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Scheduling Queues

1. Introduction to Scheduling Queues

In a multiprogramming operating system, multiple processes are managed simultaneously. To efficiently handle these processes, the OS maintains various scheduling queues that help in process scheduling.

💡 Why Scheduling Queues?

  • Organizes processes based on their states.
  • Helps the OS efficiently allocate CPU and resources.
  • Manages process execution smoothly using scheduling algorithms.

2. Types of Scheduling Queues

📌 1. Job Queue (New Queue)

✅ Stores all newly created processes before they enter the Ready Queue.
✅ A process remains here until the OS admits it into the system.
✅ Used in batch processing systems.

✔ Example:

  • A user submits multiple print jobs. These jobs first enter the Job Queue.

📌 2. Ready Queue

✅ Stores processes that are ready for execution but waiting for CPU time.
✅ The CPU Scheduler picks the highest-priority process from this queue.
✅ Uses different scheduling algorithms (e.g., FCFS, Round Robin, Priority Scheduling).

✔ Example:

  • When you open multiple applications (Chrome, MS Word, VLC), they are in the Ready Queue waiting for CPU.

📌 3. CPU Queue (Running Queue)

✅ Stores the process that is currently executing on the CPU.
✅ Only one process can be in this queue at a time in a single-core system.

✔ Example:

  • If MS Word is currently using the CPU to check spelling, it’s in the CPU Queue.

📌 4. Waiting Queue (I/O Queue or Blocked Queue)

✅ Stores processes that are waiting for I/O operations (disk read, network request).
✅ Once the I/O operation is complete, the process moves back to the Ready Queue.

✔ Example:

  • Downloading a file in a browser – the process waits for network data.

📌 5. Terminated Queue

✅ Stores processes that have completed execution or have been killed.
✅ The OS removes them from memory to free up resources.

✔ Example:

  • When you close a video player, its process moves to the Terminated Queue before being removed from memory.

3. Diagram of Scheduling Queues

Here’s how processes move between different scheduling queues:

luaCopyEdit  +-------------+      +-------------+      +-------------+      +-------------+
  |   Job Queue | ---> | Ready Queue | ---> | Running     | ---> | Terminated  |
  +-------------+      +-------------+      +-------------+      +-------------+
                               |                    |
                               v                    v
                      +----------------+      +----------------+
                      | Waiting (I/O)  |      | CPU Scheduling |
                      +----------------+      +----------------+
  • New processes enter the Job Queue.
  • OS selects them and moves them to the Ready Queue.
  • The CPU Scheduler picks a process to execute.
  • If the process needs I/O, it moves to the Waiting Queue.
  • Once execution is complete, the process enters the Terminated Queue.

4. How Scheduling Queues Work Together?

✔ Multi-level Queues: Some OSes divide the Ready Queue into different levels (Foreground, Background, etc.).
✔ Preemptive Scheduling: If a process is interrupted, it goes back to the Ready Queue.
✔ Long-Term vs. Short-Term Scheduling:

  • Long-Term Scheduler decides which processes move from Job Queue → Ready Queue.
  • Short-Term Scheduler selects which process gets CPU time from the Ready Queue.
  • Medium-Term Scheduler temporarily removes processes (swapping).

5. Conclusion

✔ Scheduling Queues help the OS manage process execution efficiently.
✔ Different queues handle processes based on their state (Ready, Running, Waiting, etc.).
✔ Schedulers (Long-term, Short-term, and Medium-term) control queue transitions.