Real Time Systems & RTOS

Concepts on Real Time Systems & RTOS By Ch. Vishwa Mohan Software Consultant & Trainer

Who am I anyway?
Name : Ch.Vishwa Mohan
Qualification : M.E ( Inst ) passed in 1994.
Experience : Over 13+ Years of Software development experience specially in Embedded/ Real Time systems in Industrial Automation, Process Control,
Semiconductor related areas.

What is an Real Time System ?
A Real time system is one, Correctness of the computations not only depends on logical correctness but also the time at which the result is produced. If the timing constrains are not meet, system failure is said to have occurred.
Computer systems those operates in an environment in which they are required to response to external events, not only in a functionally correct way, but also in a correct timely way are labeled real-time systems.
Real Time systems are one in which timelines, performance, and schedulability are essential to correctness.
Real-time systems are generally reactive in nature, usually responding to external events via interrupts.

What is an Real Time System ?
A real-time system is any information processing system which has to respond to externally generated input stimuli within a finite and specified period
The correctness depends not only on the logical result but also the time it was delivered.
Failure to respond in time is as bad as the wrong response!
The computer is a component in a larger engineering system => EMBEDDED COMPUTER SYSTEM
99% of all processors are for the embedded systems market

A simple fluid control system Pipe Flow meter Valve Interface Computer Time Input flow reading Processing Output valve angle

A Widget-Packing Station Line controller Computer Switch Assembly line Box 0 = stop 1 = run Bell Switch

A Typical Embedded System Algorithms for Digital Control Data Logging Data Retrieval and Display Operator Interface Interface Engineering System Remote Monitoring System Real-Time Clock Database Operator’s Console Display Devices Real-Time Computer

Characteristics of Real Time Systems
Large and complex — vary from a few hundred lines of assembler or C to 20 million lines of Ada estimated for the Space Station Freedom
Concurrent control of separate system components* — devices operate in parallel in the real-world; better to model this parallelism by concurrent entities in the program
Facilities to interact with special purpose hardware — need to be able to program devices in a reliable and abstract way
Extreme reliability and safe — embedded systems typically control the environment in which they operate; failure to control can result in loss of life, damage to environment or economic loss
Guaranteed response times — we need to be able to predict with confidence the worst case response times for systems; efficiency is important but predictability is essential.

Concurrent Programming
The name given to programming notation and techniques for expressing potential parallelism and solving the resulting synchronization and communication problems
Virtually all real-time systems are inherently concurrent. ( Devices operate in parallel in the real world ).
Implementation of parallelism is a topic in computer systems (hardware and software) that is essentially independent of concurrent programming.
To allow the expression of potential parallelism so that more than one computer can be used to solve the problem.

Why we need Concurrent Programming ?
To fully utilise the processor
Response time in seconds 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 2 10 1 10 -8 10 -9 10 0 human tape floppy CD memory processor

Parallelism Between CPU and I/O Devices CPU Initiate I/O Operation Interrupt I/O Routine I/O Finished I/O Device Process I/O Request Signal Completion Continue with Outstanding Requests

Airline Reservation System VDU VDU VDU VDU P P P P Process Database

Air Traffic Control

Terminology
A concurrent program is a collection of autonomous sequential processes, executing (logically) in parallel
The actual implementation (i.e. execution) of a collection of processes usually takes one of three forms.
Multiprogramming
Processes multiplex their executions on a single processor
Multiprocessing
Processes multiplex their executions on a multiprocessor system where there is access to shared memory
Distributed Processing
Processes multiplex their executions on several processors which do not share memory

Reliability, Failure and Faults
The reliability of a system is a measure of the success with which it conforms to some authoritative specification of its behaviour.
When the behaviour of a system deviates from that which is specified for it, this is called a failure
Failures result from unexpected problems internal to the system which eventually manifest themselves in the system's external behaviour
These problems are called errors and their mechanical or algorithmic cause are termed faults
Systems are composed of components which are themselves systems: hence
> failure -> fault -> error -> failure -> fault

Real Time S/W design include of:
Identification of Real time tasks.
Co-ordination between real time tasks.
Processing of system interrupts.
I/O handling to ensure no data loss.
Specifying internal/external constrains of system.
Ensuring the accuracy of data.

