Test 1 Flashcards
1
Q
Operating System
A
A program that acts as an intermediary between a user of a computer and the computer hardware
Operating system goals:
Execute user programs and make solving user problems easier
Make the computer system convenient to use
Use the computer hardware in an efficient manner
What OSs do?
Depends on the point of view
Users want convenience, ease of use and good performance
Don’t care about resource utilization
But shared computer such as mainframe or minicomputer must keep all users happy
Users of dedicated systems such as workstations have dedicated resources but frequently use shared resources from servers
Handheld computers are resource poor, optimized for usability and battery life
Some computers have little or no user interface, such as embedded computers in devices and automobiles
2
Q
4 components of Computer System Structure
A
A computer system can be divided into four components:
Hardware – provides basic computing resources
CPU, memory, I/O devices
Operating system
Controls and coordinates use of hardware among various applications and users
Application programs – define the ways in which the system resources are used to solve the computing problems of the users
Word processors, compilers, web browsers, database systems, video games
Users
People, machines, other computers
3
Q
resource allocator
A
An OS is one
Manages all resources
Decides between conflicting requests for efficient and fair resource use
4
Q
control program
A
An OS is one
Controls execution of programs to prevent errors and improper use of the computer
5
Q
kernel
A
“The one program running at all times on the computer” is the kernel.
Everything else is either
a system program (ships with the operating system) , or
an application program.
6
Q
bootstrap program
A
bootstrap program is loaded at power-up or reboot
Typically stored in ROM or EPROM, generally known as firmware
Initializes all aspects of system
Loads operating system kernel and starts execution
7
Q
Computer System operation
A
One or more CPUs, device controllers connect through common bus providing access to shared memory
Concurrent execution of CPUs and devices competing for memory cycles
I/O devices and the CPU can execute concurrently
Each device controller is in charge of a particular device type
Each device controller has a local buffer
CPU moves data from/to main memory to/from local buffers
I/O is from the device to local buffer of controller
Device controller informs CPU that it has finished its operation by causing an interrupt
8
Q
Types of Processing Units
A
CPU – Central Processing Unit
GPU – Graphics Processing Unit
TPU – Tensor Processing Unit
FPGA – Field Programmable Gate Array
ASIC – Application Specific Integrated Circuit
9
Q
CPU
A
Brain of the computer
Traditional processing unit
Responsible for most or all executed instructions
Performs operations sequentially
What’s your processor speed?
10
Q
GPU
A
Has actually been around since the 1970s
A GPU is geared toward dealing with images and graphical ‘things’
Good at matrix operations
Performs operations in parallel; has many ALUs in a single processor
GPU program: __global__ - indicates a function that runs on the device called form host code
The ‘«_space;»’ mark a call from host code to device code
11
Q
TPU (Tensor Processing Unit)
A
Designated architecture for deep learning/machine learning computations
Tasks developed using TensorFlow – a programming framework
A tensor is an n-dimensional matrix. Matrix and dense vector processing.
Designed by Google around 2016
12
Q
FGPA (Field Programmable Gate Array)
A
As the name implies, they are field programmable
Customizable for specific applications
Large datasets, high speed search (Bing search algorithms)
Can be changed to support new algorithms
13
Q
ASIC Application Specific Integrated Circuit
A
A non-standard IC designed for a specific use of application
A TPU is an ASIC
Other examples:
A chip in a stuffed animal that talks
A specialized chip for a satellite
Chip in a DVD player to decode info on the disc
14
Q
interrupt vector/interrupt driven
A
Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines
Interrupt architecture must save the address of the interrupted instruction
A trap or exception is a software-generated interrupt caused either by an error or a user request
An operating system is interrupt driven
15
Q
Interrupt handling/polling/vectored interrupt system
A
The operating system preserves the state of the CPU by storing registers and the program counter
Determines which type of interrupt has occurred:
polling (Either CPU checks at regular intervals for interrupts or it knows an interrupt has occurred and polls to see what caused it)
vectored interrupt system
Separate segments of code determine what action should be taken for each type of interrupt
Interrupts can have priorities but generally can’t be interrupted themselves.
