🧠 What is Process Scheduling?
Process Scheduling is the activity of the operating system that decides which process gets the CPU, when, and for how long.
Since there’s usually one CPU core and many processes, the scheduler’s job is to optimize CPU time by choosing which process runs next.
🔁 Why Do We Need Scheduling?
Because:
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The number of processes > number of CPUs
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Processes need to take turns to avoid starvation
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Some processes are interactive, others are CPU-heavy
Without scheduling, one process could hog the CPU, while others starve.
🎯 Goals of Process Scheduling
| Goal | Description |
|---|---|
| ⚡ Max CPU Utilization | Keep CPU as busy as possible |
| ⏳ Minimize Waiting Time | Processes shouldn’t wait too long |
| 🕒 Minimize Turnaround Time | Complete processes as soon as possible |
| 🔁 Maximize Throughput | More processes completed per second |
| 🧠 Responsiveness | Quick replies to user actions (especially in interactive systems) |
| ⚖️ Fairness | Every process gets a chance — no starvation |
⚙️ Who Handles This?
The CPU Scheduler (aka Short-Term Scheduler)
It runs every time a context switch is needed.
It chooses a process from the ready queue to assign the CPU.
🗂️ What Does It Consider?
The scheduler looks at:
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Process priority
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Time already spent on CPU
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I/O waiting
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Process type (foreground/background)
📦 Process States Involved
New → Ready → Running → (Waiting or Terminated)
↑
Preempted
The scheduler picks from the Ready Queue.
🧪 When is Scheduling Triggered?
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When a process terminates
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When a process goes from running to waiting (e.g., for I/O)
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When a process is preempted (e.g., time slice is over)
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When a process is created or moved to ready
🧰 Examples of Scheduling Algorithms
| Algorithm | Description | Preemptive? |
|---|---|---|
| FCFS (First Come First Serve) | Run in arrival order | ❌ |
| SJF (Shortest Job First) | Run shortest process first | ❌ (non-preemptive) |
| SRTF (Shortest Remaining Time First) | Preempt if a shorter job arrives | ✅ |
| Round Robin | Fixed time slices (quantum) | ✅ |
| Priority Scheduling | Run highest-priority job | ✅ or ❌ |
| Multilevel Queue | Separate queues for types of jobs | ✅ |
🧠 Interview-Ready Definition:
Process Scheduling is the OS mechanism that selects which process should run next on the CPU. It ensures efficient and fair use of the processor, maintains responsiveness, and optimizes overall system performance using scheduling algorithms like Round Robin, SJF, and Priority Scheduling.
📊 Real-World Analogy
Imagine a doctor’s clinic (CPU) with many patients (processes) in the waiting room (ready queue). The receptionist (scheduler) decides who gets to go in next — based on arrival time, urgency, or appointment type.