06-12-2012, 02:28 PM
A PRESENTATION ON PROCESS SCHEDULING
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INTRODUCTION
Multiprogramming operating system allow more than one process to be loaded into executable memory at a time and for the loaded process to share the CPU using time multiplexing.
The scheduling mechanism is the part f the process manager that handles the removal of the running process from the CPU and the selection of other process.
PROCESS CONTROL BLOCK
PCB is the data structure used by operating system, it groups all information that needs about a particular process.
The set of PCBs practically defines the state of os.
PCBs must be read and modified all the time by almost all modules in the os i.e. scheduling etc
So it is a good design practice to dedicate one level handler to the protection of the process
table then the , modules must ask this handler
any read/write access.
SCHEDULING QUEUES
When the process enters into the system, they are put into a job queue consists of all processes in the system.
Device queue is a queue for which a list of processes waiting for a particular I/O device
TWO STATE PROCESS MODEL
A process may be in of two states running
or not running.
When new process is created by os , that processes enter into the system in the not running state .
Process that are not running are kept in qu -eue , waiting there turn to execute . Each entry in a queue is a printer to particular process. Dispatcher is used in this case.
LONG TERM SCHEDULER
It is also called job scheduler. It determines which programs are admitted to system for processing.
It selects process from the queue and loads them into memory for execution.
Its aim at providing a balanced mix of jobs, s uch as I/O bounds and processor bounds.
It also controls the degree of multiprogram ing , if stable then average rate of creation must be equal to the average departure rate of processes leaving the system.
When process changes the state from new to ready, then there is a long term scheduler.
SHORT TERM SCHEDULER
It is known as CPU scheduler. It increases the system performance .
Short term scheduler is also known as dispatcher , execute most frequently and makes the fine grained decision of which process to execute next. It is much faster.