Characteristics of Real-Time (RT) Systems

• Determinism
• Responsiveness
• User control
• Reliability

Deterministic Response

• External event and timings dictate the request of service.
• OS’s response depends on
  • speed at which it can respond to interrupts
  • whether the system has sufficient capacity to handle requests.
• Can we put an upper bound on time for OS response?
  • Factor of the OS design.
• In non-RT this delay averages around 50 to 500 ms,
  • Also is usually non-deterministic.
• In an RT, delay is usually guaranteed to have an upper-bound (usually small: few μsto 1-2ms.

Responsiveness

• The time for servicing the interrupt once it has been acknowledged.
• Comprises:
  • Time to transfer control, (and context switch) and execute the ISR
• Depends upon
  • Interrupt latency of the hardware (usually very small)
  • Priority of interrupts.
    • Classic Problem: Priority inversion.
  • Priority of tasks.
• response time = f (responsiveness, determinism)