Operating Systems Index

Highest Response Next Scheduling

Highest response next (HRN) HRN drives a dynamic priority value based on the estimated run time and waiting time. The priority for each process is calculated from: Priority (P) = (waiting time + run time) / run time The process with the highest value will be selected for running. Value processes first appear in the ready queue, then the waiting time will be zero and hence P will be equal to 1, for all the process. After a short period of waiting however the shorter job will be favored. Consider two jobs P & Q with run time 10 and 50 after...

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Priority Scheduling

Priority Scheduling

Priority scheduling Each process in the system if given a priority, then the scheduling must be done according to the priority of each process. A higher priority job should get CPU whereas lower priority job can be made to wait. Priority scheduling is necessarily a form of preemptive scheduling where priority is the basic of preemption. Example: Let us consider a set of processes P1, P2, P3 having priorities ranging from 1 to 3. Let us assume that 1 is the highest priority whereas 3 is the least priority. Let us also assume that p1 arrive...

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Multilevel feedback queues

Multilevel feedback queues

Multilevel feedback Queue scheduling It is an enhancement of multilevel queue scheduling where process can move between the queues. In approach, the ready queue is partitioned into multiple queues of different priorities. The system use to assign processes to queue based on their CPU burst characteristic. If a process consumes too much CPU time, it is placed into a lower priority queue. It favors I/O bound jobs to get good input/output device utilization. A technique called aging promotes lower priority processes to the next higher priority...

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Multilevel queue scheduling

Multilevel queue scheduling

Multilevel queue scheduling Multi level queue scheduling was created for situation in which processes are easily classified into different groups. Multilevel queue scheduling has the following characteristics: Processes are divided into different queue based on their type. Process are permanently assigned to one queue, generally based on some property of process i.e. system process, interactive, batch system, end user process, memory size, process priority and process type. Each queue has its own scheduling algorithm. For example...

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Shortest Remaining Time Scheduling

Shortest Remaining Time Scheduling

Shortest remaining time (SRT) scheduling Shortest remaining time scheduling is the preemptive counter part of SJF and is useful in time sharing system. In SRT, process with the smallest estimated run time to completion is run next, in SJF once a job begin executing, it runs to completion. In SRT a running process may be preempted by a user process with a shorter estimated run time. Consider an example, where three processes arrived in the order P1, P2, P3 at the time mentioned below, and then the average waiting time using SJF scheduling...

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Shortest job first scheduling

Shortest job first scheduling

Shortest job first scheduling Key concept of this algorithm is: “CPU is allocated to the process with least CPU-burst time.” Amongst the processes in the ready queue. CPU is always assigned to the process with least CPU burst requirement. If the two processes having the same length, next CPU burst, fcfs scheduling is used i.e. one which arrives first, will be taken up first by the CPU. This algorithm can be preemptive or non-preemptive. Let us consider, the following set of processes having their CPU burst time mentioned in millisecond and...

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Round robin scheduling

Round robin scheduling

Round Robin Scheduling The basic purpose of this algorithm is to support time sharing system. This algorithm is similar to the FCFS algorithm but now it is preempted FCFS scheduling. The preempted take place after a fixed interval of time called quantum time of time slice. Its implementation requires timer interrupt based on which the preemption take place. Consider the set of the processes lined up in the ready queue. The processes are taken out of the ready queue in FCFS order. Let us suppose that a process has been taken up for execution...

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First Come First Served Algorithm

First Come First Served Algorithm

First come first served scheduling algorithm (FCFS) FCFS also termed as First-In-First-Out i.e. allocate the CPU in order in which the process arrive. When the CPU is free, it is allowed to process, which is occupying the front of the queue. Once this process goes into running state, its PCB is removed from the queue. This algorithm is non-preemptive. Advantage Suitable for batch system. It is simple to understand and code. Disadvantage Waiting time can be large if short request wait behind the long process. It is not suitable for time...

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Scheduling

Introduction Scheduling is the process of determining which processes will actually run when there are multiple runnable processes. It is very important because it can have a big effort on the resource utilization and good performance of the system. The part of operating system that makes this decision is called scheduler. The algorithm it uses is called scheduling algorithm. With batch system having input in the form of card, the scheduling algorithm was simple just a single or one process at a time and the user wait to run another process....

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File System

File System

Files The files of information in appropriate format are known as file. Each and every application programs such as ms-word, ms-excel, notepad need to store information and retrieve main requirement for application program with related with file are storage space, easy access to categories file, to provide security to file etc. so , different terminologies like file naming, file attributes, file type, file structure, file access are used. The essential requirements for long term information storage are:- 1. It must be possible to store very...

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