16
Q
I/O Structure
A
After I/O starts, control returns to user program only upon I/O completion Wait instruction idles the CPU until the next interrupt Wait loop (contention for memory access) At most one I/O request is outstanding at a time, no simultaneous I/O processing After I/O starts, control returns to user program without waiting for I/O completion System call – request to the OS to allow user to wait for I/O completion Device-status table contains entry for each I/O device indicating its type, address, and state OS indexes into I/O device table to determine device status and to modify table entry to include interrupt
17
Q
System Call
A
request to the OS to allow user to wait for I/O completion
18
Q
Device-status table
A
contains entry for each I/O device indicating its type, address, and state
19
Q
Main memory
A
Main memory – only large storage media that the CPU can access directly
Random access
Typically volatile
20
Q
Secondary storage
A
extension of main memory that provides large nonvolatile storage capacity
21
Q
Hard disks and SSDs
A
Hard disks – rigid metal or glass platters covered with magnetic recording material
Disk surface is logically divided into tracks, which are subdivided into sectors
The disk controller determines the logical interaction between the device and the computer
Solid-state disks (flash drives)– faster than hard disks, nonvolatile
22
Q
Caching
A
Caching – copying information into a faster storage system; main memory can be viewed as a cache for secondary storage
Important principle, performed at many levels in a computer (in hardware, operating system, software)
Information in use copied from slower to faster storage temporarily
Faster storage (cache) checked first to determine if information is there
If it is, information used directly from the cache (fast)
If not, data copied to cache and used there
Cache smaller than storage being cached
Cache management important design problem
Cache size and replacement policy
23
Q
Direct memory access structure
A
Used for high-speed I/O devices able to transmit information at close to memory speeds
Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention
Only one interrupt is generated per block, rather than the one interrupt per byte
There is a DMA controller…it’s like the HW can access memory and bypass the CPU
24
Q
Multiprocessors
A
Most systems use a single general-purpose processor
Most systems have special-purpose processors as well
Multiprocessors systems growing in use and importance
Also known as parallel systems, tightly-coupled systems
Advantages include:
Increased throughput
Economy of scale
Increased reliability – graceful degradation or fault tolerance
Two types:
Asymmetric Multiprocessing – each processor is assigned a specific task.
Symmetric Multiprocessing – each processor performs all tasks
25
Asymmetric Multiprocessing
each processor is assigned a specific task.
26
Symmetric Multiprocessing
each processor performs all tasks
27
multicore
One chip with more than one core
28
Clustered Systems
Like multiprocessor systems, but multiple systems working together
Usually sharing storage via a storage-area network (SAN)
Provides a high-availability service which survives failures
Asymmetric clustering has one machine in hot-standby mode
Symmetric clustering has multiple nodes running applications, monitoring each other
Some clusters are for high-performance computing (HPC)
Applications must be written to use parallelization
Some have distributed lock manager (DLM) to avoid conflicting operations
29
storage-area network (SAN)
storage sharing for clustered systems
30
Asymmetric clustering
Asymmetric clustering has one machine in hot-standby mode
31
Symmetric clustering
Symmetric clustering has multiple nodes running applications, monitoring each other
32
parallelization
Some clusters are for high-performance computing (HPC)
| Applications must be written to use parallelization
33
Multiprogramming (Batch system)
Multiprogramming (Batch system) needed for efficiency
Single user cannot keep CPU and I/O devices busy at all times
Multiprogramming organizes jobs (code and data) so CPU always has one to execute
A subset of total jobs in system is kept in memory
One job selected and run via job scheduling
When it has to wait (for I/O for example), OS switches to another job
Multiprogramming has to do with utilization of the CPU
Timesharing has to do with sharing the CPU among users
34
Timesharing (multitasking)
Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing
Response time should be < 1 second
Each user has at least one program executing in memory 🢡process
If several jobs ready to run at the same time 🢡 CPU scheduling
If processes don’t fit in memory, swapping moves them in and out to run
Virtual memory allows execution of processes not completely in memory
35
Interrupt driven (hardware and software)
Interrupt driven (hardware and software)
Hardware interrupt by one of the devices
Software interrupt (exception or trap):
Software error (e.g., division by zero)
Request for operating system service
Other process problems include infinite loop, processes modifying each other or the operating system
36
Using a timer for resource management
Timer to prevent infinite loop / process hogging resources
Timer is set to interrupt the computer after some time period
Keep a counter that is decremented by the physical clock.
