Transcript Principles of Operating System

Principles of Operating Systems
Abhishek Dubey
Daniel Balasubramanian
Slides Based On Power points and book material from William Stallings
Fall 2014
Operating System Defined
• 2 common meanings:
1. Entire package consisting of the central software
managing a computer’s resources and all of the
accompanying software tools
2. The software that manages and allocates computer
resources (CPU, RAM, DVD, display, etc)
• The second is also called a kernel; this is the
definition we will use
• The kernel itself is an executable; on Linux, this
executable is located at /boot/vmlinuz
Operating System
• A program that
controls the
execution of
application programs
and manages the
computer, which is a
set of resources for
the movement,
storage, and
processing of data
• An interface between
applications and
hardware
Figure 2.1 Computer Hardware and Software Infrastructure
Main objectives of an OS:
• Convenience
• Efficiency
• Ability to evolve
• Functions in the same way as ordinary
computer software
• Program, or suite of programs, executed by
the processor
• Frequently relinquishes control and must
depend on the processor to allow it to
regain control
What does a kernel do?
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Process scheduling
Memory management
Provides file systems
Creates and terminates processes
Provides device access
Networking
Provides a system call interface
Process Scheduling
• The CPUs execute the instructions of programs
• Linux is a preemptive, multitasking OS:
– Multiple processes simultaneously reside in
memory
– The rules governing which process receives the
CPU and for how long are determined by the OS
process scheduler.
• In other words, the kernel decides when to
run a process and how long to run it
Key Elements of an
Operating System
Memory Management
• A process must reside in main memory to run
– But RAM is limited, so the OS has to manage how
memory is allocated to processes
• Linux virtual memory management provides 2
things
– Processes are isolated from one another and from
the kernel
– Only part of a process is kept in memory at a time,
which allows more processes in RAM
simultaneously
Provides File Systems
• File system: an organized collection of regular
files and directories
• A file system provides a way to store, retrieve,
update and delete files
• The kernel must understand a variety of file
systems
– In Linux, you can see which file-system types the
kernel knows about by viewing the file
/proc/filesystems
Create and terminate processes
• The kernel loads new programs into memory
and gives them resources
– Memory, CPU, file access
• An instance of a running program is called a
process
• Once a process is finished, the kernel reclaims
its resources for subsequent reuse
Device access
• The kernel provides a standardized interface
to simplify access to devices
• Also arbitrates access to each device by many
processes
Networking
• The kernel transmits and receives network
messages for user processes.
– Includes routing network packets to the target
system
Provides a system call interface
• A system call is the way processes can request
the kernel to perform tasks on their behalf
• Equivalently, a system call is the entry point
for a process to request the kernel to perform
tasks on its behalf
Processor Modes
• Modern processors have at least 2 CPU modes: user
mode and kernel mode
– Hardware instructions switch between the two
• Areas of virtual memory are marked as user or kernel
space.
– When in user space, can only access memory in user
space; kernel mode can access both.
• Some operations are only available in kernel mode
– Halt instruction to stop system, accessing memory
management hardware.
• This hardware design prevents user programs from
affecting the kernel and executing certain instructions.
