cs0206 operating systems l t p c 3 0 0 3 prerequisite cs0201… · · 2015-11-07cs0206 operating...

CS0206-OPERATING SYSTEMS SRM UNIVERSITY,RAMAPURAM

DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING of 157

CS0206 OPERATING SYSTEMS Prerequisite CS0201, CS0203

PURPOSE Every computer professional should have a basic understanding of how an operating system controls the computing resources and provide services to the users. This course provides an introduction to the operating system functions, design and implementation. It serves as strong foundation for other courses like networks, compiler design, data base systems. INSTRUCTIONAL OBJECTIVES The students learn about: 1. Structure and functions of OS 2. Process scheduling, Deadlocks 3. Device management 4. Memory management 5. File systems UNIT 1 INTRODUCTION 9 Computer system overview-basic elements, Instruction execution, Interrupts, memory hierarchy, I/O communication techniques, operating system overview-objectives and functions, Evolution of OS Microsoft windows overview. UNIT 2 PROCESSES 9 Process description and control - process states, process description, process control; Processes and Threads, Symmetric Multiprocessing and microkernel’s. Windows Thread and SMP management. Case studies-UNIX, SOLARIS thread management UNIT 3 CONCURRENCY AND SCHEDULING 9 Principles of concurrency - mutual exclusion, semaphores, monitors, Readers/Writers problem; Deadlocks – prevention- avoidance – detection .Scheduling : Types of scheduling – scheduling algorithms. Case studies- UNIX scheduling. UNIT 4 MEMORY 9 Memory management requirements, partitioning, paging, and segmentation; Virtual memory - Hardware and control structures, operating system software, Linux memory management, case studies- WINDOWS memory management, UNIX and SOLARIS Memory management UNIT 5 INPUT/OUTPUT AND FILE SYSTEMS 9 I/O management and disk scheduling – I/O devices, organization of I/O functions; OS design issues, I/O buffering, disk scheduling, Disk cache, File management –organization, directories, file sharing, record blocking, secondary storage management; case studies-LINUX I/O, UNIX File management. TOTAL 45

L T P C 3 0 0 3



Notes for all five units Prepared by Mrs. Antony Vigil A.P[O.G]/SRMU/RMP UNIT 1

INTRODUCTION

Computer system overview-Basic elements, Instruction execution, Interrupts, Memory

hierarchy, I/O communication techniques, Operating system overview-Objectives and

functions, Evolution of OS Microsoft windows overview.

COMPUTER SYSTEM OVERVIEW

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

computer hardware is Operating System.

Operating system goals:

• Execute user programs and make solving user problems easier.

• Make the computer system convenient to use.

• Use the computer hardware in an efficient manner

Computer System Components:

1. Hardware – provides basic computing resources (CPU, memory, I/O devices).

2. Operating system – controls and coordinates the use of the hardware among the

various application programs for the various users.

3. Applications programs – define the ways in which the system resources are used

to solve the computing problems of the users (compilers, database systems, video

games, business programs).

4. Users (people, machines, other computers).



Abstract View of System Components:

Operating System Definitions: • Resource allocator – manages and allocates resources.

• Control program – controls the execution of user programs and operations of

I/O devices.

• Kernel – the one program running at all times (all else being application

programs).

Simple Batch Systems:

• Hire an operator

• User ≠ operator

• Add a card reader

• Reduce setup time by batching similar jobs

• Automatic job sequencing

• Automatically transfers control from one job to another.

• Resident monitor

• Initial control in monitor

• When job completes control transfers back to monitor



Memory Layout for a Simple Batch System:

Basic Elements

Computer =Processor + Memory + I/O modules

• Processor: Controls the operation of the computer and performs its data

processing functions. When there is only on processor, it is often

referred to as CPU.

• Main Memory: Stores data and programs.

• I/O modules: Move data between the computer and its external environments

(e.g., disk drive, network, terminals).

• System interconnection: Some structures and mechanisms that provide for

communication among processors, main memory, and I/O modules



Instruction Execution

Execute cycle involves data transfer between processor and memory (or an I/O

module), data processing or changing control flow. Most processors use pipeline

instruction execution and allow Direct Memory Access

• The processor fetches the instruction from memory

o Program counter (PC) holds address of the instruction to be fetched next

o Fetched instruction is placed in the instruction register (IR)

o Program counter is incremented after each fetch

• Processor then executes instruction in the IR

Categories of instructions:

• Processor-memory

o Transfer data between processor and memory

• Processor-I/O

o Data transferred to or from a peripheral device

• Data processing

o Arithmetic or logic operation on data

• Control

o Alter sequence of execution



Interrupts Interrupt (or exception):

• Signal sent to processor

e.g. attempt to divide by zero

e.g. illegal attempt to access address

e.g. execution of trap instruction (to make “system call”)

e.g. I/O transfer has completed

• Source and priority of interrupt are recorded

All computers provide a mechanism by which other modules may interrupt the

normal processing of the processor.

The Classes of Interrupts:

• Program: Generated by some condition that occurs as a result of an instruction.

• Timer: Generated by a timer within the processor.

• I/O: Generated by an I/O controller.

• Hardware failure: Generated by a failure such as power failure.

Interrupt is an interruption of the normal sequence of execution. After interrupt is

completed, the normal program execution is resumed. Interrupts are provided primarily to

improve processing efficiency.

E.g., Avoid CPU waiting for slow I/O devices.CPU can continue to execute other

instructions



Interrupt Cycle:

During the interrupt cycle, the processor checks to see if any interrupts have

occurred, indicated by the presence of an interrupt signal. If interrupt is pending, the

processor suspends execution of the current program and executes an Interrupt Handling

Routine. IHR determine the nature of interrupt and performs whatever actions are needed.



Program Timing: Short I/O Wait Program Timing: Long I/O Wait



Interrupt Processing:

Multiple Interrupts:

• Multiple interrupts can occur close to one another.

• To disable interrupt when executing the interrupt handler routine.

o New interrupts will have to remain pending until the completion of

interrupt

o Handler routine.

o Does not consider priority of interrupts and time-critical needs.

• To define priorities for interrupts.

o High-priority interrupts can interrupt the execution of IHR of low-priority

interrupts.



Interrupt handling:

• Contents of PC and PSW are stored automatically.

• Interrupt service routine (ISR) is executed in supervisor mode.

• ISR may store contents of other registers.

• ISR may call other operating system routines.