Operating system set the counter (privileged instruction)
When counter zero generate an interrupt
Set up before scheduling process to regain control or terminate program that exceeds allotted time
37
Dual-mode
Dual-mode operation allows OS to protect itself and other system components
User mode and kernel mode
Mode bit provided by hardware
Provides ability to distinguish when system is running user code or kernel code
Some instructions designated as privileged, only executable in kernel mode
System call changes mode to kernel, return from call resets it to user
38
multi-mode operations
Increasingly CPUs support multi-mode operations
| i.e. virtual machine manager (VMM) mode for guest VMs
39
privileged instruction
Mode bit provided by hardware
Provides ability to distinguish when system is running user code or kernel code
Some instructions designated as privileged, only executable in kernel mode
System call changes mode to kernel, return from call resets it to user
40
process management
A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity.
Process needs resources to accomplish its task
CPU, memory, I/O, files
Initialization data
Process termination requires reclaim of any reusable resources
Single-threaded process has one program counter specifying location of next instruction to execute
Process executes instructions sequentially, one at a time, until completion
Multi-threaded process has one program counter per thread
Typically system has many processes, some user, some operating system running concurrently on one or more CPUs
Concurrency by multiplexing the CPUs among the processes / threads
`
41
passive vs active entity
A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity.
42
program counter
Single-threaded process has one program counter specifying location of next instruction to execute
Process executes instructions sequentially, one at a time, until completion
Multi-threaded process has one program counter per thread
43
The operating system is responsible for the following activities in connection with process management
Creating and deleting both user and system processes
Suspending and resuming processes
Providing mechanisms for process synchronization
Providing mechanisms for process communication
Providing mechanisms for deadlock handling
44
Memory Management
To execute a program all (or part) of the instructions must be in memory
All (or part) of the data that is needed by the program must be in memory.
Memory management determines what is in memory and when
Optimizing CPU utilization and computer response to users
Memory management activities
Keeping track of which parts of memory are currently being used and by whom
Deciding which processes (or parts thereof) and data to move into and out of memory
Allocating and deallocating memory space as needed
45
Storage Management
OS provides uniform, logical view of information storage
Abstracts physical properties to logical storage unit - file
Each medium is controlled by device (i.e., disk drive, tape drive)
Varying properties include access speed, capacity, data-transfer rate, access method (sequential or random)
File-System management
Files usually organized into directories
Access control on most systems to determine who can access what
OS activities include
Creating and deleting files and directories
Primitives to manipulate files and directories
Mapping files onto secondary storage
Backup files onto stable (non-volatile) storage media
There are utilities on Linux to let you map a Windows filesystem – Samba is the most popular
46
file
Abstracts physical properties to logical storage unit - file
47
Order of fastest memory
registers > cache > main memory > SSD > Magnetic disk
48
Cache coherency
Multitasking environments must be careful to use most recent value, no matter where it is stored in the storage hierarchy
Multiprocessor environment must provide cache coherency in hardware such that all CPUs have the most recent value in their cache
Distributed environment situation even more complex
Several copies of a piece of data can exist
Various solutions covered in Chapter 17
49
I/O Subsystem
One purpose of OS is to hide peculiarities of hardware devices from the user
I/O subsystem responsible for
Memory management of I/O including buffering (storing data temporarily while it is being transferred), caching (storing parts of data in faster storage for performance), spooling (the overlapping of output of one job with input of other jobs)
General device-driver interface
Drivers for specific hardware devices
50
SPooling
Spooling can sort of be thought of as handing the job off and then continuing – we usually hear this in relation to printing. It’s like buffering, but with buffering, we’re storing data to be used by the same process.
51
Protection
any mechanism for controlling access of processes or users to resources defined by the OS
```
Systems generally first distinguish among users, to determine who can do what
User identities (user IDs, security IDs) include name and associated number, one per user
User ID then associated with all files, processes of that user to determine access control
Group identifier (group ID) allows set of users to be defined and controls managed, then also associated with each process, file
Privilege escalation (WIndows run as admin) allows user to change to effective ID with more rights
```
52
Security
Security – defense of the system against internal and external attacks
Huge range, including denial-of-service, worms, viruses, identity theft, theft of service
53
Virus
Attaches itself to a program or file (almost always an executable) enabling it to spread from one computer to another, leaving infections as it travels.
