Real-Time Scheduling in Operating System

Introduction

Real time scheduling ek aisa scheduling mechanism hai jo time-critical applications ke liye use hota hai. Isme processes ko fixed deadlines ke andar complete karna hota hai. Agar deadline miss hoti hai, toh system fail ho sakta hai.

Real-Time Systems ke Features

-> Predictability: Tasks ko fixed time ke andar execute hona hota hai.
-> Priority-Based Execution: High-priority tasks ko pehle execute kiya jata hai.
-> Minimal Jitter: Tasks execution me delay minimum hona chahiye.
-> Resource Reservation: CPU, memory, aur I/O devices ko allocate karna padta hai taaki deadlines meet ho sakein.

Types of Real-Time Systems

1. Hard Real-Time Systems

• Deadlines strict hoti hain. Agar ek bhi deadline miss ho jaye, toh system fail ho sakta hai.

• Example:

  • Airbag deployment system in cars 

  • Medical systems like pacemakers 

  • Industrial automation 

Diagram:

Task |—-|—-|—-| (strict deadlines me complete hona chahiye)

2. Soft Real-Time Systems

• Deadlines ko follow karna zaroori hai, par ek chhoti delay allow hoti hai.

• Example:

  • Online video streaming  (Frame drop allow hota hai)

  • Online gaming  (Some lag acceptable)

Diagram:

Task |—-|—-|—>| ( Thoda delay hosakta hai pr jaldi complete hona chahiye)

Real-Time Scheduling Algorithms

1. Rate Monotonic Scheduling (RMS)

• Fixed priority-based scheduling algorithm hai.

• Jitna chhota period hoga, utni high priority milegi.

• Example:

  • Suppose system me do tasks hain:

    • T1 (Period = 4ms, Execution Time = 1ms)

    • T2 (Period = 6ms, Execution Time = 2ms)

  • T1 ki priority T2 se zyada hogi kyunki uska period chhota hai.

Diagram:

 Advantages: Simple aur predictable hai.
 Disadvantages: CPU utilization ka limitation hota hai (~69%).

2. Earliest Deadline First (EDF)

• Dynamic priority scheduling algorithm hai.

• Jo task sabse pehle complete hone wala hota hai, usko highest priority milti hai.

Example:

Scheduling:

• T2 ki deadline pehle hai (4ms), isliye pehle execute hoga.

• EDF ensures ki sabhi tasks apni deadline se pehle complete ho sakein.

 Advantages: CPU utilization 100% tak ho sakta hai.
 Disadvantages: Algorithm complex ho sakta hai.

3. Least Laxity First (LLF)

• Laxity = Deadline – (Current Time + Remaining Execution Time)

• Jiska laxity sabse kam hota hai, usko highest priority milti hai.

Example:

• T2 ka laxity kam hai, toh pehle execute hoga.

 Advantages: Deadline miss hone ka risk kam hota hai.
 Disadvantages: Frequent calculations lagti hain.

Comparison of Real-Time Scheduling Algorithms

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Conclusion

• Real-Time Scheduling ka use time-critical systems me hota hai.

• Hard Real-Time Systems me deadlines strict hoti hain, jabki Soft Real-Time Systems me slight delays allow hote hain.

• RMS, EDF, aur LLF jaise scheduling algorithms system ki efficiency improve karne ke liye use hote hain.