• Eventually contents of registers may be restored and execution continued in user

mode from point of interruption

Multiprogramming:

• More than one process is active on a single processor

• There is a ready queue of processes waiting for the processor

• A process must wait after making an I/O request or after a timer interrupt



The Memory Hierarchy Advantage:

• Faster access time • Greater capacity

• Going down the hierarchy

o Decreasing cost per bit

o Increasing capacity

o Increasing access time

o Decreasing frequency of access of the memory by the processor

• Storage devices can be put in order of increasing capacity, namely, o registers, cache memory, main memory, hard disk, tape o access time also increases



o cost per bit decreases o first three are volatile caching o copying information into faster device

Secondary memory:

• Managed by the operating system • Nonvolatile • Auxiliary memory • Used to store program and data files

Cache Memory:

• Processors can execute instructions faster than instructions (and data) can be

fetched from main memory

• Cache memory provides a solution which relies on locality of reference and is

invisible to OS



I/O module

• Is an interface between the bus and a device

• Hides the complexity of the device from the processor

• The processor issues commands to an I/O module

• The I/O module controls the device to perform the requested action

• It also buffers data and maintains a status register that the processor can read

Techniques for performing I/O:

(i) Programmed I/O

(ii) Interrupt-driven I/O

(iii)Direct memory access (DMA)

(i) Programmed I/O

• I/O module performs the requested action

• No interrupts occur

• Module sets appropriate bits in the I/O status register

• Processor checks status until operation is complete

• Processor does a busy-wait for each character



(ii)Interrupt-driven I/O

• Processor is interrupted when I/O module ready to exchange data

• Processor saves context of program executing and begins executing interrupt

handler

• No needless waiting

• Involves much processor overhead because . . .

o Every character read or written passes through the processor

o One interrupt for each character



(iii)Direct memory access (DMA)

• Transfers an entire block of data directly to or from memory

• An interrupt is sent when the transfer of the entire block is complete

• One interrupt per block of data

• A DMA controller transfers data directly between a device (typically a disk) and

memory.

o The data does not pass through the processor

• The I/O module has authority to read from or write to memory

o This relieves the processor responsibility for the exchange

• The DMA competes with the processor for memory access

o This is known as cycle stealing

o Although cycle stealing halts the processor, this is not an interrupt



Operating System Overview: Objectives & Functions

An operating system (OS) is a program that controls the execution of application

programs and acts as an interface between the user of a computer and the computer

hardware.

• Convenience

o Makes the computer more convenient to use.

• Efficiency

o Allows computer system resources to be used in an efficient manner

• Ability to evolve

o Permit effective development, testing, and introduction of new system

functions without interfering with service



Convenience:

Services provided by the operating system:

• Program development

o Editors and debuggers

• Program execution

• Access to I/O devices

• Controlled access to files

• System access

o Login and passwords

• Error detection and response

o Internal and external hardware errors

o Memory error

o Device failure

o Software errors

o Arithmetic overflow

o Access forbidden memory locations

o Operating system cannot grant a request made by an application

• Accounting

o Collect usage statistics

o Monitor performance

o This information can be used . . .

o To anticipate future enhancements

o For billing purposes

Efficiency:

• The OS promotes efficiency by managing resources

o Processor(s)

o Memory

o Devices

o Files



• The OS functions same way as ordinary computer software

o It is program that is executed

o The OS frequently relinquishes control of the processor and relies on the

processor to regain control

Ability to evolve:

A major OS should be able to evolve over time in response to . . .

• Hardware upgrades and new types of hardware such as

o Paging hardware for virtual memory

o Multiple processors

• New services such as

o Overlapping windows

o Client / server computing

• Errors in the OS

The Operating System as Resource manager:

A computer is a set of resources for the movement, storage, and processing of

data and for the control of these functions. The operating system is responsible for

managing these resources. The operating system is nothing more than a computer

program.

The operating system functions in the same way as ordinary computer software;

that is, it is a program executed by the processor.

The operating system frequently relinquishes control and must depend on the

processor to allow it to regain control

Key difference between OS and other programs:

OS can direct processor in the use of other system resources and in the timing of

its execution of other program.

In order to do this, processor must cease executing OS and execute other

programs, i.e., OS relinquishes control of processor.



Evolution of operating systems

(i)Serial processing

(ii)Simple batch systems

(iii)Multiprogrammed batch systems

(iv)Time-sharing systems

(i) Serial processing:

Use card reader. No operating system.

2 main problems:

1. Manual scheduling of use

2. Setup Time

• Loading the compiler

• Loading the source program

• Saving compiled program

• Loading and linking object files

(ii)Simple batch systems:

Use a piece of software known as the monitor. So users no longer have direct

access to the machine

2 points of view:

Monitor point of view:

• Monitor software controlled a sequence of jobs

• Monitor must always in main memory and available for execution. That

portion is referred as Resident Memory.

• The rest of the monitor consists of utilities and common functions.



Interrupt Processing

Device Drivers

Job Sequencing

Control Language

Interpreter

User Program Area

Fig: Memory Layout for a Resident Memory

Processor point of view:

• Processor is executing instructions from the portion of main memory

containing the monitor

• Only one user job could run at a time

• Processor idle waiting for I/O

• But idle time between jobs and within jobs eliminated

Job Control Language (JCL):

• A special type of programming language

• Provides commands to the monitor

o Identifies new jobs

o Specifies what compiler to use

o Specifies which object files to load and link

o Specifies what data to use

• Example of JCL cards (with / /) in a deck

Monitor

Boundary



Hardware support for simple batch:

• Memory protection

o Does not allow the memory area containing the monitor to be altered

by a job

• Timer

o Prevents a job from monopolizing the system

• Privileged instructions

o Certain machine level instructions can only be executed by the monitor

l E.g. - I/O instructions (a program should not read cards of next

job)

l Note: a program “requests” that the monitor perform the

instruction

• Interrupts

o Allow processor to do something else while waiting for I/O

o Early computer models did not have this capability

• The need for memory protection and privileged instructions led to the concept

of processor modes

• A bit in the PSW register toggles the processor between user mode and kernel

mode

/ / JOB / / FORT ⋅ ⋅ ⋅ < source program cards > ⋅ ⋅ ⋅ / / LOAD / / RUN ⋅ ⋅ ⋅ < data cards > ⋅ ⋅ ⋅ / / END

Protection After an //END card or an error, the monitor flushes cards until the next //JOB card



• Each user program executes in user mode

o Certain privileged instructions may not be executed

o Only the program area may be referenced

• The monitor executes in kernel mode

o Privileged instructions may be executed

o Protected areas of memory may be accessed

(iii) Multiprogrammed batch systems:

• Several jobs resident in memory simultaneously

• Gives processor something to do while one job is waiting for I/O



(iv)Time-sharing systems

• Adds an interactive computing capability to a multiprogrammed batch system

o Processor’s time is shared among multiple interactive users

o Multiple users simultaneously access the system through terminals

• Essential for transaction processing systems

• Example: Compatible Time-Sharing System (CTSS)

o First time-sharing system

o Developed at MIT in 1961 for the IBM 709

Problems:

• Multiprogramming and time sharing led to the identification of new problems

o Memory protection

o File security

o Contention for resources

E.g. – printers



Microsoft Windows Overview

• Single-user multitasking OS that evolved out of MS-DOS

• Modular structure for flexibility

o Any module can be removed, upgraded, or replaced without rewriting the

entire system

• Executes on a variety of hardware platforms

o Pentium, Itanium, PowerPC, Alpha, etc.

o Provided by the Hardware Abstraction Layer (HAL)

o This isolates the operating system from platform-specific hardware

differences

• Supports applications written for other operating systems

o This is provided by various environment subsystems



Windows organization

• Kernel

• Consists of the most used components

o Thread scheduling

o Process switching

o Interrupt handling

o SMP

• Does not run in threads and is not preemptible

• Executive

• Contains base operating system services

o Memory management

o Process and thread management

o Security

o I/O

o Interprocess communication

• Hardware abstraction layer(HAL)

Map between hardware commands and responses

• Device Drivers

• Windowing and Graphics system

User Mode Processes:

• Special system support processes

• Service Processes

• Environment subsystems

• User Applications

Windows client-server model:

