Operating System - Mahendraguruquiz.mahendraguru.com/mahendraguru/content/Digi-Note… · ·...

Operating System

A program that acts as an intermediary between a user of a computer and the computer hardware.

Operating system has following parts

kernel is a computer program that manages input/output requests from software, and translates them into data processing instructions for the CPU

Shell is a command line interpreter (CLI). It interprets the commands the user types in and arranges for them to be carried out.

TYPES OF OPERATING SYSTEM

Single user operating system only has to deal with the requests of the single person using the computer at that time.

Eg. MS-DOS, Windows, Android etc..

Multi-user operating system allows lots of people to access the resources of the mainframe computer. It ‘slices up’ the mainframes resources and divides it out to the different users.

Eg.– UNIX,LINUX etc.

FEATURES OF OPERATING SYSTEM

Batch Processing: Batch processing is a technique in which Operating System collects one programs and data together in a batch before processing starts. Jobs are processed in the order of submission i.efirst

come first served fashion.

Multi-tasking: It means to run more than one program at once. It is the operating system which manages this process.

Multi-processing: refers to the use of two or more central processing units (CPU) within a single computer system.

PROCESS MANAGEMENT

A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity.

Process Stages

Process can have one of the following five states at a time.

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New: The process is being created.

Ready: The process is waiting to be assigned to a processor. Ready processes are waiting to have the processor allocated to them by the operating system so that they can run.

Running: Process instructions are being executed (i.e. The process that is currently being executed).

Waiting: The process is waiting for some event to occur (such as the completion of an I/O operation).

Terminated: The process has finished execution.

Process Scheduling

The problem of determining when processors should be assigned and to which processes is called processor scheduling or CPU scheduling.

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.

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: A newly arrived process is put in the ready 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.

Schedulers

Schedulers are special system softwares which handles process scheduling in various ways. Their main task is to select the jobs to be submitted into the system and to decide which process to run. Schedulers are of two types

Long Term Scheduler: It is also called job scheduler. Long term scheduler determines which programs are admitted to the system for processing.

Short Term Scheduler: It is also called CPU scheduler. Short term scheduler also known as dispatcher, execute most frequently and makes the fine grained decision of which process to execute next. Short term scheduler is faster than long term scheduler.

Context Switch

A context switch is the mechanism to store and restore the state or context of a CPU in Process Control block so that a process execution can be resumed from the same point at a later time.

Process Control Block (PCB)

It is used for storing the collection of information about the Processes and this is also called as the Data Structure which Stores the information about the process.

Scheduling Criteria

CPU utilization – keep the CPU as busy as possible

Throughput – number of processes that complete their execution per time unit

Turnaround time – amount of time to execute a particular process

Waiting time – amount of time a process has been waiting in the ready queue

Response time – amount of time it takes from when a request was submitted until the first response is produced.

Process Scheduling Algorithms

First Come First Serve (FCFS)

Jobs are executed on first come, first serve basis.

Easy to understand and implement.

Poor in performance as average wait time is high.

Shortest Job First (SJF)

Best approach to minimize waiting time. Impossible to implement

Processer should know in advance how much time process will take.

Priority Based Scheduling

Each process is assigned a priority. Process with highest priority is to be executed first and so on.

Processes with same priority are executed on first come first serve basis.

Starvation–Problem occurs which means low priority processes may never execute.

Solution is Aging– technique of gradually increasing the priority of processes that wait in the system for a long period of time.

Round Robin Scheduling

Each process is provided a fix time to execute called quantum.

Once a process is executed for given time period. Process is preempted and other process executes for given time period.

Context switching is used to save states of preempted processes.


THREADS

Thread is a light weight process that uses same address space for the execution. As each thread has its own independent resource for process execution, multiple processes can be executed in parallel by increasing number of threads.

Advantages of Thread

Thread minimize context switching time.

Use of threads provides concurrency within a process. Efficient communication.

Types of Thread

Threads are implemented in following two ways

User Level Threads-- User managed threads

Kernel Level Threads-- Operating System managed threads acting on kernel, an operating system core.


PROCESS SYNCHRONIZATION

Process Synchronization means sharing system resources by processes in a such a way that, Concurrent access to shared data is handled thereby minimizing the chance of inconsistent data.

Critical Section Problem

A Critical Section is a code segment that accesses shared variables and

has to be executed as an atomic action. It means that in a group of

cooperating processes, at a given point of time, only one process must be

executing its critical section. If any other process also wants to execute its

critical section, it must wait until the first one finishes.

Mutex Locks

It provides support for critical section code. It is a strict software approach, here in the entry section of code, a LOCK is acquired over the critical resources modified and used inside critical section, and in the exit section that LOCK is released.


Semaphores

It is a technique for managing concurrent processes by using the value of a simple integer variable. This integer variable is called semaphore.

Semaphores are mainly of two types:

Binary Semaphore: A binary semaphore is initialized to 1 and only takes the value 0 and 1 during execution of a program.

Counting Semaphores: A synchronizing tool and is accessed only through two low standard atomic operations, wait and signal designated by P() and V() respectively

Limitations of Semaphores

1. Priority Inversion is a big limitation os semaphores. 2. With improper use, a process may block indefinitely. Such a situation is

called Deadlock.

THE DEADLOCK PROBLEM

Deadlocks are a set of blocked processes each holding a resource and waiting to acquire a resource held by another process.


