Gaurav’s Notes

Scheduling Queue

3 min read

🧠 What is a Scheduling Queue?

A scheduling queue is a data structure used by the OS to track the state of processes during their lifecycle — mainly while they’re waiting for resources like the CPU or I/O.

Each queue represents a specific state or resource.

🔁 Types of Scheduling Queues

Let’s break it down:

1. 🟦 Job Queue

  • Contains all processes in the system — new, running, waiting, terminated.

  • Useful in batch processing systems (less relevant in modern interactive OSes).

Think of it as the “everything in the system” list.

2. 🟦 Ready Queue

  • Contains all processes that are:

    • Loaded into memory

    • Ready to execute

    • Waiting for CPU

  • Managed by the CPU scheduler.

This is the main queue the scheduler pulls from when selecting the next process to run.

3. 🟦 Device (I/O) Queues

  • Each I/O device (disk, printer, network) has its own queue.

  • If a process requests I/O, it is moved to the respective device’s queue.

For example, a blocked process waiting for disk read sits in the Disk I/O Queue.

4. 🟦 Waiting Queue / Blocked Queue

  • Contains processes that are blocked on some event:

    • I/O

    • Sleep

    • Wait for a signal, lock, or child process

  • Not ready for CPU.

These return to the ready queue once the event is resolved.

🧠 Visual Diagram

             [ Job Queue ]
                  ↓
             [ Ready Queue ] ←— CPU Scheduler picks from here
            ↙        ↓         ↘
   [ I/O Queue ] [ Timer Expired ] [ Waiting Queue ]
                     ↓
                 [ Back to Ready Queue ]

⚙️ How Queues Work Together in Real Life

  1. A process is created → enters the job queue.

  2. OS admits it into memory → moves to ready queue.

  3. Scheduler picks it → moves it to CPU.

  4. It either:

    • Finishes → terminates

    • Gets blocked → enters I/O / waiting queue

    • Gets preempted → back to ready queue

📚 Real Example

Let’s say you open Chrome:

  • Chrome process enters job queue.

  • OS loads it → moves to ready queue.

  • CPU picks it → runs it.

  • Chrome makes a network request → process goes to network I/O queue.

  • When data is received → process returns to ready queue.

🧠 Interview-Ready Definition:

A scheduling queue is a collection of processes that are in similar states of execution — such as ready to run, waiting for I/O, or blocked on an event.
The OS uses these queues to manage process scheduling and resource allocation efficiently.

Graph View

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