Real-time operating systems (RTOS) are crucial for embedded systems with strict timing requirements. They ensure tasks meet deadlines and provide deterministic behavior, which is essential for applications like aircraft control systems and medical devices.

RTOS performance is measured by response time, jitter, throughput, and resource utilization. Key components include the kernel, tasks, and synchronization mechanisms like semaphores and mutexes. Scheduling algorithms like priority-based and earliest deadline first help manage task execution efficiently.

Real-time Operating System Fundamentals

Real-time System Characteristics

• Real-time operating system (RTOS) designed to support applications with strict timing requirements and deterministic behavior
• Hard real-time systems have critical deadlines that must be met to avoid system failure (aircraft control systems, medical devices)
• Soft real-time systems have more flexible deadlines where occasional misses are tolerable (multimedia streaming, gaming)
• Determinism ensures predictable and consistent execution times for tasks, allowing the system to meet real-time constraints
• Deadline represents the time by which a task must complete its execution to maintain system integrity and functionality

RTOS Performance Metrics

• Response time measures how quickly the RTOS reacts to events or interrupts and starts executing the corresponding task
• Jitter refers to the variation in the timing of periodic tasks, which should be minimized in real-time systems for predictability
• Throughput represents the amount of work or tasks completed by the RTOS within a given time period
• Resource utilization efficiency of the RTOS in managing system resources (CPU, memory) to maximize performance

Scheduling and Multitasking

Task Scheduling Mechanisms

• Preemptive multitasking allows the RTOS to interrupt and switch between tasks based on priority and time constraints
• Priority-based scheduling assigns priorities to tasks, with higher-priority tasks executing before lower-priority ones
• Round-robin scheduling allocates fixed time slices to tasks, ensuring fair distribution of CPU time among tasks of equal priority
• Earliest deadline first (EDF) scheduling prioritizes tasks based on their deadlines, executing tasks with the nearest deadlines first

Real-time Performance Factors

• Interrupt latency time taken by the RTOS to respond to an interrupt and start executing the associated interrupt service routine (ISR)
• Context switching overhead involved in saving and restoring task states when switching between tasks, which should be minimized
• Scheduling overhead time consumed by the RTOS to make scheduling decisions and manage task queues
• Memory footprint size of the RTOS code and data structures, which should be optimized to fit within the limited memory of embedded systems

RTOS Components and Synchronization

Core RTOS Elements

• Kernel central component of the RTOS responsible for task management, scheduling, and resource allocation
• Task basic unit of execution in an RTOS, representing a specific function or piece of code that can be scheduled and executed independently
• Task control block (TCB) data structure maintained by the RTOS to store task-related information (state, priority, stack pointer)
• Interrupt service routine (ISR) special task executed in response to an interrupt, typically having higher priority than regular tasks

Synchronization Mechanisms

• Semaphore synchronization primitive used to control access to shared resources and coordinate task execution
• Binary semaphore (mutex) restricts access to a shared resource to a single task at a time, preventing race conditions
• Counting semaphore allows multiple tasks to access a shared resource simultaneously, up to a specified maximum count
• Mutex (mutual exclusion) similar to a binary semaphore but with additional ownership and priority inheritance mechanisms to prevent priority inversion