RTOS

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Contents

Real Time System Fundamentals

Real Time Operating System (RTOS)

Linux as RTOS

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What are Real-time Systems ?

Real-time systems

Those systems in which the correctness of the system depends not only on the logical result of computation, but also on the time at which the results are produced.


Types of Real Time Systems

Based on Hard deadline: Penalty due to missing deadline is a higher order of magnitude than the Reward in meeting the deadline.

Based on Soft deadline: Penalty often equal / lesser magnitude than Reward.


A Sample Real Time System (1)



Mission: Reaching the destination safely.

Controlled System: Car.

Operating environment: Road conditions.

Controlling System - Human driver: Sensors - Eyes and Ears of the driver. - Computer: Sensors - Cameras, Infrared receiver, and Laser telemeter.


A Sample Real Time System (3)Controls: Accelerator, Steering wheel, Break-pedal.

Actuators: Wheels, Engines, and Brakes.

Critical tasks: Steering and breaking

Non-critical tasks: Turning on radio



Performance is not an absolute one. It measures the goodness of the outcome relative to the best outcome possible under a given circumstance.

Cost of fulfilling the mission → Efficient solution.

Reliability of the driver → Fault-tolerance is a must.


Contents



Linux as RTOS


RTOS KernelRTOS Kernel provides an Abstraction layer that hides from application software the hardware details of the processor / set of processors upon which the application software shall run.


Application Software

RTOS

Hardware

RTOS Kernel Functions


Task Management

Set of services used to allow application software developers to design their software as a number of separate chunks of software each handling a distinct topic, a distinct goal, and sometimes its own real-time deadline.

Main service offered is Task Scheduling controls the execution of application software

tasks can make them run in a very timely and

responsive fashion.


Task SchedulingNon Real -time systems usually use Non-preemptive Scheduling Once a task starts executing, it completes its full

execution

Most RTOS perform priority-based preemptive task scheduling.

Basic rules for priority based preemptive task scheduling The Highest Priority Task that is Ready to Run, will

be the Task that Must be Running.13

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Priority based Preemptive Task Scheduling

Every Task in a software application is assigned a priority.

Higher Priority = Higher Need for Quick Response.

Follows nested preemption


Nested Preemption


Timeline for Priority-based Preemptive Scheduling

Task Switch (1)

Each time the priority-based preemptive scheduler is alerted by an External world trigger / Software trigger it shall go through the following steps that constitute a Task Switch:

Determine whether the currently running task

should continue to run. Determine which task should run next. Save the environment of the task that was

stopped (so it can continue later). Set up the running environment of the task that

will run next. Allow the selected task to run.


Task Switch (2)

A Non Real time operating system might do task switching only at timer tick times.

Even with preemptive schedulers a large array of tasks is searched before a task switch.

A Real time OS shall use Incrementally arranged tables to save on time.


Task Switch (3)

Task Switching Timing


Intertask Communication & Synchronization

These services makes it possible to pass information from one task to another without information ever being damaged.

Makes it possible for tasks to coordinate & productively cooperate with each other.


Inter-Task communication & Synchronization

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

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


MessageProducer Task

Message Receiver Task

Message passing in OSMost General Purpose OS actually copy messages twice as they transfer them from task to task via a message queue.

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Message Sender Task RAM

Message

RAMMessage Receiver

Task

Message

Message passing in RTOSIn 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 with message-sender task.


Message Sender Task

RAM

Message Receiver Task

RTOSmsg_ptr

msg_ptr

Message Message

Dynamic Memory Allocation in General Purpose OS

Non-real-time operating systems offer memory allocation services from what is termed a Heap.

Heaps suffer from a phenomenon called External Memory Fragmentation.

Fragmentation problem is solved by Garbage collection / Defragmentation.

Garbage collection algorithms are often wildly non-deterministic.


Dynamic Memory Allocation in RTOS

RTOS does it by a mechanism known as Pools.

Pools memory allocation mechanism allows application software to allocate chunks of memory of 4 to 8 different buffer sizes per pool.

Pools avoid external memory fragmentation, by not permitting a buffer that is returned to the pool to be broken into smaller buffers in the future.

When a buffer is returned the pool, it is put onto a free buffer list of buffers of its own size that are available for future re-use at their original buffer size 24

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Contents



Linux as RTOS


Dark Side of Linux in Real-Time

The Linux kernel (< version 2.6) is neither preemptive nor reentrant by user processes.

Processes are heavy-weight: Linux has insufficient resource handling.

Fair share scheduling algorithm .

Unbound amount of CPU time used by interrupt handlers under Linux, and these run at a higher priority than any user process.


Typical Solutions for Linux

3 ways to deal with Linux’ s poor real-time performance :

Ignore the problem (only for soft real-time).

Work around and run real-time applications under an RTOS with Linux itself as a separate task. e.g. RT-Linux.

Adapt Linux and run a Linux compatible RTOS kernel instead of the Linux kernel.


Ignore the problem Approach

The most popular approach is to ignore the problem.

Mostly works for soft real-time systems.

Design recommendations to reduce latency If possible, address all real-time response

needs directly with interrupt service routines. Avoid known excessive interrupt-off periods in

Linux. If a process component is required in the real-

time control path, then consider aggregate system loading & Adapt processes.


Work Around Approach

Linux kernel runs as a task under a small real-time executive.

Real-time tasks are run directly under the real-time executive

Non-real-time tasks are run under Linux.

Applications that work on such a system can include machine control, process control, and instrumentation applications.


Work Around - Advantages

The real-time executive can be small and simple making it easier to verify its real-time performance.

The Linux kernel running non-real-time tasks is "standard" Linux so it is compatible with other Linux distributions and can easily be updated.

Supports hard real-time applications.


Work Around - Disadvantages

Not applicable if real-time part of the software is large / not easily separated from the code that needs a Linux environment

Tasks running under the real-time executive do not have access to the Linux facilities, device drivers, etc.

Tasks running under the Linux kernel and can access the facilities are not real-time.

Programmers tend to increase the number of facilities in the real-time kernel until the real-time executive replicates facilities of Linux.


Adapt LinuxThe original Linux kernel is replaced with a compatible kernel with hard real-time performance characteristics.

The kernel is only a small part of a whole operating system. If just the Linux kernel is replaced and the libraries, utilities, and file structure remain the same.

The real-time kernel must support all the facilities of a Linux kernel while still remaining fully preemptive and reentrant.


Adapt Linux Approach Advantages & Disadvantages

Advantages: No limits on the size and complexity of the

real-time application code. Third party software not originally envisioned

for real-time use can be invoked by real-time tasks

With the same programming interfaces, programmers who understand Linux would already be experts in the real-time environment.

Disadvantages: With modified Linux you are NOT sure of full

compatibility with standard Linux application


Any Questions?

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