Topics interest in RT System Includes:
Responding to external events
Priorities and scheduling
Synchronization requirements
Deterministic response times.

Metrics of Real Time Systems Include:
Predictability fast responses to urgent events: The device should perform its intended function in a predictable manner, while supporting other functions.
High degree of schedulability: The timing requirements of the system must be satisfied at high degrees of utilization.
Stability under transient overload: When the system is overloaded by events and it is impossible to meet all the deadlines, the deadlines of selected critical tasks must still be guaranteed.

Problems for Real Time Software Developers:
Real-time software developers have all the problems of non-real time developers plus several more:
Timeliness:
This timeliness is the key difference in real time systems. Many people thing of real time system as really-fast, but it is a misconception.
In real-time systems, the right answer at the wrong time is the wrong answer.
Designers can distinguish hard, soft and firm real time systems*.
Low Resource Availability ( Memory, CPU speed, etc.,)
Reliability Issues: Should be more reliable, No Ctrl + Alt + Del
Safety Issues: Need to use Dual memory storage, CRC, Watch dog timer, etc.,
Compiler and Tool Availability:

Hard, Soft & Firm Real Time Systems:
Hard Real Time Systems: In Hard real time systems the processing deadlines are missed, the system had said to be failed.
(Eg: Missile may not attack the target. Aircraft may crash, Nuclear Power plant may explode).
Soft Real Time Systems: In soft real time systems must meet the deadline but only in the average case.
(Eg: Screen switching at 250 m sec, Maintain and update of flight plans in commercial airlines, Live audio/Video systems)
A soft real-time operating system is one that has reduced constraints on 'lateness,' but still must operate quickly within fairly consistent time constraints."
Firm Real Time Systems: These systems that are primarily soft, but do have a hard deadline as well.

It is important to note that hard versus soft real-time does not necessarily relate to the length of time available.
A machine may overheat if a processor does not turn on cooling within 15 minutes . Still it is a Hard real time system.
A network interface card may lose buffered data if it is not read within a fraction of a second , but the data can be resent over the network without major adverse consequences. It is a Soft Real time system.

Hard real-time — systems where it is absolutely imperative that responses occur within the required deadline. E.g. Flight control systems.
Soft real-time — systems where deadlines are important but which will still function correctly if deadlines are occasionally missed. E.g. Data acquisition system.
Real real-time — systems which are hard real-time and which the response times are very short. E.g. Missile guidance system.
Firm real-time — systems which are soft real-time but in which there is no benefit from late delivery of service.
A single system may have all hard, soft and real real-time subsystems, In reality many systems will have a cost function associated with missing each deadline.

RTOS
( Real Time Operating Systems )

RTOS ( Real Time Operating System )
What is an RTOS?
RTOS is the brain of the Real Time System.
Respond to predictable way to unpredictable events.
Strict timing constraints.
Which would ensure fast interrupt response and real time scheduling.
Primary role of an OS is to manage resources so as to meet the demand of target applications. In addition to this Real time applications demands Timelines and Predictability . The OS targeting real time applications meet the above demands by paying special attention to a host of OS features Like:
Multitasking & Synchronization
Interrupt and Event Handling
Inter Task Communication
Timer & Clocks
Input/Output
Memory Management.
Modern real time systems are based on complementary concept of multitasking and Inter task communication.

