Types of Scheduling Queues

Process Queues play an important role in process scheduling.

This article will discuss different types of Scheduling Queues in detail along with their characteristics.

Types of Scheduling Queues

We will look at the numerous types of Scheduling Queues used in computer systems in the following sub-topics.

Job Queue (In Disk)

This queue is known as the job queue, it contains all the processes or jobs in the list that are waiting to be processed. Job: When a job is created, it goes into the job queue and waits until it is ready for processing.

• Contains all submitted jobs.
• Processes are stored here in a wait state until they are ready to go to the execution stage.
• This is the first and most basic state that acts as a default storage of new jobs added to a scheduling system.
• Long Term Scheduler Picks a process from Job Queue and moves to ready queue.

Ready Queue (In Main Memory)

The Stand-by queue contains all the processes ready to be fetched from the memory, for execution. When the process is initiated, it joins the ready queue to wait for the CPU to be free. The operating system assigns a process to the executing processor from this queue based on the scheduling algorithm it implements.

• Contains processes (mainly their PCBs) waiting for the CPU to execute various processes it contains.
• They are controlled using a scheduling algorithm like FCFS, SJF, or Priority Scheduling.
• Short Term Scheduler picks a process from Ready Queue and moves the selected process to running state.

Block or Device Queues (In Main Memory)

The processes which are blocked due to unavailability of an I/O device are added to this queue. Every device has its own block queue.

Flow of Movement in the above Queues

The below diagram shows movements of processes in different queues.

• All processes are initially in the Job Queue.
• A new process is initially put in the Ready queue by scheduler. It waits in the ready queue until it is selected for execution(or dispatched). Once the process is assigned to the CPU and is executing, one of the following several events can occur:
  1) The process could issue an I/O request, and then be placed in a Device queue.
  2) The process could create a new subprocess and wait for its termination.
  3) The process could be removed forcibly from the CPU, as a result of an interrupt, and be put back in the ready queue.

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Article Tags :

• Operating Systems
• Operating Systems-Process Management