Normally doesn’t infect your computer unless it is run
Cannot spread without human interaction (like running it)
Attaches to an executable file, requires human action to spread
54
Worm
```
Subclass of a virus
Capability to travel without human interaction - takes advantage of file or information transport features
Example: Worm to send a copy of itself to everyone listed in your e-mail address book. Then, the worm replicates and repeats.
May consume resources and slow computer down severely
Can replicate itself on system, does not require human action to spread.
```
55
Computing Environments (traditional)
Stand-alone general purpose machines
But blurred as most systems interconnect with others (i.e., the Internet)
Portals provide web access to internal systems
Network computers (thin clients) are like Web terminals
Mobile computers interconnect via wireless networks
Networking has become almost ubiquitous – even home systems use firewalls to protect home computers from Internet attacks
56
Computing Environments (mobile)
Handheld smartphones, tablets, etc
What is the functional difference between them and a “traditional” laptop?
Extra features – more OS features (GPS, gyroscope)
Allows new types of apps like augmented reality
Use IEEE 802.11 wireless, or cellular data networks for connectivity
Leaders are Apple iOS and Google Android
57
Distributed computing environment
Distributed computiing
Collection of separate, possibly heterogeneous, systems networked together
Network is a communications path, TCP/IP most common
Local Area Network (LAN)
Wide Area Network (WAN)
Metropolitan Area Network (MAN)
Personal Area Network (PAN)
Network Operating System provides features between systems across network
Communication scheme allows systems to exchange messages
Illusion of a single system
58
Network
```
Network is a communications path, TCP/IP most common
Local Area Network (LAN)
Wide Area Network (WAN)
Metropolitan Area Network (MAN)
Personal Area Network (PAN)
```
59
Network Operating System
Network Operating System provides features between systems across network
Communication scheme allows systems to exchange messages
Illusion of a single system
60
Client-Server Computing
Client-Server Computing
Dumb terminals supplanted by smart PCs
Many systems now servers, responding to requests generated by clients
Compute-server system provides an interface to client to request services (i.e., database)
File-server system provides interface for clients to store and retrieve files
61
Peer-to-Peer computing environment
Another model of distributed system
P2P does not distinguish clients and servers
Instead all nodes are considered peers
May each act as client, server or both
Node must join P2P network
Registers its service with central lookup service on network, or
Broadcast request for service and respond to requests for service via discovery protocol
Examples include Napster and Gnutella, Voice over IP (VoIP) such as Skype
62
Virtualization
Virtualization – OS natively compiled for CPU, running guest OSes also natively compiled
Consider VMware running WinXP guests, each running applications, all on native WinXP host OS
VMM (virtual machine Manager) provides virtualization services
Allows operating systems to run applications within other OSes
Vast and growing industry
Use cases involve laptops and desktops running multiple OSes for exploration or compatibility
Apple laptop running Mac OS X host, Windows as a guest
Developing apps for multiple OSes without having multiple systems
QA testing applications without having multiple systems
Executing and managing compute environments within data centers
VMM can run natively, in which case they are also the host
There is no general purpose host then (VMware ESX and Citrix XenServer)
63
Emulation
Emulation used when source CPU type different from target type (i.e. PowerPC to Intel x86)
Generally slowest method
When computer language not compiled to native code – Interpretation
64
Cloud Computing
Delivers computing, storage, even apps as a service across a network
Logical extension of virtualization because it uses virtualization as the base for its functionality.