CONDITIONS FOR DEADLOCK

There are four conditions that are necessary to achieve deadlock

Mutual Exclusion - At least one resource must be held in a non-sharable mode; If any other process requests this resource, then that process must wait for the resource to be released.

Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process.

No preemption - Once a process is holding a resource ( i.e. once its request has been granted ), then that resource cannot be taken away from that process until the process voluntarily releases it.

Circular Wait - A set of processes { P0, P1, P2, . . ., PN } must exist such that every P[ i ] is waiting for P[ ( i + 1 )

HANDLING DEADLOCK

Deadlocks can be prevented by avoiding at least one of the four conditions. Mutual Exclusion

Hold and Wait

No Preemption

Circular Wait

Deadlock can be avoided by using banker’s algorithm.


Memory management

Memory management is the functionality of an operating system which

handles or manages primary memory. Memory management keeps track

of each and every memory location either it is allocated to some process

or it is free. It checks how much memory is to be allocated to processes. It

decides which process will get memory at what time. It tracks whenever

some memory gets freed or unallocated and correspondingly it updates

the status.

Dynamic Loading

In dynamic loading, a routine of a program is not loaded until it is called by the program. All routines are kept on disk in a re-locatable load format. The main program is loaded into memory and is executed. Other routines methods or modules are loaded on request.

Dynamic Linking

Linking is the process of collecting and combining various modules of code and data into a executable file that can be loaded into memory and executed.

Swapping

Swapping is a mechanism in which a process can be swapped temporarily out of main memory to a backing store, and then brought back into memory for continued execution.


CONTIGUOUS ALLOCATION

In contiguous memory allocation each process is contained in a single contiguous block of memory. The free blocks of memory are known as holes. The set of holes is searched to determine which hole is best to

allocate

O O O O

process 5 process 5 process 5 process 5

process 8

process 9 process 9

process 10

process 2 process 2 process 2 process 2

Dynamic Storage-Allocation Problem

First-fit: Allocate the first hole that is big enough

Best-fit: Allocate the smallest hole that is big enough; must search entire

list, unless ordered by size

Worst-fit: Allocate the largest hole; must also search entire list

Fragmentation

As processes are loaded and removed from memory, the free memory space is broken into little pieces. It happens after sometimes that processes can not be allocated to memory blocks considering their small size and memory blocks remains unused. This problem is known as Fragmentation.


Fragmentation is of two types

External fragmentation: Total memory space is enough to satisfy a request or to reside a process in it, but it is not contiguous so it can not be used. This can be controlled by Compaction Technique.

Internal fragmentation: Memory block assigned to process is bigger. Some portion of memory is left unused as it can not be used by another process.

Paging

External fragmentation is avoided by using paging technique. Paging is a technique in which physical memory is broken into blocks of the same size called pages. When a process is to be executed, it's corresponding pages are loaded into any available memory frames.

Logical address space of a process can be non-contiguous and a process is allocated physical memory whenever the free memory frame is available. Operating system keeps track of all free frames. Operating system needs n free frames to run a program of size n pages.



Address generated by CPU is divided into

Page number (p) -- page number is used as an index into a page table which contains base address of each page in physical memory.

Page offset (d) -- page offset is combined with base address to define the physical memory address.

Segmentation

Segmentation is a technique to break memory into logical pieces where each piece represents a group of related information. For example ,data segments or code segment for each process, data segment for operating system and so on. Segmentation can be implemented using or without using paging.

Unlike paging, segment are having varying sizes and thus eliminates internal fragmentation. External fragmentation still exists but to lesser extent.


Address generated by CPU is divided into

Segment number (s) -- segment number is used as an index into a segment table which contains base address of each segment in physical memory and a limit of segment.

Segment offset (o) -- segment offset is first checked against limit and then is combined with base address to define the physical memory address.

Virtual memory

Virtual memory is a technique that allows the execution of processes which are not completely available in memory.This separation allows an extremely large virtual memory to be provided for programmers when only a smaller physical memory is available.



Virtual memory is commonly implemented by demand paging. It can also be implemented in a segmentation system. Demand segmentation can also be used to provide virtual memory.

Demand Paging

It is a type of swapping in which pages of data are not copied from disk to RAM until they are needed.

A demand paging system is quite similar to a paging system with swapping. When we want to execute a process, we swap it into memory. Rather than swapping the entire process into memory, however, we use a lazy swapper called pager.

Frame # valid-invalid bit

v

v

v

v

i

i

i

page

PAGE FAULT

An interrupt that occurs when a program requests data that is not in memory. The interrupt triggers the operating system to fetch the data from a virtual memory and load it into RAM.


PAGE REPLACEMENT

Page replacement algorithms are the techniques using which Operating System decides which memory pages to swap out, write to disk when a page of memory needs to be allocated.

First In First Out (FIFO) algorithm

Oldest page in main memory is the one which will be selected for replacement.

Easy to implement, keep a list, replace pages from the tail and add new pages at the head.

Optimal Page algorithm

An optimal page-replacement algorithm has the lowest page-fault rate of all algorithms. An optimal page-replacement algorithm exists, and has been called OPT or MIN.

Replace the page that will not be used for the longest period of time . Use the time when a page is to be used.



Least Recently Used (LRU) algorithm

Page which has not been used for the longest time in main memory is the one which will be selected for replacement.

Easy to implement, keep a list, replace pages by looking back into time.

THRASHING

Thrashing is a condition in which excessive paging operations are taking place. A system that is thrashing can be perceived as either a very slow system or one that has come to a halt.

This leads to low CPU utilization.

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