chapter 2 - os services

8/11/2019 Chapter 2 - OS Services

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Operating-System Services


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2.2 Silberschatz, Galvin and Gagne 2005Operating System Concepts

Operating System Services

One set of operating-system services provides functions that arehelpful to the user:

User interface - Almost all operating systems have a user interface (UI)

Varies between Command-Line (CLI), Graphics User Interface(GUI), Batch

Program execution - The system must be able to load a program intomemory and to run that program, end execution, either normally orabnormally (indicating error)

I/O operations - A running program may require I/O, which may involvea file or an I/O device.

File-system manipulation - The file system is of particular interest.Obviously, programs need to read and write files and directories, createand delete them, search them, list file Information, permissionmanagement.


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Operating System Services (Cont.)

One set of operating-system services provides functions that arehelpful to the user (Cont):

CommunicationsProcesses may exchange information, on the samecomputer or between computers over a network

Communications may be via shared memory or through messagepassing (packets moved by the OS)

Error detectionOS needs to be constantly aware of possible errors

May occur in the CPU and memory hardware, in I/O devices, in userprogram

For each type of error, OS should take the appropriate action to

ensure correct and consistent computing Debugging facilities can greatly enhance the users and

programmers abilities to efficiently use the system


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Operating System Services (Cont.)

Another set of OS functions exists for ensuring the efficient operation of thesystem itself via resource sharing

Resource allocation - When multiple users or multiple jobs runningconcurrently, resources must be allocated to each of them

Many types of resources - Some (such as CPU cycles,mainmemory,and file storage) may have special allocation code, others (such as I/Odevices) may have general request and release code.

Accounting -To keep track of which users use how much and what kindsof computer resources

Protection and security - The owners of information stored in a multiuseror networked computer system may want to control use of that information,concurrent processes should not interfere with each other

Protectioninvolves ensuring that all access to system resources is

controlled Securityof the system from outsiders requires user authentication,

extends to defending external I/O devices from invalid access attempts

If a system is to be protected and secure, precautions must beinstituted throughout it.


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User Operating System Interface - CLI

CLI allows direct command entry Sometimes implemented in kernel, sometimes by systems

program

Sometimes multiple flavors implementedshells

Primarily fetches a command from user and executes it

Sometimes commands built-in, sometimes just names ofprograms

If the latter, adding new features doesnt require shell

modification


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User Operating System Interface - GUI

User-friendly desktopmetaphor interface

Usually mouse, keyboard, and monitor

Iconsrepresent files, programs, actions, etc

Various mouse buttons over objects in the interface causevarious actions (provide information, options, execute function,

open directory (known as a folder) Invented at Xerox PARC

Many systems now include both CLI and GUI interfaces

Microsoft Windows is GUI with CLI command shell

Apple Mac OS X as Aqua GUI interface with UNIX kernel

underneath and shells available

Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)


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Processes


8/362.8 Silberschatz, Galvin and Gagne 2005Operating System Concepts

Process Concept

Processa program in execution; process execution mustprogress in sequential fashion

are active entities which executed in sequentialfashion.

A program residing on your disk will only be named aprocess when it already demands execution.

is more than the program code. It also includes thecurrent state, activity and other relevant information.

*Operating system processes execute system code and userprocesses execute user code.



Process State

As a process executes, it changes state new: The process is being created

running: Instructions are being executed

waiting: The process is waiting for some event tooccur

ready: The process is waiting to be assigned to aprocessor

terminated: The process has finished execution



Diagram of Process State



Process State

Transitions:

dispatch transition from READY to RUNNING. This is theassignment of the CPU to the process.

block (I/O or event wait)transition from RUNNING toWAITING. This happens when a process voluntarily

relinquishes control of the CPU due to some event. wakeup (I/O or event complet ion) transition from

WAITING to READY. This happens when the event beingwaited by a process occurs, e.g. I/O completion.

preempt (interrupt)transition from RUNNING to READY.

This happens when a process is forced out of the CPUbecause a higher priority job is ready to take control of theCPU or in time-sharing systems the process consumes itstime share.


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Process Control Block (PCB)

Process State

defines the current activity of the process.

state may be new, ready, waiting, blocked, or halted.

Program counter

indicates the address of the next instruction to be executed forthis purpose.

CPU Register

the values of the registers must be saved when an interruptoccurs to ensure that the process can progress afterwards.

CPU scheduling information

This information contains various scheduling parameters anddetails. e.g. process priority.

Accounting information

this includes the information like amount of CPU used, time limits,and so on.

I/O status information

This would consist of the list of resources allocated to theprocess.




PCB- simply acts as a repository for any informationthat may vary form process to process.

2 characteristics of process:

1. execution process state, program counter,

scheduling

2. ownership memory management, andresource management.



CPU Switch From Process to Process



Threads

A process is a program that performs a single THREADof execution.

a basic unit of CPU utilization( basic unit of execution).lightweight process

consists of program counter, a register set and a stackspace.

operate like processes in many aspects. ( has severalstates, executes in sequential fashion, can use CPU at a

time)are not independent of one another.designed to aid one another.



Threads

Multithreadingthe ability of an operating system to support multiple threadsof execution within a single process.