Amazon EC2 has thousands of servers, millions of virtual machines, petabytes of storage available across the Internet, pay based on usage
Many types
Public cloud – available via Internet to anyone willing to pay
Private cloud – run by a company for the company’s own use
Hybrid cloud – includes both public and private cloud components
Software as a Service (SaaS) – one or more applications available via the Internet (i.e., word processor)
Platform as a Service (PaaS) – software stack ready for application use via the Internet (i.e., a database server)
Infrastructure as a Service (IaaS) – servers or storage available over Internet (i.e., storage available for backup use)
Cloud computing environments composed of traditional OSes, plus VMMs, plus cloud management tools
Internet connectivity requires security like firewalls
Load balancers spread traffic across multiple applications
65
Real-time embedded systems
Real-time embedded systems most prevalent form of computers
Vary considerably, special purpose, limited purpose OS, real-time OS
Use expanding
Many other special computing environments as well
Some have OSes, some perform tasks without an OS
Real-time OS has well-defined fixed time constraints
Processing must be done within constraint
Correct operation only if constraints met
66
Open-Source Operating Systems
Operating systems made available in source-code format rather than just binary closed-source
Counter to the copy protection and Digital Rights Management (DRM) movement
Started by Free Software Foundation (FSF), which has “copyleft” GNU Public License (GPL)
Examples include GNU/Linux and BSD UNIX (including core of Mac OS X), and many more
Can use VMM like VMware Player (Free on Windows), Virtualbox (open source and free on many platforms - http://www.virtualbox.com)
Use to run guest operating systems for exploration
67
OPerating system services (helpful to user)
Operating systems provide an environment for execution of programs and services to programs and users
One set of operating-system services provides functions that are helpful to the user:
User interface - Almost all operating systems have a user interface (UI).
Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch
Program execution - The system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error)
I/O operations - A running program may require I/O, which may involve a file or an I/O device
68
User interface
User interface - Almost all operating systems have a user interface (UI).
Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch
69
Program execution
Program execution - The system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error)
70
I/O operations
I/O operations - A running program may require I/O, which may involve a file or an I/O device
71
File-system manipulation
File-system manipulation - The file system is of particular interest. Programs need to read and write files and directories, create and delete them, search them, list file Information, permission management.
72
Communications
Communications – Processes may exchange information, on the same computer or between computers over a network
Communications may be via shared memory or through message passing (packets moved by the OS)
73
Error detection
Error detection – OS needs to be constantly aware of possible errors
May occur in the CPU and memory hardware, in I/O devices, in user program
For each type of error, OS should take the appropriate action to ensure correct and consistent computing
Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system
74
set of OS functions exists for ensuring the efficient operation of the system itself via resource sharing
Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them
Many types of resources - CPU cycles, main memory, file storage, I/O devices.
Accounting - To keep track of which users use how much and what kinds of computer resources
Protection and security - The owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other
Protection involves ensuring that all access to system resources is controlled
Security of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts
75
CLI or command line interpreter
CLI or command line interpreter allows direct command entry
Sometimes implemented in kernel, sometimes by systems program
Sometimes multiple flavors implemented – shells
Primarily fetches a command from user and executes it
Sometimes commands built-in, sometimes just names of programs
If the latter, adding new features doesn’t require shell modification
76
User-friendly desktop metaphor interface
User-friendly desktop metaphor interface
Usually mouse, keyboard, and monitor
Icons represent files, programs, actions, etc
Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function, open directory (known as a folder)
Invented at Xerox PARC
Many systems now include both CLI and GUI interfaces
Microsoft Windows is GUI with CLI “command” shell
Apple Mac OS X is “Aqua” GUI interface with UNIX kernel underneath and shells available
Unix and Linux have CLI with optional GUI interfaces (CDE, KDE, GNOME)
77
System Calls
Programming interface to the services provided by the OS
Typically written in a high-level language (C or C++)
Mostly accessed by programs via a high-level Application Programming Interface (API) rather than direct system call use
Three most common APIs are Win32 API for Windows, POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM)
78
System-call interface
Typically, a number associated with each system call
System-call interface maintains a table indexed according to these numbers
The system call interface invokes the intended system call in OS kernel and returns status of the system call and any return values
The caller need know nothing about how the system call is implemented
