Transcript Processes and OS Basics

Processes
and OS basics
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 computer – I think of a computer in abstract
terms
• A bit like interfaces and implementations…
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OS Basics
A computer
monitor…
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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
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OS Basics
• As application developers, we interface
with the OS, not the
hardware
• Still need a basic
understanding of the
way an OS works
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OS Basics
• Main tasks for an OS:
– Hardware operation
– Software operation
– Memory management
– File system management
– Security
– Networking
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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
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Process management
• Class / Program
• Object / Process
– A specification of
behavior
– Passive collection of
instructions
– Only one definition
– Resides in secondary
storage (hard disk)
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– A ”living” entity
– Active execution of
instructions
– Multiple instances can
coexist (usually)
– Resides in primary
storage (RAM)
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Process management
• The OS manages the
life-cycle of a process
– Starting the process
– Managing the process
while active
– Terminating the process
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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
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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”
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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
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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
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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
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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
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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
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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
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Process life-cycle
Created
Waiting
Blocked
Running
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Terminated
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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)?
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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
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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)
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Memory management
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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
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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
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Memory management
Heap data
Stack data
Program data
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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
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Memory management
• Whenever a process accesses (virtual)
memory, the OS looks up the corresponding 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…?
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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
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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
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Memory management
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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
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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?
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