Processesand OS basics

RHS – SOC 2

OS Basics

• An Operating System (OS) is essentially an abstraction of a computer

• As a user or programmer, I do not care too much about the specifics of a given com-puter – I think of a computer in abstract terms

• A bit like interfaces and implementations…

RHS – SOC 3

OS Basics

A computer monitor…

RHS – SOC 4

OS Basics

• An Operating System shields us from dealing with concrete details of a computer

• We can think of a computer in terms of– A file system– Memory– Input/output devices

• …and not worry about details

RHS – SOC 5

OS Basics

• As application deve-lopers, we interface with the OS, not the hardware

• Still need a basic understanding of the way an OS works

RHS – SOC 6

OS Basics

• Main tasks for an OS:– Hardware operation– Software operation– Memory management– File system management– Security– Networking

RHS – SOC 7

Process management

• Software operation more specifically means process management

• What is a process…?

• A process is a running instance of a computer program

• Similar to the relation between a class and an object

RHS – SOC 8

Process management

• Class / Program– A specification of

behavior– Passive collection of

instructions– Only one definition– Resides in secondary

storage (hard disk)

• Object / Process– A ”living” entity– Active execution of

instructions– Multiple instances can

coexist (usually)– Resides in primary

storage (RAM)

RHS – SOC 9

Process management

• The OS manages the life-cycle of a process– Starting the process– Managing the process

while active– Terminating the process

RHS – SOC 10

Process management

• The complexity of life-cycle management depends on the OS category

• Single-tasking OS – only one process can be active at any time

• Multi-tasking OS – many processes can be active at any time

• Almost all modern OS are multi-tasking – we will focus on that category

RHS – SOC 11

Process management

• In a multi-tasking OS, we can start many tasks, but only have one CPU available

• CPU resources – and other resources – must thus be shared among processes

• Managing this is a key OS task!

• A process may thus be in more states than just ”running” or ”not running”

RHS – SOC 12

Process life-cycle

• A multi-tasking OS will always include a process scheduler

• The process scheduler must decide when resources can be assigned to a specific process, thus making it able to execute

RHS – SOC 13

Process life-cycle

• Process scheduling is not trivial!

• What is the overall goal…?– Fairness– Responsiveness– Meeting a deadline– Minimising waiting time– …and other possible objectives

RHS – SOC 14

Process life-cycle

• In a modern OS, processes can be assigned a priority

• The lower priority, the fewer resources assigned to the process

• Enables the OS to do certain tasks ”in the background”, like– Virus scan– Disk defragmentation

RHS – SOC 15

Process life-cycle

• Starting a process:– First, the process is created – this involves

loading a copy of the program from secondary storage into RAM

– The process is then put in a waiting state by the process scheduler

– The process will remain in the waiting state, until the resources needed by the process become available

RHS – SOC 16

Process life-cycle

• Running a process:– At some point, the needed resources ar

assigned to the process, and the process can start executing. The process is now running

– During the execution, the process can become blocked or again become waiting

– A process becomes blocked if it has to wait for some other action to complete, like opening a file

RHS – SOC 17

Process life-cycle

• Terminating a process– At some point, the process has completed its

task (perhaps stopped by user)– The state of the process then becomes terminated

– The OS can then reclaim the memory used by the (now terminated) process

RHS – SOC 18

Process life-cycle

Created

Waiting Blocked

Running Terminated

RHS – SOC 19

Exercises

• What is the primary purpose of an operating system?• What is the relation between a program and a process?• Try to press Ctrl+Shift+Esc, which brings up the task manager. Go

to Processes – how many processes are (approximately) running on your PC? How many of them can you recognise?

• What makes a multi-tasking OS complex?• What type of priority should a Virus Scanner run with (high or low)?

RHS – SOC 20

Memory management

• A very important part of managing a process is memory management

• The OS has to make sure that memory is available in a transparent and efficient manner for the process

• The OS uses a technique called virtual memory for enable this

RHS – SOC 21

Memory management

• When a process is started, the OS will set up a virtual memory address space for the process

• The process only interacts with the virtual memory adress space

• The OS maps the virtual memory adress space to physical memory (either RAM or secondary storage, e.g hard drive)

RHS – SOC 22

Memory management

RHS – SOC 23

Memory management

• Since the OS cannot predict how much memory a process will need, it is typically given a large virtual address space

• 32-bit OS: Up to 4 GB

• Sum of virtual address spaces often much larger than available RAM

RHS – SOC 24

Memory management

• In the virtual address space, we have three types of data– Program data; the program itself– Stack data; data which is allocated when

methods are called, etc. (local variables)– Heap data; data which is dynamically

allocated, using the new statement

RHS – SOC 25

Memory management

Program data

Stack data

Heap data

RHS – SOC 26

Memory management

• Using RAM as physical memory is much more efficient than using the hard drive

• OS will continuously try to map as much virtual memory to RAM as possible

• Memory is divided into pages (typically less than 1 Mb) – RAM can be considered a cache of most used pages

RHS – SOC 27

Memory management

• Whenever a process accesses (virtual) memory, the OS looks up the correspon-ding page of physical memory– If the page is already in RAM, fine (page hit)– If the page is in secondary storage, it is

swapped into RAM (page fault)

• What page is then swapped out…?

RHS – SOC 28

Memory management

• A page fault is expensive, since it involves copying data from secondary storage

• OS tries to minimise number of page faults

• Usually, the OS will swap out the Least Recently Used (LRU) page

RHS – SOC 29

Memory management

• In general, data placement is always a compromise between speed and volume

• Most modern CPUs have several layers of internal memory, with a similar strategy for memory management

• Managed by the CPU, not the OS

RHS – SOC 30

Memory management

RHS – SOC 31

Memory management

• Other aspects of memory management– Security; preventing exploits such as buffer

overruns or other malicious attacks– Inter-process communication; when two

processes need to exchange data– Optimisation; OS cleans up memory when

processes are terminated, and relocates memory to larger contiguous blocks

RHS – SOC 32

Exercises

• What are the advantages of using Virtual Memory address spaces?• What happens if the running processes use more virtual memory

than the amount of available physical memory?• What is a page hit? a page fault?• Why should the OS try to minimise the number of page faults?• Can you think of other strategies for swapping out memory pages

than the LRU (Least Recently Used) strategy?• See if you can find some information about a modern CPU on the

Internet (e.g Intel Core i7). How many layers of memory cache are on the chip? How much memory is in each layer?