Just needs to obey API and understand what OS will do as a result call
Most details of OS interface hidden from programmer by API
Managed by run-time support library (set of functions built into libraries included with compiler)
79
System call parameter passing
Often, more information is required than simply identity of desired system call
Exact type and amount of information vary according to OS and call
Three general methods used to pass parameters to the OS
Simplest: pass the parameters in registers
In some cases, may be more parameters than registers
Parameters stored in a block, or table, in memory, and address of block passed as a parameter in a register
This approach taken by Linux and Solaris
Parameters placed, or pushed, onto the stack by the program and popped off the stack by the operating system
Block and stack methods do not limit the number or length of parameters being passed
80
Status information system program
Status information
Some ask the system for info - date, time, amount of available memory, disk space, number of users
Others provide detailed performance, logging, and debugging information
Typically, these programs format and print the output to the terminal or other output devices
Some systems implement a registry - used to store and retrieve configuration information
81
System program features
File modification
Text editors to create and modify files
Special commands to search contents of files or perform transformations of the text
Programming-language support - Compilers, assemblers, debuggers and interpreters sometimes provided
Program loading and execution- Absolute loaders, relocatable loaders, linkage editors, and overlay-loaders, debugging systems for higher-level and machine language
Communications - Provide the mechanism for creating virtual connections among processes, users, and computer systems
Allow users to send messages to one another’s screens, browse web pages, send electronic-mail messages, log in remotely, transfer files from one machine to another
82
System program background services
```
Background Services
Launch at boot time
Some for system startup, then terminate
Some from system boot to shutdown
Provide facilities like disk checking, process scheduling, error logging, printing
Run in user context not kernel context
Known as services, subsystems, daemons
```
83
system program Application programs
```
Application programs
Don’t pertain to system
Run by users
Not typically considered part of OS
Launched by command line, mouse click, finger poke
```
84
User goals and System goals
User goals and System goals
User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast
System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient
85
Policy vs Mechanism
Important principle to separate
Policy: What will be done?
Mechanism: How to do it?
Mechanisms determine how to do something, policies decide what will be done
The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later (example – timer)
Specifying and designing an OS is highly creative task of software engineering
86
OS languages
Much variation
Early OSes in assembly language
Then system programming languages like Algol, PL/1
Now C, C++
Actually usually a mix of languages
Lowest levels in assembly
Main body in C
Systems programs in C, C++, scripting languages like PERL, Python, shell scripts
More high-level language easier to port to other hardware
But slower
Emulation can allow an OS to run on non-native hardware
87
Layered Approach
The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.
With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers
88
Microkernel System Structure
Moves as much from the kernel into user space as possible
Mach example of microkernel
Mac OS X kernel (Darwin) partly based on Mach
Communication takes place between user modules using message passing
Benefits:
Easier to extend a microkernel
Easier to port the operating system to new architectures
More reliable (less code is running in kernel mode)
More secure
Detriments:
Performance overhead of user space to kernel space communication
89
loadable kernel modules
Many modern operating systems implement loadable kernel modules
Uses object-oriented approach
Each core component is separate
Each talks to the others over known interfaces
Each is loadable as needed within the kernel
Overall, similar to layers but more flexible
Linux, Solaris, etc
90
Hybrid Systems
Most modern operating systems are actually not one pure model
Hybrid combines multiple approaches to address performance, security, usability needs
Linux and Solaris kernels in kernel address space, so monolithic, plus modular for dynamic loading of functionality
Windows mostly monolithic, plus microkernel for different subsystem personalities
Apple Mac OS X hybrid, layered, Aqua UI plus Cocoa programming environment
Below is kernel consisting of Mach microkernel and BSD Unix parts, plus I/O kit and dynamically loadable modules (called kernel extensions)
monolithic
In kernel architecture, describes a kernel without structure (such as layers or modules).
91
Monolithic kernel
A monolithic kernel is an operating system architecture where the entire operating system is working in kernel space
92
OS Debugging
Debugging is finding and fixing errors, or bugs
OS generate log files containing error information
Failure of an application can generate core dump file capturing memory of the process
Operating system failure can generate crash dump file containing kernel memory
Beyond crashes, performance tuning can optimize system performance
Sometimes using trace listings of activities, recorded for analysis
Profiling is periodic sampling of instruction pointer to look for statistical trends
Kernighan’s Law: “Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.”