• Simplifies the Executive

o Possible to construct a variety of APIs

• Improves reliability

o Each service runs on a separate process with its own partition of memory

o Clients cannot not directly access hardware



• Provides a uniform means for applications to communicate via Local Procedure

Call (LPC)

• Provides base for distributed computing

Windows threads and SMP:

• Operating system routines can run on any available processor or simultaneously

on different processors

• Multiple threads of execution within a single process may execute on different

processors simultaneously

• Server processes may use multiple threads to process requests from multiple

processes simultaneously

• Mechanisms provided to share data and resources between processes

Windows Objects:

• Encapsulation

• Object Class and instance

• Inheritance

• Polymorphism



UNIT 2

PROCESSES

Process description and control - Process states, Process description, Process control,

Processes and Threads, Symmetric Multiprocessing and Microkernels, Windows

Thread and SMP Management. Case studies-UNIX, SOLARIS thread management

Process description and control

Processes:

• A program in execution

• An instance of a program running on a computer

• An entity that can be assigned to and executed on the computer

• A process is comprised of:

o Program code (possibly shared)

o A set of data

o A number of attributes describing the state of the process

Process Control Block (PCB)

• While the process is running it has a number of elements including

o Identifier

o State

o Priority

o Program counter

o Memory pointers

o Context data

o I/O status information

o Accounting information



Process States

• The behavior of an individual process is shown by listing the sequence of

instructions that are executed

• This list is called a Trace

• Dispatcher is a small program which switches the processor from one process to

another

• Each process runs to completion

Program Counter

8000



Two-State Process Model:

• Process may be in one of two states

o Running

o Not-running



The creation and Termination of Processes:

Creation Termination

New batch job Normal Completion

Interactive Login Memory unavailable

Created by OS to provide a service Protection error

Spawned by existing process Operator or OS Intervention

Process Creation

• The OS builds a data structure to manage the process

• Traditionally, the OS created all processes

o But it can be useful to let a running process create another

• This action is called process spawning

o Parent Process is the original, creating, process

o Child Process is the new process

Reasons for Process Creation:

• New Batch job

• Interactive Logon

• Created by OS to provide a service

• Spawned by existing process

Process Termination

• There must be some way that a process can indicate completion.

• This indication may be:

o A HALT instruction generating an interrupt alert to the OS.

o A user action (e.g. log off, quitting an application)

o A fault or error

o Parent process Terminating



Five-State Process Model



Suspended Processes:

• Processor is faster than I/O so all processes could be waiting for I/O

o Swap these processes to disk to free up more memory and use processor

on more processes

• Blocked state becomes suspend state when swapped to disk

• Two new states

o Blocked/Suspend

o Ready/Suspend



Reason for Process Suspension

Reason Comment

Swapping The OS needs to release sufficient main memory to bring in a

process that is ready to execute.

Other OS Reason OS suspects process of causing a problem.

Interactive User Request e.g. debugging or in connection with the use of a resource.

Timing A process may be executed periodically (e.g., an accounting or

system monitoring process) and may be suspended while waiting

for the next time.

Parent Process Request A parent process may wish to suspend execution of a descendent

to examine or modify the suspended process, or to coordinate the

activity of various descendants.

Process Description

Processes and Resources:



Operating System Control Structures:

• For the OS is to manage processes and resources, it must have information about

the current status of each process and resource.

• Tables are constructed for each entity the operating system manages

OS Control Tables

Memory Tables

• Memory tables are used to keep track of both main and secondary memory.

• Must include this information:

o Allocation of main memory to processes

o Allocation of secondary memory to processes

o Protection attributes for access to shared memory regions

o Information needed to manage virtual memory



I/O Tables

• Used by the OS to manage the I/O devices and channels of the computer.

• The OS needs to know

o Whether the I/O device is available or assigned

o The status of I/O operation

o The location in main memory being used as the source or destination of

the I/O transfer

File Tables

• These tables provide information about:

o Existence of files

o Location on secondary memory

o Current Status

o Other attributes.

• Sometimes this information is maintained by a file management system

Process Tables

• To manage processes the OS needs to know details of the processes

o Current state

o Process ID

o Location in memory

• Process control block

o Process image is the collection of program. Data, stack, and attributes

Process Attributes

• We can group the process control block information into three general categories:

o Process identification

o Processor state information

o Process control information



Process Identification

• Each process is assigned a unique numeric identifier.

• Many of the other tables controlled by the OS may use process identifiers to

cross-reference process tables

Processor State Information

• This consists of the contents of processor registers.

o User-visible registers

o Control and status registers

o Stack pointers

• Program status word (PSW)

o contains status information

o Example: the EFLAGS register on Pentium processors

Pentium II EFLAGS Register:



Structure of Process Images in Virtual Memory:



Process Control

Modes of Execution:

Most processors support at least two modes of execution

• User mode

o Less-privileged mode

o User programs typically execute in this mode

• System mode

o More-privileged mode

o Kernel of the operating system

Process Creation:

• Once the OS decides to create a new process it:

o Assigns a unique process identifier

o Allocates space for the process

o Initializes process control block

o Sets up appropriate linkages

o Creates or expand other data structures

Process switch is switch the process state between the status like read, blocked,

suspend. Mode switch is the switch the process privilege between the mode like use

mode, kernel mode. Generally a mode switch is considered less expensive compared to a

process switch.

Process Switching:

• Several design issues are raised regarding process switching

o What events trigger a process switch?

o We must distinguish between mode switching and process switching.

o What must the OS do to the various data structures under its control to

achieve a process switch?



When to switch processes?

A process switch may occur any time that the OS has gained control from the

currently running process. Possible events giving OS control are:

Mechanism Cause Use

Interrupt External to the execution of

the current instruction

Reaction to an asynchronous

external event

Trap Associated with the execution

of the current instruction

Handling of an error or an

exception condition

Supervisor call Explicit request Call to an operating system

function

The steps in a process switch are:

1. Save context of processor including program counter and other registers

2. Update the process control block of the process that is currently in the

Running state

3. Move process control block to appropriate queue – ready; blocked;

ready/suspend

4. Select another process for execution

5. Update the process control block of the process selected

6. Update memory-management data structure

7. Restore context of the selected process



Execution of the Operating System



Non-process Kernel (a)

• Execute kernel outside of any process

• The concept of process is considered to apply only to user programs

o Operating system code is executed as a separate entity that operates in

privileged mode

Execution within User Processes (b)

o Operating system software within context of a user process

o No need for Process Switch to run OS routine

Process-based Operating System(c)

• Process-based operating system

o Implement the OS as a collection of system process



Processes and Threads

• Resource ownership - process includes a virtual address space to hold the process

image

• Scheduling/execution- follows an execution path that may be interleaved with

other processes

• These two characteristics are treated independently by the operating system

o Dispatching is referred to as a thread or lightweight process

Multithreading:

• Operating system supports multiple threads of execution within a single process

• MS-DOS supports a single thread

• UNIX supports multiple user processes but only supports one thread per process

• Windows, Solaris, Linux, Mach, and OS/2 support multiple threads

Process:

• Have a virtual address space which holds the process image

• Protected access to processors, other processes, files, and I/O resources

Thread:

• An execution state (running, ready, etc.)

