What Is Process Scheduling? Can You Enlist Common Forms Of The Scheduling Queues?

Process scheduling is a technique that is used when there are limited resources and many processes are competing for them; Multiprogramming tries to ensure that there is some process running at all times. This is done to .utilize the CPU as much as possible. In timesharing system, the CPU switches so frequently between jobs that the user does not feel that the machine is being shared by many processes or even many users.

If the system has more than one processor, then it is possible to execute more than one process at the same time. In a single processor system, only one process can be executed at any given time. If there are more processes than processors, then the operating system must schedule the processes. It means that some processes will be executed and others will have to wait. There are many strategies for deciding which process should be assigned to the CPU.

Scheduling Queues
Below is a list of the most common types of queues and their purpose.
• Job Queue - Each entering process goes into job queue. Processes in job queue reside on mass storage and awaits the allocation of main memory.
• Ready Queue - The set of all processes that are in main memory and are waiting for CPU time, are kept in ready queue.
• Waiting (Device) Queues - The set of processes waiting for allocation of certain I/O devices, are kept in waiting device queue.

Process scheduling is the process of managing several processes which want to access CPU.
There are two forms of queues in an operating system or you can also say that there are two classes of queues exist.
* Ready Queue: Ready queue contains all the processes that are ready to execute and are waiting for the CPU. Each process is represented by a PCB, and the PCBs can be linked together to form a ready queue.

* I/O request queues: When a process is allowed to use CPU, then that process executes for some time and eventually quits, or it waits for a particular event to occur, such as the completion of an I/O request. In the case of I/O request, a request may be a shared device, such as a disk. If I/O requests of other processes are using disk then the process have to wait for the disk become available. Process waiting for a particular I/O device is called a device queue.

A new process is putted into the ready queue. It has to wait until CPU becomes free. One of the several events could occur when the process is assigned to the CPU. The process can issue an I/O request, and then it will be entered into I/O queue. It is also possible that process creates a new sub process and wait for its termination. The CPU can issue an interrupt request and in this case process has to stop its working and it will be entered into waiting state.