Real Time Applications

• Scheduling– Priority Driven Approach

• Optimize average case response time

– Interactive Processes Given Highest Priority• Aim to reduce response times of processes

– Real Time Processes• Processes with high priority

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RT Linux

• RT kernel intercepts and attends to all interrupts– If an interrupt is to cause a RT task to run, the

RT kernel preempts Linux (if Linux is running that time) and lets the RT task run

Hard-ware

RT Kernel

Linux

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References:

1. Real-Time Systems Theory and Practice Rajib Mall

2. Real-Time Systems, Jane W.S. Liu

3. Real-Time Operating System, Frank Kolnick

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questions

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