• Saved thread context when not running

• Has an execution stack

• Some per-thread static storage for local variables

• Access to the memory and resources of its process

o All threads of a process share this



Benefits of Threads

• Takes less time to create a new thread than a process

• Less time to terminate a thread than a process

• Less time to switch between two threads within the same process

• Since threads within the same process share memory and files, they can

communicate with each other without invoking the kernel

Uses of Threads in a Single-User Multiprocessing System

• Foreground to background work

• Asynchronous processing

• Speed of execution

• Modular program structure

Threads

• Suspending a process involves suspending all threads of the process since all

threads share the same address space

• Termination of a process, terminates all threads within the process

Thread States:

• States associated with a change in thread state

o Spawn

• Spawn another thread

o Block

o Unblock

o Finish

• Deallocate register context and stacks



Adobe PageMaker

User-Level Threads

• All thread management is done by the application

• The kernel is not aware of the existence of threads



Kernel-Level Threads:

• Windows is an example of this approach

• Kernel maintains context information for the process and the threads

• Scheduling is done on a thread basis

Combined Approaches

• Example is Solaris

• Thread creation done in the user space

• Bulk of scheduling and synchronization of threads within application



Symmetric Multiprocessing and Microkernels

• Kernel can execute on any processor

• Typically each processor does self-scheduling form the pool of available process

or threads

SMP Architecture:

Categories of Computer Systems:

• Single Instruction Single Data (SISD) stream

o Single processor executes a single instruction stream to operate on data

stored in a single memory

• Single Instruction Multiple Data (SIMD) stream

o Each instruction is executed on a different set of data by the different

processors

• Multiple Instruction Single Data (MISD) stream

o A sequence of data is transmitted to a set of processors, each of which

executes a different instruction sequence. Never implemented

• Multiple Instruction Multiple Data (MIMD)

o A set of processors simultaneously execute different instruction sequences

on different data sets



Multiprocessor Operating System Design Considerations:

• Simultaneous concurrent processes or threads

• Scheduling

• Synchronization

• Memory management

• Reliability and fault tolerance

Microkernels:

• Small operating system core

• Contains only essential core operating systems functions

• Many services traditionally included in the operating system are now external

subsystems

o Device drivers

o File systems

o Virtual memory manager

o Windowing system

o Security services



Benefits of a Microkernel Organization:

• Uniform interface on request made by a process

o Don’t distinguish between kernel-level and user-level services

o All services are provided by means of message passing

• Extensibility

o Allows the addition of new services

• Flexibility

o New features added

o Existing features can be subtracted

• Portability

o Changes needed to port the system to a new processor is changed in the

microkernel - not in the other services

• Reliability

o Modular design

o Small microkernel can be rigorously tested



• Distributed system support

o Message are sent without knowing what the target machine is

• Object-oriented operating system

o Components are objects with clearly defined interfaces that can be

interconnected to form software

Microkernel Design:

• Low-level memory management

o Mapping each virtual page to a physical page frame

• Interprocess Communication

• I/O and Interrupt Management



Windows Thread and SMP Management

• Implemented as objects

• An executable process may contain one or more threads

• Both processes and thread objects have built-in synchronization capabilities



Windows Process Object Windows Thread Object

Windows 2000 Thread States

• Ready

• Standby

• Running

• Waiting

• Transition

• Terminated



CASE STUDIES: SOLARIS, UNIX, LINUX

• Process includes the user’s address space, stack, and process control block

• User-level threads

• Lightweight processes (LWP)

• Kernel threads



Solaris Lightweight Data Structure:

• Identifier

• Priority

• Signal mask

• Saved values of user-level registers

• Kernel stack

• Resource usage and profiling data

• Pointer to the corresponding kernel thread

• Pointer to the process structure



Linux Task Data Structure:

• State

• Scheduling information

• Identifiers

• Interprocess communication

• Links

• Times and timers

• File system

• Address space

• Processor-specific context

Linux States of a Process:

• Running

• Interruptable

• Uninterruptable

• Stopped

• Zombie



UNIT 3

CONCURRENCY AND SCHEDULING

Principles of concurrency - Mutual exclusion, Semaphores, Monitors, Readers/Writers

Problem; Deadlocks –Prevention- Avoidance – Detection; Scheduling: Types of

scheduling – Scheduling Algorithms, Case studies-UNIX scheduling.

Concurrency

3 different contexts:

• Multiple applications: Allow processing time to be shared

• Structured applications: set of concurrent processes

• Operating system structure: implemented as set of processes or threads.



Principles of Concurrency

In single processor multiprogramming system, process is interleaved in time to

yield the appearance of simultaneous execution.

Difficulties of Concurrency:

• Sharing of global resources

• Operating system managing the allocation of resources optimally

• Difficult to locate programming errors

Process Interaction:



Competition among Processes:

• Mutual Exclusion

o Critical sections

• Only one program at a time is allowed in its critical section

• Example only one process at a time is allowed to send

command to the printer

• Deadlock

• Starvation

Mutual Exclusion

Hardware Support:

• Interrupt Disabling

o A process runs until it invokes an operating system service or until it

is interrupted

o Disabling interrupts guarantees mutual exclusion

o Processor is limited in its ability to interleave programs

o Multiprocessing

• disabling interrupts on one processor will not guarantee

mutual exclusion

• Test and Set Instruction

boolean testset (int i) {

if (i == 0) {

i = 1;

return true;

}

else {

return false;

}

}



• Exchange Instruction

void exchange(int register,int memory)

{

int temp;

temp = memory;

memory = register;

register = temp;

}



Semaphores

• Special variable called a semaphore is used for signaling

• If a process is waiting for a signal, it is suspended until that signal is sent

• Semaphore is a variable that has an integer value

o May be initialized to a nonnegative number

o Wait operation decrements the semaphore value

o Signal operation increments semaphore value

Semaphore Primitives:



Binary Semaphore Primitives:



Mutual Exclusion Using Semaphores:



Producer/Consumer Problem:

• One or more producers are generating data and placing these in a buffer

• A single consumer is taking items out of the buffer one at time

• Only one producer or consumer may access the buffer at any one time



Monitors

• Monitor is a software module

• Chief characteristics

o Local data variables are accessible only by the monitor

o Process enters monitor by invoking one of its procedures

o Only one process may be executing in the monitor at a time



Synchronization:

• Sender and receiver may or may not be blocking (waiting for message)

• Blocking send, blocking receive

o Both sender and receiver are blocked until message is delivered

o Called a rendezvous

• Nonblocking send, blocking receive

o Sender continues on

o Receiver is blocked until the requested message arrives

• Nonblocking send, nonblocking receive

o Neither party is required to wait

Addressing:

• Direct addressing

o Send primitive includes a specific identifier of the destination process

o Receive primitive could know ahead of time which process a message is

expected

o Receive primitive could use source parameter to return a value when the

receive operation has been performed

• Indirect addressing

o Messages are sent to a shared data structure consisting of queues

o Queues are called mailboxes

o One process sends a message to the mailbox and the other process picks

up the message from the mailbox



Message Format:



Readers/Writers Problem

• Any number of readers may simultaneously read the file

• Only one writer at a time may write to the file

• If a writer is writing to the file, no reader may read it

Deadlock

• Permanent blocking of a set of processes that either compete for system resources

or communicate with each other

• No efficient solution

• Involve conflicting needs for resources by two or more processes



Reusable Resources:

