Lecture 09 (1)

8/4/2019 Lecture 09 (1)

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Operating Systems Slide 1 of 22

Operating Systems

Processes

8/4/2019 Lecture 09 (1)



Some Basics

Process: a program in execution

● Running a program may spawn (create) more

than one process

The terms job and process are often used

interchangeably

8/4/2019 Lecture 09 (1)



Process

● Process is more than program code (sometimes

called the text section)

● It also includes current activity, such as:

●

Program counter● Contents of registers

● Process stack (function parameters, return addresses,

local variables)

● Data section (contains global variables)

● Heap (memory dynamically allocated during run-

time)

8/4/2019 Lecture 09 (1)



Process in Memory

Text

Data

Heap

Stack● Two or more instances of

the same program are

treated distinctly

● Their text section may bethe same, but the data

section, heap and stack

will be different

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Process State

As a process executes it changes state

The state of a process is defined in part by what

it is currently doing

● New: the process is being created

● Running: instructions are being executed

● Waiting: process is waiting for some event to

occur● Ready: waiting to be assigned to a processor

● Terminated: finished execution

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Process State Diagram

TerminatedNew

Ready Running

Waiting

admitted

scheduler dispatch

interrupt

event completion event wait

exit

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Process Control Block

Each process in the OS is represented by a Process

Control Block (aka task control block)

Contains information about the process:

●

Process state● Program counter

● CPU registers

● CPU-scheduling information

● Memory-management information

● Accounting information

● I/O status information

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CPU Switch from Process to Process

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Threads

Process model discussed so far implies that

processes execute in a single thread of execution

● Means only one thing can happen at a time

● Think of a thread as a sequence of execution

● Threads are a type of concurrency within a

process

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Process Scheduling

Objective of multiprogramming is to have a

process running at all times, to maximize CPU

utilization

Objective of multitasking is to switch frequently& fast between processes so that user can

interact with all of them

Process Scheduler is the part of the OS thatselects one of the available processes for

execution on the CPU

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Scheduling Queues

● As processes enter the system, they are put

into a job queue

● Processes in main memory that are ready &

waiting are put in the ready queue– Usually stored as a linked list

– Ready-queue header contains pointers to first & last

PCBs in the list

– Each PCB has a field that points to the next in the list

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Scheduling Queues

● When processes need to use an I/O device and

have to wait for it, they enter its device

queue

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Scheduling Queues: Visualization

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Queuing Interactions

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Schedulers

Processes move between various scheduling

queues during their lifetime

● Which process in any given queue should be

picked next?

The selection of a process from all the processes

in a queue is the job of the scheduler

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Long- & Short-Term Schedulers

The Long-Term Scheduler (or job scheduler)

selects processes from the job pool and loads

them into memory for execution

The Short-Term Scheduler (or CPUscheduler) selects from among the processes

that are ready-to-execute for allocating the CPU

to

● Basic difference is in the frequency of execution

● Speed more important for CPU scheduler

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Some Definitions

● Long-term scheduler controls the degree of

multiprogramming (the number of

processes in memory)

●

I/O-bound processes spend more of theirtime doing I/O

● CPU-bound processes spend more of their

time doing computations

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Intricacies of Scheduling

● LTS has to choose a good mix of CPU-bound

& I/O-bound processes

● Too many CPU-bound processes – I/O devices

are under-utilized● Too many I/O-bound processes – CPU is

under-utilized

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Medium-Term Scheduler

Medium-Term Scheduler manages the degree of

multiprogramming

– Removes processes from memory (& thus from CPU

contention)

– Later process can be re-loaded & execution continued

– This unloading & reloading process is called swapping

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Switching Between Processes

● Interrupts cause OS to change CPU from

current task to run kernel routine

● Such events happen frequently

● When interrupt happens, OS needs to save

the context of currently-executing process so

that its state can be restored

● Essentially suspending the process & resuming it

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Context of a Process

The context of a process is represented in its

PCB

● Value of CPU registers

● Process state

● Memory management information

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Context Switch

When switching from one process to another,

the OS performs a state save and then a state

restore

This process is called a context switch– This is pure overhead: no useful work is being done

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Lecture 09 (1)