93
Performance Tuning
Improve performance by removing bottlenecks
OS must provide means of computing and displaying measures of system behavior
For example, “top” program or Windows Task Manager
DTrace tool in Solaris, FreeBSD, Mac OS X allows live instrumentation on production systems
94
System Boot
When power initialized on system, execution starts at a fixed memory location
Firmware ROM used to hold initial boot code
Operating system must be made available to hardware so hardware can start it
Small piece of code – bootstrap loader, stored in ROM or EEPROM locates the kernel, loads it into memory, and starts it
Sometimes two-step process where boot block at fixed location loaded by ROM code, which loads bootstrap loader from disk
Common bootstrap loader, GRUB, allows selection of kernel from multiple disks, versions, kernel options
Kernel loads and system is then running
95
Process Concept (parts of process)
An operating system executes a variety of programs:
Batch system – jobs
Time-shared systems – user programs or tasks
Textbook uses the terms job and process almost interchangeably
Process – a program in execution; process execution must progress in sequential fashion
Multiple parts
The program code, also called text section
Current activity including program counter, processor registers
Stack containing temporary data
Function parameters, return addresses, local variables
Data section containing global variables
Heap containing memory dynamically allocated during run time
Program is passive entity stored on disk (executable file), process is active
Program becomes process when executable file loaded into memory
Execution of program started via GUI mouse clicks, command line entry of its name, etc
One program can be several processes
Consider multiple users executing the same program
96
Process State
As a process executes, it changes state
new: The process is being created
running: Instructions are being executed
waiting: The process is waiting for some event to occur
ready: The process is waiting to be assigned to a processor
terminated: The process has finished execution
97
Process Control Block (PCB)
Information associated with each process
(also called task control block)
Process state – running, waiting, etc
Program counter – location of instruction to next execute
CPU registers – contents of all process-centric registers
CPU scheduling information- priorities, scheduling queue pointers
Memory-management information – memory allocated to the process
Accounting information – CPU used, clock time elapsed since start, time limits
I/O status information – I/O devices allocated to process, list of open files
98
Threads
So far, process has a single thread of execution
Consider having multiple program counters per process
Multiple locations can execute at once
Multiple threads of control -> threads
Must then have storage for thread details, multiple program counters in PCB
See next chapter
99
Processor Scheduling
Maximize CPU use, quickly switch processes onto CPU for time sharing
Process scheduler selects among available processes for next execution on CPU
Maintains scheduling queues of processes
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
Processes migrate among the various queues
100
Schedulers
Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU
Sometimes the only scheduler in a system
Short-term scheduler is invoked frequently (milliseconds) ⇒ (must be fast)
Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue
Long-term scheduler is invoked infrequently (seconds, minutes) ⇒ (may be slow)
The long-term scheduler controls the degree of multiprogramming
Processes can be described as either:
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
Long-term scheduler strives for good process mix
101
Mobile multitasking
Some mobile systems (e.g., early version of iOS) allow only one process to run, others suspended
Due to screen real estate, user interface limits iOS provides for a
Single foreground process- controlled via user interface
Multiple background processes– in memory, running, but not on the display, and with limits
Limits include single, short task, receiving notification of events, specific long-running tasks like audio playback
Android runs foreground and background, with fewer limits
Background process uses a service to perform tasks
Service can keep running even if background process is suspended
Service has no user interface, small memory use
102
Context Switch
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 via a context switch
Context of a process represented in the PCB
Context-switch time is overhead; the system does no useful work while switching
The more complex the OS and the PCB 🡺 the longer the context switch
Time dependent on hardware support
Some hardware provides multiple sets of registers per CPU 🡺 multiple contexts loaded at once
103
Process Creation
Parent process create children processes, which, in turn create other processes, forming a tree of processes
Generally, process identified and managed via a process identifier (pid)
Resource sharing options
Parent and children share all resources
Children share subset of parent’s resources
Parent and child share no resources
Execution options
Parent and children execute concurrently
Parent waits until children terminate
```
Address space
Child duplicate of parent
Child has a 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
```
104
Process termination
Process executes last statement and then asks the operating system to delete it using the exit() system call.
Returns status data from child to parent (via wait())
Process’ resources are deallocated by operating system
Parent may terminate the execution of children processes using the abort() system call. Some reasons for doing so:
Child has exceeded allocated resources
Task assigned to child is no longer required
The parent is exiting and the operating systems does not allow a child to continue if its parent terminates
105
cascading termination
Some operating systems do not allow child to exists if its parent has terminated. If a process terminates, then all its children must also be terminated.
cascading termination. All children, grandchildren, etc. are terminated.
The termination is initiated by the operating system.