• Used by only one process at a time and not depleted by that use

• Processes obtain resources that they later release for reuse by other processes

• Processors, I/O channels, main and secondary memory, devices, and data

structures such as files, databases, and semaphores

• Deadlock occurs if each process holds one resource and requests the other

Example of Deadlock:

Resource Allocation Graphs:

• Directed graph that depicts a state of the system of resources and processes



Conditions for Deadlock:

• Mutual exclusion

o Only one process may use a resource at a time

• Hold-and-wait

o A process may hold allocated resources while awaiting assignment of

others

• No preemption

o No resource can be forcibly removed from a process holding it

• Circular wait

A closed chain of processes exists, such that each process holds at least one resource

needed by the next process in the chain

Deadlock Prevention

• Mutual Exclusion

o Must be supported by the operating system

• Hold and Wait

o Require a process request all of its required resources at one time

• No Preemption

o Process must release resource and request again

o Operating system may preempt a process to require it releases its

resources

• Circular Wait

o Define a linear ordering of resource types



Deadlock Avoidance

• A decision is made dynamically whether the current resource allocation request

will, if granted, potentially lead to a deadlock

• Requires knowledge of future process request

Two Approaches to Deadlock Avoidance:

• Do not start a process if its demands might lead to deadlock

• Do not grant an incremental resource request to a process if this allocation might

lead to deadlock

Deadlock Avoidance:

• Maximum resource requirement must be stated in advance

• Processes under consideration must be independent; no synchronization

requirements

• There must be a fixed number of resources to allocate

• No process may exit while holding resources

Deadlock Detection



Strategies once Deadlock Detected:

• Abort all deadlocked processes

• Back up each deadlocked process to some previously defined checkpoint, and

restart all process

o Original deadlock may occur

• Successively abort deadlocked processes until deadlock no longer exists

• Successively preempt resources until deadlock no longer exists

Selection Criteria Deadlocked Processes:

• Least amount of processor time consumed so far

• Least number of lines of output produced so far

• Most estimated time remaining

• Least total resources allocated so far

• Lowest priority

Strengths and Weaknesses of the Strategies:



Scheduling

Aim of Scheduling:

• Assign processes to be executed by the processor(s)

• Response time

• Throughput

• Processor efficiency

Long-Term Scheduling

• Determines which programs are admitted to the system for processing

• Controls the degree of multiprogramming

• More processes, smaller percentage of time each process is executed

Medium-Term Scheduling

• Part of the swapping function

• Based on the need to manage the degree of multiprogramming



Short-Term Scheduling

• Known as the dispatcher

• Executes most frequently

• Invoked when an event occurs

o Clock interrupts

o I/O interrupts

o Operating system calls

o Signals



Scheduling Algorithms

• First-Come-First-Serve (FCFS)

• Round-Robin

• Shortest Process Next

• Shortest Remaining Time

• Highest Response Ratio Next (HRRN)



First-Come-First-Serve (FCFS):

• Each process joins the Ready queue

• When the current process ceases to execute, the oldest process in the Ready

queue is selected

• A short process may have to wait a very long time before it can execute

• Favors CPU-bound processes

• I/O processes have to wait until CPU-bound process completes

Round-Robin (RR):

• Uses preemption based on a clock

• An amount of time is determined that allows each process to use the

processor for that length of time

• Clock interrupt is generated at periodic intervals

• When an interrupt occurs, the currently running process is placed in the

read queue

o Next ready job is selected

• Known as time slicing



Shortest Process Next (SPN):

• Nonpreemptive policy

• Process with shortest expected processing time is selected next

• Short process jumps ahead of longer processes

• Predictability of longer processes is reduced

• If estimated time for process not correct, the operating system may abort it

• Possibility of starvation for longer processes

Shortest Remaining Time (SRT):

• Preemptive version of shortest process next policy

• Must estimate processing time



Highest Response Ratio Next (HRRN):

• Choose next process with the greatest ratio

CASE STUDY-UNIX Scheduling

• Multilevel feedback using round robin within each of the priority queues

• If a running process does not block or complete within 1 second, it is

preempted

• Priorities are recomputed once per second

• Base priority divides all processes into fixed bands of priority levels

Bands

• Decreasing order of priority

o Swapper

o Block I/O device control

o File manipulation

o Character I/O device control

o User processes



UNIT 4

MEMORY

Memory management requirements, Partitioning, Paging, and Segmentation; Virtual

memory - Hardware and control structures, operating system software- Linux memory

management, Case studies- WINDOWS memory management, UNIX and SOLARIS

Memory management

Memory Management:

• Subdividing memory to accommodate multiple processes

• Memory needs to be allocated to ensure a reasonable supply of ready processes to

consume available processor time

Memory Management Requirements

• Relocation

o Programmer does not know where the program will be placed in memory

when it is executed

o While the program is executing, it may be swapped to disk and returned to

main memory at a different location (relocated)

o Memory references must be translated in the code to actual physical

memory address

• Protection

o Processes should not be able to reference memory locations in another

process without permission

o Impossible to check absolute addresses at compile time

o Must be checked at rum time

o Memory protection requirement must be satisfied by the processor

(hardware) rather than the operating system (software)

o Operating system cannot anticipate all of the memory references a

program will make



• Sharing

o Allow several processes to access the same portion of memory

o Better to allow each process access to the same copy of the program rather

than have their own separate copy

• Logical Organization

o Programs are written in modules

o Modules can be written and compiled independently

o Different degrees of protection given to modules (read-only, execute-only)

o Share modules among processes

• Physical Organization

o Memory available for a program plus its data may be insufficient

• Overlaying allows various modules to be assigned the same region

of memory

o Programmer does not know how much space will be available



Partitioning

Fixed Partitioning:

• Equal-size partitions

o Any process whose size is less than or equal to the partition size can be

loaded into an available partition

o If all partitions are full, the operating system can swap a process out of a

partition

o A program may not fit in a partition. The programmer must design the

program with overlays

o Main memory use is inefficient. Any program, no matter how small,

occupies an entire partition. This is called internal fragmentation.



Placement Algorithm with Partitions:

• Equal-size partitions

o Because all partitions are of equal size, it does not matter which partition

is used

• Unequal-size partitions

o Can assign each process to the smallest partition within which it will fit

o Queue for each partition

o Processes are assigned in such a way as to minimize wasted memory

within a partition



Dynamic Partitioning

• Partitions are of variable length and number

• Process is allocated exactly as much memory as required

• Eventually get holes in the memory. This is called external fragmentation

• Must use compaction to shift processes so they are contiguous and all free

memory is in one block



Dynamic Partitioning Placement Algorithm:

Operating system must decide which free block to allocate to a process

• Best-fit algorithm

o Chooses the block that is closest in size to the request

o Worst performer overall

o Since smallest block is found for process, the smallest amount of

fragmentation is left

o Memory compaction must be done more often

• First-fit algorithm

o Scans memory form the beginning and chooses the first available block

that is large enough

o Fastest

o May have many process loaded in the front end of memory that must be

searched over when trying to find a free block

• Next-fit

o Scans memory from the location of the last placement

o More often allocate a block of memory at the end of memory where the

largest block is found

o The largest block of memory is broken up into smaller blocks

o Compaction is required to obtain a large block at the end of memory



Buddy System:

