BITS Pilani K K Birla Goa Campus

Operating Systems

Dr. Lucy J. Gudino Dept. of CS and IS

BITS Pilani K K Birla Goa Campus

Operating System: Process Management

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As a process executes, it changes state

new: The process is being created.

ready: The process is waiting to be assigned to a process.

running: Instructions are being executed.

waiting(Blocking) : The process is waiting for some event to occur.

terminated (exit): The process has finished execution.

Contd Five State Model

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4 reasons for process creation

System initialization.

A user request to create a new process. Example : user request for file print

Batch environment: a process is created in response to the submission of a job

Interactive environment : a process is created when a new user attempts to log on

Contd

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Normal completion

Time limit exceeded

Memory unavailable

Protection error

Arithmetic error

Bounds violation

I/O failure

Invalid instruction

Privileged instruction

Parent termination

Parent request

Reasons for process termination

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Job queue set of all processes in the system.

Ready queue set of all processes residing in main memory, ready and waiting to execute.

Device queues set of processes waiting for an I/O device.

Process Scheduling Queues

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Queuing Model

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Ready queue and device queue

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Representation of Process Scheduling

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OS must have information about the current state of

processes.

OS must have information about the current state of

resources in the system.

OS must keep track of utilization of resources by

processes.

OS constructs and maintains tables (control structure) of

information about each entity it is managing.

OS as process manager

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Contd

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Operating System Control Tables

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Memory Table

The allocation of main memory to processes

The allocation of secondary memory to processes

Any protection attributes of blocks of main or virtual memory, such as which processes may access certain shared memory regions

Any information needed to manage virtual memory

I/O Table

Status of I/O Operation

the location in main memory being used as the source or destination of the I/O transfer

Tables

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File table

the existence of files,

their location on secondary memory,

current status

other attributes

Process Table

process image : collection of program, data, stack, and PCB

Tables

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Types:

Long-term scheduler (or job scheduler) Decides which processes should be brought into the ready queue.

Short-term scheduler (or CPU scheduler) Decides which process should be executed next and allocates

CPU.

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

The long-term scheduler controls the degree of multiprogramming (the number of processes in memory).

invoked only when a process leaves the system

can be slow

Schedulers

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Long term scheduler I/O bound process and CPU bound process

I/O bound process: spends more time doing I/O than

computations, many short CPU bursts. CPU-bound process spends more time doing

computations; few very long CPU bursts.

Should select a good process mix of I/O bound and CPU bound

System should be balanced

Medium term scheduler Swapping is necessary to improve the process mix

Scheduler

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Addition of Medium Term Scheduling

Operating System Concepts

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When CPU switches to another process, the system must save the state of the old process and load the

saved state for the new process.

Kernel saves context of old process in its PCB and loads the context of new process

Context-switch time is overhead; the system does no useful work while switching.

deciding factors: memory speed, number of registers that must be copied etc.

Time dependent on hardware support. some systems provide multiple sets of registers

change the pointer to the current register set

Context Switch

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

Processes are identified using unique process identifier (pid) : integer number

each process has one parent but zero, one, two, or more children

Process Creation

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Resource sharing

Parent and children share all resources.

Children share subset of parents resources.

Parent and child share no resources.

When a process is created, initialization data (input) may be passed along by the parent process to the child process

Execution : 2 possibilities

Parent and children execute concurrently.

Parent waits until some or all its children terminate.

Process Creation (Cont)

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Address space : two possibilities

Child is a duplicate of parent. (program and data same)

Child has a new program loaded into it.

UNIX examples

fork system call creates new process

exec system call used after a fork to replace the process memory space with a new program.

ps command can be used to list processes

Process Creation (Cont)

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Process executes last statement and asks the operating system to delete it (exit). status of the terminating process is return to parent process

(wait() system call)

Process resources are deallocated by operating system.

Parent may terminate execution of children processes (abort). Child has exceeded allocated resources.

Task assigned to child is no longer required.

Parent is exiting.

Operating system does not allow child to continue if its parent terminates.

Cascading termination.

Process Termination