🧠 What is a Scheduler?
A Scheduler in an Operating System is the component that selects which process gets access to the CPU or resources next, based on a defined policy or algorithm.
In short: it decides what to run, when, and for how long.
🔁 Types of Schedulers
There are 3 primary types, each working at a different stage of process lifecycle:
1. 🧱 Long-Term Scheduler (Admission Scheduler)
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Decides which jobs/processes to admit into the system (i.e., into memory and ready queue).
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Controls degree of multiprogramming.
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Used mostly in batch systems or when the OS is under heavy load.
| Role | Admit new processes from Job Queue → Ready Queue |
| Frequency | Infrequent |
| Controlled By | OS policy (e.g., memory limits, system load) |
2. ⚡ Short-Term Scheduler (CPU Scheduler)
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The most important scheduler in modern OS.
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Decides which ready process gets the CPU next.
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Operates very frequently (milliseconds or less).
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Must be fast and efficient.
| Role | Picks from Ready Queue → CPU |
| Frequency | Very high (every context switch, timer interrupt) |
| Examples | Round Robin, Priority Scheduling, SJF, etc. |
3. 🔄 Medium-Term Scheduler
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Optional — used in swapping systems.
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Temporarily removes processes from memory (RAM) and puts them on disk to free up space.
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Later, it can resume these processes.
| Role | Moves processes in and out of memory (Ready ↔ Suspended) |
| Used When | System is low on RAM |
| Rare | Used in embedded/legacy or highly loaded systems |
🎯 Goals of Scheduling
| Goal | Description |
|---|---|
| ⏱️ Maximize CPU Utilization | Keep CPU as busy as possible |
| ⚖️ Fairness | No starvation |
| 🚀 Maximize Throughput | Complete as many processes as possible |
| 🕰️ Minimize Turnaround Time | From submission → completion |
| ⌛ Minimize Waiting Time | Time spent in ready queue |
| 🔁 Minimize Response Time | First visible output for interactive users |
🧩 Real World Example
Let’s say 5 processes are in the ready queue:
P1, P2, P3, P4, P5
Depending on the scheduling algorithm:
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FCFS → pick
P1 -
Priority → pick the one with highest priority
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Round Robin → rotate every
xms -
SJF → pick the one with shortest burst time
The CPU Scheduler (short-term) handles all this, extremely fast.
⚙️ CPU Scheduling Flow Diagram
[ Ready Queue ] → Scheduler → [ CPU ]
↑
Based on Scheduling Algorithm
🧠 Interview-Ready Definition:
A Scheduler is an OS module that makes decisions about which process or task to run next, based on priority, fairness, and system efficiency. It can be long-term (job admission), short-term (CPU allocation), or medium-term (memory swapper).