• Entire space available is treated as a single block of 2U

• If a request of size s such that 2U-1 < s <= 2U, entire block is allocated

o Otherwise block is split into two equal buddies

o Process continues until smallest block greater than or equal to s is

generated



Relocation

• When program loaded into memory the actual (absolute) memory locations are

determined

• A process may occupy different partitions which means different absolute

memory locations during execution (from swapping)

• Compaction will also cause a program to occupy a different partition which

means different absolute memory locations

Addresses

• Logical

o Reference to a memory location independent of the current assignment of

data to memory

o Translation must be made to the physical address

• Relative

o Address expressed as a location relative to some known point

• Physical

o The absolute address or actual location in main memory



Paging

• Partition memory into small equal fixed-size chunks and divide each process into

the same size chunks

• The chunks of a process are called pages and chunks of memory are called frames

• Operating system maintains a page table for each process

o Contains the frame location for each page in the process

o Memory address consist of a page number and offset within the page

Assignment of Process Pages to Free Frames:



Segmentation

• All segments of all programs do not have to be of the same length

• There is a maximum segment length

• Addressing consist of two parts - a segment number and an offset

• Since segments are not equal, segmentation is similar to dynamic partitioning



Virtual Memory

Hardware and Control Structures:

• Memory references are dynamically translated into physical addresses at run time

o A process may be swapped in and out of main memory such that it

occupies different regions

• A process may be broken up into pieces that do not need to located contiguously

in main memory

• All pieces of a process do not need to be loaded in main memory during execution

Execution of a Program:

• Operating system brings into main memory a few pieces of the program

• Resident set - portion of process that is in main memory

• An interrupt is generated when an address is needed that is not in main memory

• Operating system places the process in a blocking state

• Piece of process that contains the logical address is brought into main memory

o Operating system issues a disk I/O Read request

o Another process is dispatched to run while the disk I/O takes place

o An interrupt is issued when disk I/O complete which causes the operating

system to place the affected process in the Ready state

Types of Memory:

• Real memory

o Main memory

• Virtual memory

o Memory on disk

o Allows for effective multiprogramming and relieves the user of tight

constraints of main memory

Thrashing

• Swapping out a piece of a process just before that piece is needed

• The processor spends most of its time swapping pieces rather than executing user

instructions



Principle of Locality

• Program and data references within a process tend to cluster

• Only a few pieces of a process will be needed over a short period of time

• Possible to make intelligent guesses about which pieces will be needed in the

future

• This suggests that virtual memory may work efficiently

Paging

• Each process has its own page table

• Each page table entry contains the frame number of the corresponding page in

main memory

• A bit is needed to indicate whether the page is in main memory or not

Modify Bit in Page Table:

• Modify bit is needed to indicate if the page has been altered since it was last

loaded into main memory

• If no change has been made, the page does not have to be written to the disk when

it needs to be swapped out



Inverted Page Table:

• Page number

• Process identifier

• Control bits

• Chain pointer



Translation Lookaside Buffer:

• Each virtual memory reference can cause two physical memory accesses

o One to fetch the page table

o One to fetch the data

• To overcome this problem a high-speed cache is set up for page table entries

o Called a Translation Lookaside Buffer (TLB)

• Given a virtual address, processor examines the TLB

• If page table entry is present (TLB hit), the frame number is retrieved and the real

address is formed

• If page table entry is not found in the TLB (TLB miss), the page number is used

to index the process page table

• First checks if page is already in main memory

o If not in main memory a page fault is issued

• The TLB is updated to include the new page entry



Page Size:

• Smaller page size, less amount of internal fragmentation

• Smaller page size, more pages required per process

• More pages per process means larger page tables

• Larger page tables means large portion of page tables in virtual memory

• Secondary memory is designed to efficiently transfer large blocks of data so a

large page size is better

• Small page size, large number of pages will be found in main memory

• As time goes on during execution, the pages in memory will all contain portions

of the process near recent references. Page faults low.

• Increased page size causes pages to contain locations further from any recent

reference. Page faults rise.

Segmentation:

• May be unequal, dynamic size

• Simplifies handling of growing data structures

• Allows programs to be altered and recompiled independently

• Lends itself to sharing data among processes

• Lends itself to protection

Segment Tables:

• Corresponding segment in main memory

• Each entry contains the length of the segment

• A bit is needed to determine if segment is already in main memory

• Another bit is needed to determine if the segment has been modified since it was

loaded in main memory



Combined Paging and Segmentation:

• Paging is transparent to the programmer

• Segmentation is visible to the programmer

• Each segment is broken into fixed-size pages



Basic Replacement Algorithms:

• Optimal policy

o Selects for replacement that page for which the time to the next reference

is the longest

o Impossible to have perfect knowledge of future events

• Least Recently Used (LRU)

o Replaces the page that has not been referenced for the longest time

o By the principle of locality, this should be the page least likely to be

referenced in the near future

o Each page could be tagged with the time of last reference. This would

require a great deal of overhead



• First-in, first-out (FIFO)

o Treats page frames allocated to a process as a circular buffer

o Pages are removed in round-robin style

o Simplest replacement policy to implement

o Page that has been in memory the longest is replaced

o These pages may be needed again very soon

• Clock Policy

o Additional bit called a use bit

o When a page is first loaded in memory, the use bit is set to 1

o When the page is referenced, the use bit is set to 1

o When it is time to replace a page, the first frame encountered with the use

bit set to 0 is replaced.

o During the search for replacement,each use bit set to 1 is changed to 0



Cleaning Policy:

• Demand cleaning

o A page is written out only when it has been selected for replacement

• Precleaning

o Pages are written out in batches

Load Control:

• Determines the number of processes that will be resident in main memory

• Too few processes, many occasions when all processes will be blocked and much

time will be spent in swapping

• Too many processes will lead to thrashing



Operating System Softwares

Linux Memory Management

2 components:

1. First, Physical memory management system deals with allocating and freeing

pages, groups of pages and small blocks of memory.

2. The second component handles virtual memory, which is memory mapped into

the address space of running processes.

Management of Physical Memory:

Page allocator is responsible for allocating and freeing all physical pages and is

capable of allocating ranges of physically contiguous pages on request.

Buddy-Heap Algorithm:

• Keep track of available physical pages

• Pairs adjacent units of allocatable together.

Fig: Splitting of Memory in a Buddy Heap

Address Translation:

• Page directory

• Page middle directory

• Page table

16KB

4 KB

8 KB

8 KB 8 KB

4 KB



Linux processes:

• Each process gets 3GB virtual memory

• Remaining 1GB for kernel and page tables

• Virtual address space composed of areas with same protection, paging properties

(pageable or not, direction of growth)

• Each process has a linked list of areas, sorted by virtual address (text, data,

memory-mapped-files,…)

Linux main memory management:

• Kernel never swapped

• The rest: user pages, file system buffers, variable-size device drivers

• The buddy algorithm is used. In addition:

o Linked lists of same-size free blocks are maintained

o To reduce internal fragmentation, a second memory allocation scheme

manages smaller units inside buddy-blocks

• Demand paging (no pre-paging)

• Dynamic backing store management.