The shell
• Special program that reads commands you
type and executes commands in response
– Also called a command interpreter
• User level process
– Therefore it is not considered part of the OS
kernel
– Several different shells: Bourne shell (sh), C shell
(csh), Korn shell (ksh), Bourne again shell (bash)
Different Architectural Approaches
• Demands on operating systems require new
ways of organizing the OS
Different approaches and design elements have been tried:
• Microkernel architecture
• Multithreading
• Symmetric multiprocessing
• Distributed operating systems
• Object-oriented design
Microkernel Architecture vs the
monolithic kernel approach
• Assigns only a few essential functions to the
kernel:
address
spaces
interprocess
communication
(IPC)
basic
scheduling
– The approach:
simplifies
implementation
provides
flexibility
is well suited to a
distributed
environment
Key Interfaces
• Instruction set architecture (ISA)
• Application binary interface (ABI)
• Application programming interface (API)
Evolution of Operating Systems
 A major OS will evolve over time for a
number of reasons:
Hardware upgrades
New types of hardware
New services
Fixes
Evolution of
Operating Systems
Multiprogrammed
Batch Systems
Simple Batch
Systems
Serial
Processing
Time
Sharing
Systems
Serial Processing: 1940s – mid 1950s
Earliest Computers:
• No operating system
• programmers interacted
directly with the
computer hardware
• Computers ran from a
console with display lights,
toggle switches, some form
of input device, and a
printer
• Users have access to the
computer in “series”
Problems:
• Scheduling:
– most installations used a
hardcopy sign-up sheet to
reserve computer time
– time allocations could
run short or long,
resulting in wasted
computer time
– Setup time
– a considerable amount of
time was spent just on
setting up the program to
run
Simple Batch Systems: 1950s –
1960s
• Early computers were very expensive
– important to maximize processor utilization
• Monitor
– user no longer has direct access to processor
– job is submitted to computer operator who batches them
together and places them on an input device
– program branches back to the monitor when finished
• Monitor controls the sequence of
events
• Resident Monitor is software
always in memory
• Monitor reads in job and gives
control
• Job returns control to monitor
• Processor executes instruction from the memory
containing the monitor
• Executes the instructions in the user program until it
encounters an ending or error condition
• “control is passed to a job” means processor is fetching
and executing instructions in a user program
• “control is returned to the monitor” means that the
processor is fetching and executing instructions from the
monitor program
Memory protection for monitor
• while the user program is executing, it must not alter the memory area
containing the monitor
Timer
• prevents a job from monopolizing the system
Privileged instructions
• can only be executed by the monitor
Interrupts
• gives OS more flexibility in controlling user programs
Simple Batch System Overhead
• Processor time alternates between execution of user
programs and execution of the monitor
• Sacrifices:
– some main memory is now given over to the monitor
– some processor time is consumed by the monitor
– Despite overhead, the simple batch system improves
utilization of the computer
Multiprogrammed
Batch Systems
• Processor is
often idle
• even with
automatic
job
sequencing
• I/O devices
are slow
compared
to
processor
• The processor spends a certain amount of
time executing, until it reaches an I/O
instruction; it must then wait until that I/O
instruction concludes before proceeding
•
There must be enough memory to hold the OS (resident monitor)
and one user program
•
When one job needs to wait for I/O, the processor can switch to
the other job, which is likely not waiting for I/O
Effects on Resource Utilization
Table 2.2 Effects of Multiprogramming on Resource Utilization
• Can be used to handle multiple interactive jobs
• Processor time is shared among multiple users
• Multiple users simultaneously access the system
through terminals, with the OS interleaving the
execution of each user program in a short burst
or quantum of computation
Table 2.3 Batch Multiprogramming versus Time Sharing
• Operating Systems are among the most
complex pieces of software ever developed
Major advances in
development include:
• Processes/Multi Threading
• Memory management
• Information protection and security
• Scheduling and resource
management
• System structure
• Technique in which a process, executing an application, is
divided into threads that can run concurrently
Thread
• dispatchable unit of work
• includes a processor context and its own data area to enable subroutine branching
• executes sequentially and is interruptible
Process
• a collection of one or more threads and associated system resources
• programmer has greater control over the modularity of the application and the timing
of application related events
Causes of Errors
• Improper
synchronization
– a program must wait until
the data are available in a
buffer
– improper design of the
signaling mechanism can
result in loss or duplication
• Failed mutual exclusion
– more than one user or
program attempts to make
use of a shared resource at
the same time
– only one routine at at time
allowed to perform an
update against the file
• Nondeterminate program
operation
– program execution is
interleaved by the processor
when memory is shared
– the order in which programs
are scheduled may affect
their outcome
• Deadlocks
– it is possible for two or
more programs to be hung
up waiting for each other
– may depend on the chance
timing of resource
allocation and release