Lesson 2 : Scheduling Concept
2.1. Learning Objectives
On completion of the lesson you will be able to know :
On completion of the lesson you will be able to know :
i) scheduling and scheduling queues
ii) what a scheduler is
iii) different types of schedulers.
Scheduling is a fundamental operating system function, since almost all computer resources are scheduled before use. The CPU is of course, one of the primary computer resources. Thus its scheduling is central to operating system design.
When more than one process is run-able, the OS must decide which one to run first. That part of the OS concerned with this decision is called scheduler and the algorithm it uses is called scheduling algorithm.
Scheduling refers to a set of policies and mechanism into operating system that govern the order in which the work to be done by a computer system is complicated.
Scheduler is an OS module that selects the next job to be admitted into the system and the next process to run. The primary objective of scheduling is to optimize system performance in accordance with the criteria deemed most important by the system designer. Before discussing about schedulers, we have to know about scheduling queues. Let's look at scheduling queues.
2.2.1. Scheduling Queues
The ready queues is used to contain all processes that are ready to be placed on to the CPU. The processes which are ready and waiting to execute are kept on a list called the ready queue. This list is generally a linked list. A ready queue header will contain pointers to the first and last PCBs in the list. Each PCB has a pointer field which points to the next process in the ready queue The ready queue is not necessarily a first-in-first-out (FIFO) queue. A ready queue may be implemented as a FIFO queue, a priority queue, a tree, a stack, or simply an unordered list. Conceptually, however, all of the processes in the ready are lined up waiting for a chance to run on the CPU.
The list of processes waiting for a particular I/O device is called a device queue. Each device has its own device queue. If the device is a dedicated device, the device queue will never have more than one process in it. If the device is sharable, several processes may be in the device queue.
A common representation for a discussion of CPU scheduling is a queuing diagram such as Fig. 3.3. Each rectangular box represents a queue. Two types of queues are present : the ready queue and a set of device queues. The circles represent the resources which serve the queues, and the arrows indicate the flow of processes in the system.
A process enters the system from the outside world and is put in the ready queue. It waits in the ready queue until it is selected for the CPU. After running on the CPU, it waits for an I/O operation by moving to an I/O queue. Eventually, it is served by the I/O device and returns to the ready queue. A process continue this CPU-I/O cycle until it finishes; then it exits from the system.
Since our main concern at this time is CPU scheduling, we can replace with one I/O waiting queue and I/O server.
The ready queues is used to contain all processes that are ready to be placed on to the CPU. The processes which are ready and waiting to execute are kept on a list called the ready queue. This list is generally a linked list. A ready queue header will contain pointers to the first and last PCBs in the list. Each PCB has a pointer field which points to the next process in the ready queue The ready queue is not necessarily a first-in-first-out (FIFO) queue. A ready queue may be implemented as a FIFO queue, a priority queue, a tree, a stack, or simply an unordered list. Conceptually, however, all of the processes in the ready are lined up waiting for a chance to run on the CPU.
The list of processes waiting for a particular I/O device is called a device queue. Each device has its own device queue. If the device is a dedicated device, the device queue will never have more than one process in it. If the device is sharable, several processes may be in the device queue.
A common representation for a discussion of CPU scheduling is a queuing diagram such as Fig. 3.3. Each rectangular box represents a queue. Two types of queues are present : the ready queue and a set of device queues. The circles represent the resources which serve the queues, and the arrows indicate the flow of processes in the system.
A process enters the system from the outside world and is put in the ready queue. It waits in the ready queue until it is selected for the CPU. After running on the CPU, it waits for an I/O operation by moving to an I/O queue. Eventually, it is served by the I/O device and returns to the ready queue. A process continue this CPU-I/O cycle until it finishes; then it exits from the system.
Since our main concern at this time is CPU scheduling, we can replace with one I/O waiting queue and I/O server.
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