Windows Memory Management

Virtual Address Space:

• Virtual address space layout for 3 user processes

• White areas are private per process

• Shaded areas are shared among all processes



Memory Management Concepts:

• Each virtual page can be in one of following states:

o Free/invalid – Currently not in use, a reference causes access violation

o Committed – code/data was mapped to virtual page

o Reserved – allocated to thread, not mapped yet. When a new thread starts,

1MB of process space is reserved to its stack

o Readable/writable/executable

• Dynamic (just-in-time) backing store management

o Improves performance of writing modified data in chunks

o Up to 16 pagefiles

• Supports memory-mapped files

• Can use 4K or 4M pages

Implementation of Memory Management:

Fig: A page table entry for a mapped page on the Pentium



Physical Memory Management:

Fig: Various page lists and transitions between them



CASE STUDIES

UNIX and SOLARIS Memory Management

Paging system:

• Page replacement:

o Page frame data table used for page replacement

o Lists are created within table using pointers (e.g., list of free

frames available; when number of frames on this table drops below

a threshold, the kernel will steal a number of pages to compensate)

o Page replacement algorithm in SVR4 is a modified clock policy

algorithm, the two-handed clock algorithm

• It uses the reference bit in the page table entry for each

page in memory that is eligible (not locked) to be swapped

out

• Bit is set to 0 when page is first brought in and set to 1

when page is referenced for read or write

• The front-hand of the algorithm sweeps through the eligible

pages and sets reference bits to 1

• The backhand, later, sweeps same list and checks the

referenced bit: if 0, page is placed on the list to be paged

out

• Parameters: Scan rate and Handspead

• Kernel memory allocator:

• Requirement

o Kernel generates and destroys frequently small tables and buffers

(e.g, file descriptor blocks) which require dynamic memory

allocation

o These tables and buffers are much smaller than typical machine

page size: paging mechanism would be inefficient



• Solution: SVR4 uses the lazy buddy system

o Observation: demand for blocks of particular size varies slowly in

time

o Solution: defer coalescing blocks until it seems likely that it is

needed and then coalesce as many blocks as possible

o Strategy: try to maintain a pool of locally free blocks and only

invoke coalescing if the number of free blocks exceeds a threshold

o Criterion for coalescing: the number of locally free blocks of a

given size should not exceed the number of allocated blocks of that

size



UNIX Page Daemon:

• It is assumed useful to keep a pool of free pages

• freeing of page frames is done by a pagedaemon - a process that sleeps most of

the time

• awakened periodically to inspect the state of memory - if less than ¼ 'th of page

frames are free, then it frees page frames

• this strategy performs better than evicting pages when needed (and writing the

modified to disk in a hurry)

• The net result is the use of all of available memory as page-pool

• Uses a global clock algorithm – two-handed clock



UNIT 5

INPUT/OUTPUT AND FILE SYSTEMS

I/O management and disk scheduling – I/O devices, organization of I/O functions; OS

design issues, I/O Buffering, disk scheduling, Disk cache, File management –

Organization, Directories, File sharing, Record blocking, Secondary storage

management; Case studies-LINUX I/O, UNIX File management.

I/O Management and Disk Scheduling

I/O Devices

Categories of I/O Devices:

• Human readable

o Used to communicate with the user

o Printers

o Video display terminals

o Display

o Keyboard

o Mouse

• Machine readable

o Used to communicate with electronic equipment

o Disk and tape drives

o Sensors

o Controllers

o Actuators

• Communication

o Used to communicate with remote devices

o Digital line drivers

o Modems

Differences in I/O Devices

• Data rate

o May be differences of several orders of magnitude between the data

transfer rates



• Application

o Disk used to store files requires file management software

o Disk used to store virtual memory pages needs special hardware and

software to support it

o Terminal used by system administrator may have a higher priority

• Complexity of control

• Unit of transfer

o Data may be transferred as a stream of bytes for a terminal or in larger

blocks for a disk

• Data representation

o Encoding schemes

• Error conditions

o Devices respond to errors differently

Organization of the I/O Function

• Programmed I/O

o Process is busy-waiting for the operation to complete

• Interrupt-driven I/O

o I/O command is issued

o Processor continues executing instructions

o I/O module sends an interrupt when done

• Direct Memory Access (DMA)

o DMA module controls exchange of data between main memory and the

I/O device

o Processor interrupted only after entire block has been transferred



Evolution of the I/O Function:

• Controller or I/O module with interrupts

o Processor does not spend time waiting for an I/O operation to be

performed

• Direct Memory Access

o Blocks of data are moved into memory without involving the processor

o Processor involved at beginning and end only

• I/O module is a separate processor

• I/O processor

o I/O module has its own local memory

o It’s a computer in its own right

Direct Memory Access:

• Processor delegates I/O operation to the DMA module

• DMA module transfers data directly to or form memory

• When complete DMA module sends an interrupt signal to the processor



Operating System Design Issues

• Efficiency

o Most I/O devices extremely slow compared to main memory

o Use of multiprogramming allows for some processes to be waiting on I/O

while another process executes

o I/O cannot keep up with processor speed

o Swapping is used to bring in additional Ready processes which is an I/O

operation

• Generality

o Desirable to handle all I/O devices in a uniform manner

o Hide most of the details of device I/O in lower-level routines so that

processes and upper levels see devices in general terms such as read,

write, open, close, lock, unlock



I/O Buffering

Reasons for buffering:

o Processes must wait for I/O to complete before proceeding

o Certain pages must remain in main memory during I/O

• Block-oriented

o Information is stored in fixed sized blocks

o Transfers are made a block at a time

o Used for disks and tapes

• Stream-oriented

o Transfer information as a stream of bytes

o Used for terminals, printers, communication ports, mouse and other

pointing devices, and most other devices that are not secondary storage

Single Buffer

• Block-oriented

o User process can process one block of data while next block is read in

o Swapping can occur since input is taking place in system memory, not

user memory

o Operating system keeps track of assignment of system buffers to user

processes

• Stream-oriented

o Used a line at time

o User input from a terminal is one line at a time with carriage return

signaling the end of the line

o Output to the terminal is one line at a time



Double Buffer

• Use two system buffers instead of one

• A process can transfer data to or from one buffer while the operating system

empties or fills the other buffer

Circular Buffer

• More than two buffers are used

• Each individual buffer is one unit in a circular buffer

• Used when I/O operation must keep up with process



Disk Scheduling

Disk Performance Parameters

o To read or write, the disk head must be positioned at the desired track and

at the beginning of the desired sector

o Seek time

o Time it takes to position the head at the desired track

o Rotational delay or rotational latency

o Time it takes for the beginning of the sector to reach the head

Timing of a Disk I/O Transfer



Disk Performance Parameters:

• Access time

o Sum of seek time and rotational delay

o The time it takes to get in position to read or write

• Data transfer occurs as the sector moves under the head

Disk Scheduling Policies:

• Seek time is the reason for differences in performance

• For a single disk there will be a number of I/O requests

• If requests are selected randomly, we will poor performance

Disk Scheduling Algorithms:

