Real time Operating System
- 1. REAL TIME OPERATING SYSTEM
- 2. TOPICS OF DISCUSSION. • WHAT IS RTOS. • COMPARISON BETWEEN RTOS AND GENERAL OPERATING SYSTEMS. • TYPES OF RTOS. • CHARACTERISTICS OF RTOS. • FUNCTIONS OF RTOS. • APPLICATIONS OF RTOS. • EXAMPLE OF SOME RTOS • CONCLUSION.
- 3. What is Real Time ? • “ Real time in operating systems: The ability of the operating system to provide a required level of service in a bounded response time.” - POSIX Standard 1003.1
- 4. WHAT IS RTOS. • It responds to inputs immediately(Real-Time). • Here the task is completed within a specified time delay. • In real life situations like controlling traffic signal or a nuclear reactor or an aircraft, • The operating system has to respond quickly.
- 5. What a RTOS is not • Real time computing is equivalent to fast computing. • Real time systems operate in a static environment • Real time programming involves assembly coding, priority interrupt programming, writing device drivers.
- 6. Soft RTOS… • In a soft real-time system, it is considered undesirable, but not catastrophic, if deadlines are occasionally missed. • Also known as “best effort” systems • Most modern operating systems can serve as the base for a soft real time systems. • Examples: – multimedia transmission and reception, – networking, telecom (cellular) networks, – web sites and services – computer games.
- 7. Hard RTOS… • A hard real-time system has time-critical deadlines that must be met; otherwise a catastrophic system failure can occur. • Absolutely, positively, first time every time • Requires formal verification/guarantees of being to always meet its hard deadlines (except for fatal errors). • Examples: – air traffic control – vehicle subsystems control – Nuclear power plant control
- 8. CHARACTERISTICS OF RTOS. Consistency Reliability Scalability Predictability Performance
- 9. FUNCTIONS OF RTOS • Task management • Scheduling. • Resource Allocation. • Interrupt Handling.
- 10. Task management • In Real Time Applications the Process is called as Task which takes execution time and occupies memory. • Task management is the process of managing tasks through its life cycle.
- 11. Task States Pended Ready Delayed Suspended Run
- 12. Task/Process States • Each task/Process can belong to one and only one state • The Scheduler only operates on the processes in the Ready state • There is a single process in the Run/current state at any time. • Transitions to and from the Ready queue are affected as a part of the execution of the RTOS resource/object services or as a result of timing events
- 13. Typical Task Operations • creating and deleting tasks, • controlling task scheduling, and • obtaining task information.
- 14. Scheduling in RTOS • More information about the tasks are known – No of tasks – Resource Requirements – Release Time – Execution time – Deadlines • Being a more deterministic system better scheduling algorithms can be devised.
- 15. Scheduling Algorithms in RTOS • Clock Driven Scheduling • Weighted Round Robin Scheduling • Priority Scheduling (Greedy / List / Event Driven)
- 16. Scheduling Algorithms in RTOS (contd) • Clock Driven – All parameters about jobs (release time/ execution time/deadline) known in advance. – Schedule can be computed offline or at some regular time instances. – Minimal runtime overhead. – Not suitable for many applications.
- 17. Scheduling Algorithms in RTOS (contd) • Weighted Round Robin – Jobs scheduled in FIFO manner – Time quantum given to jobs is proportional to it’s weight – Example use : High speed switching network • QOS guarantee. – Not suitable for precedence constrained jobs. • Job A can run only after Job B. No point in giving time quantum to Job B before Job A.
- 18. Scheduling Algorithms in RTOS (contd) • Priority Scheduling (Greedy/List/Event Driven) – Processor never left idle when there are ready tasks – Processor allocated to processes according to priorities – Priorities • static - at design time • Dynamic - at runtime
- 19. Priority Scheduling • Earliest Deadline First (EDF) – Process with earliest deadline given highest priority • Least Slack Time First (LSF) – slack = relative deadline – execution left • Rate Monotonic Scheduling (RMS) – For periodic tasks – Tasks priority inversely proportional to it’s period
- 20. Resource Allocation in RTOS • Resource Allocation – The issues with scheduling applicable here. – Resources can be allocated in • Weighted Round Robin • Priority Based • Some resources are non preemptible – Example : semaphores • Priority Inversion if priority scheduling is used
- 21. 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 – No Protection
- 22. INTERRUPTS HANDLING OF RTOS. • An interrupt is a signal from a device attached to a computer or from a program with in a computer that causes the main program that is operating system to stop and figure out what to do next. • Interrupts cause the processor to suspend the operations whatever it is doing instead execute the code that will respond to the event whatever caused the interrupt.
- 23. APPLICATIONS OF RTOS. • Almost all the modern telecommunication systems make use of RTOS . • Radar systems, network switching control systems, satellite monitoring systems, satellite launch-control and maneuvering mechanisms, global positioning systems all have their roots in RTOS. • Now a days RTOS are increasingly finding use in strategic and military operations. These are used in guided missile launching units, track-and-trace spy satellites, etc.
- 24. Comparison of RTOS VXWorks pSOS eCos Scheduler Preemptive Preemptive Preemptive Synchronizatio No condition Y Y n mechanism variable POSIX support Y Y Linux Scalable Y Y Y Custom hw BSP BSP HAL, I/O support package Kernel size - 16KB - Multiprocessor VxMP/ PSOS+m kernel Y/only basic support VxFusion support (SMP) (accessories)
- 25. VxWorks • Created by Wind River. • Current Version: VxWorks 6.0 • VxWorks is the most established and most widely deployed device software operating system. • Currently there are more than 300 million devices that are VxWorks enabled. • The core attributes of VxWorks, include high performance, reliability, determinism, low latency and scalability.
- 26. VxWorks (contd..) • Enhanced error management • Backward compatibility to previous versions features for exception handling and template support • Extensive POSIX 1003.1, .1b, .1c compatibility • Scheduling – Uses preemptive priority with round robin scheduling to accommodate for both • Real time processes • Non-real time processes
- 27. VxWorks (contd..) • Memory Protection – MMU based memory protection. • Reduced Context Switch time – Saves only those register windows that are actually in use – When a task’s context is restored, only the relevant register window is restored – To increase response time, it saves the register windows in a register cache – useful for recurring tasks
- 29. microkernel • Several types of semaphores – binary, – counting – mutual exclusion with priority inheritance • 256 priorities • POSIX compliant
- 30. Microkernel features (cont.) • High scalability • Incremental linking and loading of components • Fast, efficient interrupt and exception handling • Optimized floating-point support • Dynamic memory management • System clock and timing facilities
- 31. A note on POSIX • Portable Operating System Interface • set of standards under ISO/ IEEE charter • POSIX standard 1003.1b, (formerly called 1003.4) for RTOS • makes it easier to move applications from one operating system to another.
- 32. CONCLUSION. RTOS have been the heroes in most of the technological areas, right from fuel injection system to nuclear reactor control, satellite control, global positioning systems, and fully equipped high-tech warfare aircrafts. And the best is yet to come!
Editor's Notes
- #7: Computer failureThe term catastrophic failure is occasionally used in computer software to indicate an unexpected error from which the system cannot meaningfully recover. This usually results in a screen of death. This however, is likely to be recovered by a simple pressing of the reset button, unlike most of the examples above.
- #9: In computing, preemption (more correctly pre-emption) is the act of temporarily interrupting a task being carried out by a computer system, without requiring its cooperation, and with the intention of resuming the task at a later time. Such a change is known as a context switch. It is normally carried out by a privileged task or part of the system known as a preemptive scheduler, which has the power to preempt, or interrupt, and later resume, other tasks in the system.