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Real-Time Operating Systems Real-Time Operating Systems RTOS .ppt
- 1. Real-Time Operating Systems RTOS BY Abebe B. For Embedded systems
- 2. Real-Time Systems • Result in severe consequences if logical and timing correctness are not met • Two types exist – Soft real-time • Tasks are performed as fast as possible • Late completion of jobs is undesirable but not fatal. • System performance degrades as more & more jobs miss deadlines.
- 3. Real-Time Systems (cont.) – Hard real-time • Tasks have to be performed on time • Failure to meet deadlines is fatal • Example : – Flight Control System
- 4. Hard and Soft Real Time Systems (Operational Definition) • Hard Real Time System – Validation by provably correct procedures or extensive simulation that the system always meets the timings constraints • Soft Real Time System – Demonstration of jobs meeting some statistical constraints suffices. • Example – Multimedia System – 25 frames per second on an average
- 5. Most Real-Time Systems are embedded • An embedded system is a computer built into a system but not seen by users as being a computer • Examples – FAX machines – Copiers – Printers – Scanners – Routers – Robots
- 6. Role of an OS in Real Time Systems • Standalone Applications – Often no OS involved – Micro controller based Embedded Systems • Some Real Time Applications are huge & complex – Multiple threads – Complicated Synchronization Requirements – File system / Network / Windowing support – OS primitives reduce the software design time
- 7. Features of Real Time OS (RTOS) • Scheduling. • Resource Allocation. • Interrupt Handling. • Other issues like kernel size.
- 8. Scheduling in RTOS • More information about the tasks are known – Number of tasks – Resource Requirements – Execution time – Deadlines • Being a more deterministic system better scheduling algorithms can be devised.
- 9. Scheduling Algorithms in RTOS • Clock Driven Scheduling • Weighted Round Robin Scheduling • Priority Scheduling
- 10. Scheduling Algorithms in RTOS (cont.) • Clock Driven – All parameters about jobs (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.
- 11. Scheduling Algorithms in RTOS (cont.) • Weighted Round Robin – Jobs scheduled in FIFO manner – Time quantum given to jobs is proportional to it’s weight
- 12. Scheduling Algorithms in RTOS (cont.) • Priority Scheduling – Processor never left idle when there are ready tasks – Processor allocated to processes according to priorities – Priorities • Static - at design time • Dynamic - at runtime
- 13. 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
- 14. Schedulers • Also called “dispatchers” • Schedulers are parts of the kernel responsible for determining which task runs next • Most real-time kernels use priority- based scheduling – Each task is assigned a priority based on its importance – The priority is application-specific
- 15. Preemptive Kernels • The highest-priority task ready to run is always given control of the CPU – If an ISR makes a higher-priority task ready, the higher-priority task is resumed (instead of the interrupted task) • Most commercial real-time kernels are preemptive
- 17. Advantages of Preemptive Kernels • Execution of the highest-priority task is deterministic
- 18. Resource Allocation in RTOS • Resource Allocation – The issues with scheduling applicable here. – Resources can be allocated in • Weighted Round Robin • Priority Based • Priority inversion problem may occur if priority scheduling is used
- 19. Assigning Task Priorities • Not trivial • In most systems, not all tasks are critical – Non-critical tasks are obviously low- priorities • Most real-time systems have a combination of soft and hard requirements
- 20. A Technique for Assigning Task Priorities • Rate Monotonic Scheduling (RMS) – Priorities are based on how often tasks execute
- 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. 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 • There was no notion of deadlines
- 23. Interrupt Handling in Linux • Interrupts were disabled in ISR/critical sections of the kernel • There was no worst case bound on interrupt latency avaliable – eg: Disk Drivers might disable interrupt for few hundred milliseconds
- 24. Interrupt Handling in Linux (cont.) • Linux was not suitable for Real Time Applications – Interrupts may be missed
- 25. Other Problems with Linux • Processes were not preemtible in Kernel Mode – System calls like fork take a lot of time – High priority thread might wait for a low priority thread to complete it’s system call • Processes are heavy weight – Context switch takes several hundred microseconds
- 26. Part II BY Abebe B.
- 27. Characteristics of a real-time operating system • Real-time operating systems generally have the following characteristics: – Small footprint. Compared to general OSes, real-time operating systems are lightweight. – High performance. RTOSes are typically fast and responsive. – Determinism. Repeating inputs end in the same output.