• First-in, first-out (FIFO)

o Process request sequentially

o Fair to all processes

o Approaches random scheduling in performance if there are many

processes

• Priority

o Goal is not to optimize disk use but to meet other objectives

o Short batch jobs may have higher priority

o Provide good interactive response time

• Last-in, first-out

o Good for transaction processing systems

• The device is given to the most recent user so there should be little

arm movement

o Possibility of starvation since a job may never regain the head of the line



• Shortest Service Time First

o Select the disk I/O request that requires the least movement of the disk

arm from its current position

o Always choose the minimum Seek time

• SCAN

o Arm moves in one direction only, satisfying all outstanding requests until

it reaches the last track in that direction

o Direction is reversed

• C-SCAN

o Restricts scanning to one direction only

o When the last track has been visited in one direction, the arm is returned to

the opposite end of the disk and the scan begins again



• N-step-SCAN

o Segments the disk request queue into subqueues of length N

o Subqueues are processed one at a time, using SCAN

o New requests added to other queue when queue is processed

• FSCAN

o Two queues

o One queue is empty for new requests

Disk Cache

• Buffer in main memory for disk sectors

• Contains a copy of some of the sectors on the disk

• Least Recently Used

Least Recently Used:

• The block that has been in the cache the longest with no reference to it is replaced

• The cache consists of a stack of blocks

• Most recently referenced block is on the top of the stack

• When a block is referenced or brought into the cache, it is placed on the top of the

stack



File Management

• File management system consists of system utility programs that run as privileged

applications

• Input to applications is by means of a file

• Output is saved in a file for long-term storage

File System Properties:

• Long-term existence

• Sharable between processes

• Structure

File Operations:

• Create

• Delete

• Open

• Close

• Read

• Write



Terms Used with Files:

• Field

o Basic element of data

o Contains a single value

o Characterized by its length and data type

• Record

o Collection of related fields

o Treated as a unit

Example: employee record

• File

o Collection of similar records

o Treated as a single entity

o Have file names

o May restrict access

• Database

o Collection of related data

o Relationships exist among elements

Typical Operations:

• Retrieve All

• Retrieve One

• Retrieve Next

• Retrieve Previous

• Insert One

• Delete One

• Update One

• Retrieve Few



File Management Systems

• The way a user of application may access files

• Programmer does not need to develop file management software

• Meet the data management needs and requirements of the user

• Guarantee that the data in the file are valid

• Optimize performance

• Provide I/O support for a variety of storage device types

• Minimize or eliminate the potential for lost or destroyed data

• Provide a standardized set of I/O interface routines

• Provide I/O support for multiple users



File Management Functions:

• Identify and locate a selected file

• Use a directory to describe the location of all files plus their attributes

• On a shared system describe user access control

• Blocking for access to files

• Allocate files to free blocks

• Manage free storage for available blocks



File Organization

Criteria for File Organization:

• Short access time

o Needed when accessing a single record

o Not needed for batch mode

• Ease of update

o File on CD-ROM will not be updated, so this is not a concern

• Economy of storage

o Should be minimum redundancy in the data

o Redundancy can be used to speed access such as an index

• Simple maintenance

• Reliability

File Organization:

• The Pile

o Data are collected in the order they arrive

o Purpose is to accumulate a mass of data and save it

o Records may have different fields

o No structure

o Record access is by exhaustive search



• The Sequential File

o Fixed format used for records

o Records are the same length

o All fields the same (order and length)

o Field names and lengths are attributes of the file

o One field is the key filed

• Uniquely identifies the record

• Records are stored in key sequence

o New records are placed in a log file or transaction file

o Batch update is performed to merge the log file with the master file

• Indexed Sequential File

• Index provides a lookup capability to quickly reach the vicinity of the

desired record

o Contains key field and a pointer to the main file

o Indexed is searched to find highest key value that is equal to or

precedes the desired key value

o Search continues in the main file at the location indicated by the

pointer



• New records are added to an overflow file

• Record in main file that precedes it is updated to contain a pointer to the

new record

• The overflow is merged with the main file during a batch update

• Multiple indexes for the same key field can be set up to increase efficiency

• Indexed File

• Uses multiple indexes for different key fields

• May contain an exhaustive index that contains one entry for every record

in the main file

• May contain a partial index



• The Direct or Hashed File

o Directly access a block at a known address

o Key field required for each record

File Directories

• Contains information about files

o Attributes

o Location

o Ownership

• Directory itself is a file owned by the operating system

• Provides mapping between file names and the files themselves

Simple Structure for a Directory:

• List of entries, one for each file

• Sequential file with the name of the file serving as the key

• Provides no help in organizing the files

• Forces user to be careful not to use the same name for two different files



Two-level Scheme for a Directory:

• One directory for each user and a master directory

• Master directory contains entry for each user

o Provides address and access control information

• Each user directory is a simple list of files for that user

• Still provides no help in structuring collections of files

Hierarchical or Tree-Structured Directory:

• Master directory with user directories underneath it

• Each user directory may have subdirectories and files as entries

• Files can be located by following a path from the root, or master, directory down

various branches

• This is the pathname for the file

• Can have several files with the same file name as long as they have unique path

names

• Current directory is the working directory

• Files are referenced relative to the working directory



File Sharing

• In multiuser system, allow files to be shared among users

• Two issues

o Access rights

o Management of simultaneous access

Access Rights:

• None

o User may not know of the existence of the file

o User is not allowed to read the user directory that includes the file

• Knowledge

o User can only determine that the file exists and who its owner is

• Execution

o The user can load and execute a program but cannot copy it

• Reading

o The user can read the file for any purpose, including copying and

execution

• Appending

o The user can add data to the file but cannot modify or delete any of the

file’s contents

• Updating

o The user can modify, deleted, and add to the file’s data. This includes

creating the file, rewriting it, and removing all or part of the data

• Changing protection

o User can change access rights granted to other users

• Deletion

o User can delete the file

• Owners

o Has all rights previously listed

o May grant rights to others using the following classes of users

• Specific user

• User groups



• All for public files

Management of simultaneous Access:

• User may lock entire file when it is to be updated

• User may lock the individual records during the update

• Mutual exclusion and deadlock are issues for shared access

Record Blocking

Fixed Blocking:



Variable Blocking-Spanned:

Variable Blocking-Unspanned:



Secondary Storage Management

• Space must be allocated to files

• Must keep track of the space available for allocation

Preallocation:

• Need the maximum size for the file at the time of creation

• Difficult to reliably estimate the maximum potential size of the file

• Tend to overestimated file size so as not to run out of space

Methods of File Allocation:

• Contiguous allocation

o Single set of blocks is allocated to a file at the time of creation

o Only a single entry in the file allocation table

• Starting block and length of the file

o External fragmentation will occur

o Need to perform compaction



• Chained allocation

• Allocation on basis of individual block

• Each block contains a pointer to the next block in the chain

• Only single entry in the file allocation table

o Starting block and length of file

• No external fragmentation

• Best for sequential files

• No accommodation of the principle of locality



• Indexed allocation

o File allocation table contains a separate one-level index for each file

o The index has one entry for each portion allocated to the file

o The file allocation table contains block number for the index



CASE STUDIES

LINUX FILE MANAGEMENT

Linux Virtual File System:

• Uniform file system interface to user processes

• Represents any conceivable file system’s general feature and behavior

• Assumes files are objects that share basic properties regardless of the target file

system



Primary Objects in VFS:

• Superblock object

o Represents a specific mounted file system

• Inode object

o Represents a specific file

• Dentry object

o Represents a specific directory entry

• File object

o Represents an open file associated with a process



UNIX File Management

Types of files:

o Regular, or ordinary

o Directory

o Special

o Named pipes

o Links

o Symbolic links

Inodes:

• Index node

• Control structure that contains key information for a particular file

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