Different RTOS’s available :
VxWorks
QNX
pSOS
RT Linux
eCOS
Win CE (& Win CE.NET)
 C/OS-II
etc.,

Real-time Programming Languages
Assembly languages
Sequential systems implementation languages — e.g. RTL/2, C.
Both normally require operating system support.
High-level concurrent languages. Impetus from the software crisis. e.g. Ada, Chill, Modula-2, Java.
No operating system support! We will consider:
Java/Real-Time Java
C and Real-Time POSIX
Ada 95
Also Modula-1 for device driving

RTOS Characteristics:
The OS must be multithreaded and preemptive*.
The notion of thread priority must exists.
A system of Priority inheritance* must be exists.
The OS must support predictable thread synchronization mechanism.
Mechanism to avoid Priory Inversion*.
Sufficient Priority levels.
The interrupt latency must be predictable.
In addition to all the above, OS behavior must be predictable,
Contd…,

RTOS Characteristics:
In addition to previous characteristics, Real time system developers must have detailed information about the system interrupt levels, system calls and timings.
The maximum time during which interrupt are masked by the OS and device drivers must be known.
The maximum time the device drivers use to process an interrupt and specific IRQ information relating to these device drivers must be known.
The interrupt latency* ( time from interrupt occur to interrupt task run) must be predictable and compatible with application requirements.
The time for every system call should be predictable and must be independent of no of objects present in the system.

Different Types of Multitasking Systems:
Preemptive Multitasking system
The task can be stopped by its execution whenever higher priority task is ready to execute called preemptive multi tasking .
Non-Preemptive Multitasking System
Co-operative Multitasking System

Priority Inheritance
Priority Inversion: It refers to a situation where the use of a resource by a low-priority thread delays the execution of a high-priority thread, when both are contending for the same resources. To address this you have following solutions:
Priority Inheritance Protocol: It temporarily raises the priority of the low-priority thread to match that of the blocked thread until low priority thread releases the resource.
Priority Ceiling Protocol: Here the priority of the low-priority thread is raised immediately when it lock the resource, rather then waiting for a sub sequent lock attempt by a high priority thread.

Deadlocks and Race Conditions
Deadlocks: A deadlock occurs when each thread is waiting for the other to complete an action.
Race Condition: A race condition occurs when the threads are sharing a value and the final result of the value depends on the scheduling order of the thread.

Definition on Latencies & Delays:
Interrupt Latency: The amount of time that elapses from the time that an external interrupt arrives at the processor until the time that the interrupt processing* begins.
Context Switch Delay: The amount of time required to scheduling a task. It involves scheduler determines which task to run saving the current task context and restoring the new task.
ISR Latency: Time from the point when an IRQ is set at the CPU to the point when the ISR begins to run.
IST Latency: IST latency is the period from the point when an ISR finishes execution (signals a thread) to the point when the IST begins execution.

Interrupt Latencies:
In a typical RTOS these ISR and IST latencies are measured in single digit micro seconds.

Interrupt & Context Switch Latency:

Steps Involved in Interrupt Processing:
Suspends the active thread.
Saves thread-related data that will be needed when the thread is resumed.
Transfers control to an ISR.
Performs some processing in the ISR to determine what action is needed.
Retrieves and saves any critical (incoming) data associated with the interrupt.
Sets any required device-specific (output) values.
Determines which thread should execute given the change in environment created by the interrupt and its processing.
Clears the interrupt hardware to allow the next interrupt to be recognized.
Transfers control to the selected thread, including retrieval of its environmental data that was saved when it was last interrupted.

Considerations Selecting RTOS
Real Time Capabilities
( Hard real time interrupt handling and Premptive scheduling services).
Foot Print
Target CPU
Interrupt Latencies & Context Switch times.
Available API’s
BSP’s
Development Environment and Debugging Tools
Licensing

Considerations Selecting RTOS
Flexible Choice of Scheduling Algorithms ( Eg: FIFO, Round Rabin, Rate-Monotonic, EDF, POSIX 1.b Scheduler, Sporadic, etc., )
Guaranteed CPU availability : ( RTOS supporting Partitioning Scheduler, etc., ).
GUI Support: ( Eg: Win CE)
Tools for remote diagnostics:
Internet capabilities:
Standard APIs:
Source code and Driver Development Kit support:

Factors Selecting Commercial Operating Systems:

Thank you ! You can reach me at:

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Real Time Systems & RTOS

by Vishwa Mohan on Mar 27, 2011

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