- 28. Cont.… • Safety and security. Safety-critical and security standards are typically the highest priority, as RTOSes are frequently used in critical systems. • Priority-based scheduling.Tasks that are assigned a high priority are executed first followed by lower-priority jobs. • Timing information. RTOSes are responsible for timing and providing application programming interface
- 29. What are Real-Time Operating System? There are several examples of RTOS that are commonly used in embedded systems: • FreeRTOS • VxWorks • QNX • ThreadX • Micrium µC/OS • etc
- 30. What are the Best RTOS for Embedded Systems? • Choosing the best RTOS for an embedded system depends on – the specific requirements of the project, – Including; • size of the system, • performance, • memory footprint, • available resources, and real time requirements.
- 31. What are the Best RTOS for Embedded Systems? • FreeRTOS: – It is a popular open-source, real-time operating system that is designed to be small, portable, and easy to use. – FreeRTOS provides a rich set of features, including task management, interrupt handling, and synchronization mechanisms. – It is widely used in various industries, including automotive, aerospace, and consumer electronics. https://www.freertos.org/index.html
- 32. What are the Best RTOS for Embedded Systems? • Micrium µC/OS: – It is a scalable, real-time operating system that provides a robust set of features for embedded systems. Micrium µC/OS is widely used in various industries, including medical devices, industrial automation, and telecommunications. https://blackberry.qnx.com/en
- 33. What are the Best RTOS for Embedded Systems? • ThreadX: – It is a small, fast, and efficient real-time operating system that provides a reliable and predictable environment for running real- time applications. – ThreadX is widely used in various industries, including automotive, aerospace, and consumer electronics. https://www.embeddedtechnology.com/doc/threadx -rtos-0001
- 34. What are the Best RTOS for Embedded Systems? • VxWorks: – It is a real-time operating system that provides a robust set of features for embedded systems. – VxWorks is widely used in various industries, including aerospace, defense, and telecommunications. – For nearly 35 years, VxWorks has been used to ensure the security, safety, and reliability you need to design and build mission-critical embedded systems that simply must work. https://www.windriver.com/products/vxworks
- 35. What are the Best RTOS for Embedded Systems? • QNX: – It is a real-time operating system that provides a reliable and predictable environment for running real-time applications. – QNX is widely used in various industries, including automotive, medical devices, and industrial automation.
- 36. Description and Application RTOS Description Common Applications FreeRTOS An open-source RTOS with a lightweight kernel. Widely used in embedded systems and microcontrollers. Embedded devices, IoT, satellites, drones, robotics, industrial automation. VxWorks A commercial RTOS known for reliability and scalability. Used in aerospace, automotive, and more. Aerospace systems, automotive control units, industrial machinery. QNX Commercial RTOS with robustness and fault tolerance. Supports multi-core processors. Automotive infotainment, medical devices, industrial automation. ThreadX Compact and efficient RTOS suitable for resource-constrained devices. Consumer electronics, networking equipment, IoT devices. Nucleus RTOS Small-footprint RTOS by Mentor Graphics. Used in various embedded systems. Telecommunications, automotive, consumer electronics. These RTOS options cater to different needs, from open-source flexibility to commercial reliability, making them essential for time-critical applications.
- 37. Compare RTOS Description Pros Cons FreeRTOS An open-source RTOS with a lightweight kernel. Widely used in embedded systems and microcontrollers. - Open-source: Easily accessible and customizable. - Small footprint: Suitable for resource-constrained devices. - Limited features: Basic functionality; lacks some advanced features. - Community support: May vary in quality. VxWorks A commercial RTOS known for reliability and scalability. Used in aerospace, automotive, and more. - Robustness: Proven track record in critical applications. - Scalability: Supports complex systems. - Cost: Commercial license required. - Learning curve: Complex configuration and setup. QNX Commercial RTOS with robustness and fault tolerance. Supports multi- core processors. - Fault tolerance: High reliability and fault recovery. - Multi-core support: Efficient utilization. - Cost: Commercial license required. - Vendor dependency: Tied to BlackBerry. ThreadX Compact and efficient RTOS suitable for resource-constrained devices. - Low overhead: Minimal impact on system resources. - Certified: Used in safety-critical applications. - Proprietary: Closed- source with licensing fees. - Limited community: Smaller user base. Nucleus RTOS Small-footprint RTOS by Mentor Graphics. Used in various embedded systems. - Small footprint: Ideal for memory-constrained devices. - Scalability: Supports various architectures. - Commercial license: Requires licensing. - Vendor-specific: Tied to Mentor Graphics.
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Editor's Notes
- Aerospace Systems: Aerospace systems refer to a wide range of technologies, vehicles, and equipment related to both aeronautics (the study of flight within Earth’s atmosphere) and astronautics (the study of space travel beyond Earth’s atmosphere). These systems encompass everything from commercial airplanes and satellites to rockets, drones, and spacecraft. Aerospace systems play a critical role in transportation, communication, scientific exploration, national defense, and more.