Ex. Java run-time environment, windows 2000, Solaris,Linux and OS/2.

Advantages of multithreading:1.Expensive context switching in traditional processes is avoidedsince there is an extensive sharing of resources among peerthreads.2. It takes a far less time to create a new thread in an existingprocess than to create a brand new process.

3.It takes less time to terminate a thread than a process.4.Threads enhance efficient communication between differentexecuting programs.



Process Scheduling

Question: How do we run processes concurrently if we

only have one CPU?

Answer: Process Scheduling (by having an order ofexecution )

Scheduling Queues:

Job queueset of all processes in the system Ready queueset of all processes residing in main

memory, ready and waiting to execute

Device queuesset of processes waiting for an I/Odevice. Each device has its own device queue.

Processes migrate among the various queues



Ready Queue And Various I/O Device Queues



Representation of Process Scheduling

Queueing Diagram

Rectangular box- queueCircle-resourcesArrows- flow of processes



Schedulers

A process migrates between the various schedulingqueues throughout its lifetime.

Long-term scheduler (or job scheduler)selects which

processes should be brought into the ready queue Short-term scheduler (or CPU scheduler)selects

which process should be executed next and allocatesCPU


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Schedulers (Cont.)

Short-term scheduler is invoked very frequently(milliseconds) (must be fast)

Long-term scheduler is invoked very infrequently (seconds,minutes) (may be slow)

The long-term scheduler controls the degree of

mu l t iprogramm ing ( the number of processes in memo ry)

Processes can be described as either:

I/O-bound processspends more time doing I/O thancomputations, many short CPU bursts

CPU-bound process

spends more time doingcomputations; few very long CPU bursts


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Context Switch

When CPU switches to another process, the system mustsave the state of the old process and load the saved statefor the new process

Context-switch time is overhead; the system does no usefulwork while switching

Time dependent on hardware support


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Process Creation

Parent processcreating process

Child processthe new process created by the parent process

Parent process create children processes, which, in turn createother processes, forming a tree of processes

Resource sharing

Parent and children share all resources

Children share subset of parents resources

Parent and child share no resources

Execution

Parent and children execute concurrently

Parent waits until children terminate


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Process Creation (Cont.)

Address space

Child duplicate of parent

Child has a program loaded into it

UNIX examples

forksystem call creates new process execsystem call used after a forkto replace the

process memory space with a new program

* When a process is created, the OS creates a PCB for thesaid process and append this to the list of existing PCBs in

the system. Then, the code for the process is located(usually residing in the secondary store). If at least one ofthese steps is unsuccessful, i.e. no space for PCB isavailable or the code can not be found, then the process isnot created (undoing whatever has been done).


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A tree of processes on a typical Solaris


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Process Termination

5 types of term inat ion:

1. Normal termination - due to a call by the process to a terminationstatement.

- uses exit system call.

2. Abnormal termination - is the forced termination of the process bythe OS due to some unrecoverable error like bound violation,arithmetic error, invalid instructions, I/O failure, data misuse, andother fault conditions.

3. Cascading termination - a phenomenon of terminating childrenprocesses because the parent process is finished.

4. A parent process has already finished execution and childrenprocesses are no longer needed.

- uses abort system call.

5. Process may be killed due to operating system or user starvation.


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Process Termination

To handle process termination, the OS deletes the PCB ofthe process to free the space and if the process is occupyingmemory space such is declared free.

A parent may terminate the execution of one of its childrenfor a variety of reasons, such as:

The child has exceeded its usage of some of the resources it

has been allocated. The task assigned to the child is no longer required.


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Interprocess Communication (IPC)

Mechanism for processes to communicate and to

synchronize their actions (useful in distributed environment,e.g. chat)

Message passing systemprocesses communicate witheach other without resorting to shared variables

IPC facility provides two operations:

send(message)message size fixed or variable

receive(message)

If Pand Qwish to communicate, they need to:

establish a communicationlinkbetween them

exchange messages via send/receive

Implementation of communication link

physical (e.g., shared memory, hardware bus)

logical (e.g., logical properties)


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Types of Message Passing Systems


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Direct Communication

Processes must name each other explicitly:

send(P, message)send a message to process P

receive(Q, message)receive a message from processQ

Properties of communication link

Links are established automatically

A link is associated with exactly one pair ofcommunicating processes

Between each pair there exists exactly one link

The link may be unidirectional, but is usually bi-directional


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Synchronization

Message passing may be either blocking or non-blocking

Blockingis considered synchronous

Blocking send : The sending process is blocked untilthe message is received

Blocking receive : The receiver blocks until a message

is available

Non-blockingis considered asynchronous

Non-blocking send: The sending process sends themessage and resumes operation

Non-blocking receive: The receiver retrieves either avalid message or null


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Buffering

another issue is whether to store messages in the link if theprocess on the other side is not ready to receive it. If themessage is stored, the sender can continue executionimmediately after sending the message.

3 Methods:

1. Zero-capacity does not allow buffering so the otherprocess must wait until other process is ready toreceive the message. Has a queue with length 0

2. Bounded capacityhas a queue with length n and canstore messages until it is full.

3. Unbounded capacity has an infinite queue and thesender is never delayed.

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