Scheduling Queues

Scheduling queues refers to queues of processes or devices. When the process enters into the system, then this process is put into a job queue. This queue consists of all processes in the system. The operating system also maintains other queues such as device queue. Device queue is a queue for which multiple processes are waiting for a particular I/O device. Each device has its own device queue.

This figure shows the queuing diagram of process scheduling.

• Queue is represented by rectangular box.
• The circles represent the resources that serve the queues.
• The arrows indicate the process flow in the system.

Queues are of two types

• Ready queue
• Device queue

A newly arrived process is put in the ready queue. Processes waits in ready queue for allocating the CPU. Once the CPU is assigned to a process, then that process will execute. While executing the process, any one of the following events can occur.

• The process could issue an I/O request and then it would be placed in an I/O queue.
• The process could create new sub process and will wait for its termination.
• The process could be removed forcibly from the CPU, as a result of interrupt and put back in the ready queue.