The parent process may wait for termination of a child process by using the wait()system call. The call returns status information and the pid of the terminated process
pid = wait(&status);
If no parent waiting (did not invoke wait()) process is a zombie
If parent terminated without invoking wait , process is an orphan
If the parent didn’t invoke wait, the child exit status is in the process table – reading that code is called “reaping” the child. The child is a zombie until that happens.
If the parent terminates before the child, the child is considered an orphan.
106
Interprocess Communication
Processes within a system may be independent or cooperating
Cooperating process can affect or be affected by other processes, including sharing data
Reasons for cooperating processes:
Information sharing
Computation speedup
Modularity
Convenience
Cooperating processes need interprocess communication (IPC)
Two models of IPC
Shared memory
Message passing
Independent process cannot affect or be affected by the execution of another process
Cooperating process can affect or be affected by the execution of another process
107
Producer Consumer Problem
Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process
unbounded-buffer places no practical limit on the size of the buffer
bounded-buffer assumes that there is a fixed buffer size -> can only use n - 1 of buffer size b/c buffer is empty when in == out
108
Shared Memory
An area of memory shared among the processes that wish to communicate
The communication is under the control of the users processes not the operating system.
Major issue is to provide mechanism that will allow the user processes to synchronize their actions when they access shared memory.
Synchronization is discussed in great details in Chapter 5.
109
Message Passing
Mechanism for processes to communicate and to synchronize their actions
Message system – processes communicate with each other without resorting to shared variables
IPC facility provides two operations:
send(message)
receive(message)
The message size is either fixed or variable
If processes P and Q wish to communicate, they need to:
Establish a communication link between them
Exchange messages via send/receive
```
Implementation of communication link
Physical:
Shared memory
Hardware bus
Network
Logical:
Direct or indirect
Synchronous or asynchronous
Automatic or explicit buffering
```
110
Direct Communication
Processes must name each other explicitly:
send (P, message) – send a message to process P
receive(Q, message) – receive a message from process Q
Properties of communication link
Links are established automatically
A link is associated with exactly one pair of communicating processes
Between each pair there exists exactly one link
The link may be unidirectional, but is usually bi-directional
111
Indirect Communication
Messages are directed and received from mailboxes (also referred to as ports)
Each mailbox has a unique id
Processes can communicate only if they share a mailbox
Properties of communication link
Link established only if processes share a common mailbox
A link may be associated with many processes
Each pair of processes may share several communication links
Link may be unidirectional or bi-directional
Operations
create a new mailbox (port)
send and receive messages through mailbox
destroy a mailbox
Primitives are defined as:
send(A, message) – send a message to mailbox A
receive(A, message) – receive a message from mailbox A
Mailbox sharing
P1, P2, and P3 share mailbox A
P1, sends; P2 and P3 receive
Who gets the message?
Solutions
Allow a link to be associated with at most two processes
Allow only one process at a time to execute a receive operation
Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.
112
Synchronization
Message passing may be either blocking or non-blocking
Blocking is considered synchronous
Blocking send -- the sender is blocked until the message is received
Blocking receive -- the receiver is blocked until a message is available
Non-blocking is considered asynchronous
Non-blocking send -- the sender sends the message and continue
Non-blocking receive -- the receiver receives:
A valid message, or
Null message
Different combinations possible
If both send and receive are blocking, we have a rendezvous
113
Buffering
Queue of messages attached to the link.
implemented in one of three ways
1. Zero capacity – no messages are queued on a link.
Sender must wait for receiver (rendezvous)
2. Bounded capacity – finite length of n messages
Sender must wait if link full
3. Unbounded capacity – infinite length
Sender never waits
114
Sockets
A socket is defined as an endpoint for communication
Concatenation of IP address and port – a number included at start of message packet to differentiate network services on a host
The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8
Communication consists between a pair of sockets
All ports below 1024 are well known, used for standard services
Special IP address 127.0.0.1 (loopback) to refer to system on which process is running
Three types of sockets
Connection-oriented (TCP)
Connectionless (UDP)
MulticastSocket class– data can be sent to multiple recipients
115
Remote Procedure Calls
Remote procedure call (RPC) abstracts procedure calls between processes on networked systems
Again uses ports for service differentiation
Stubs – client-side proxy for the actual procedure on the server
The client-side stub locates the server and marshalls the parameters
The server-side stub receives this message, unpacks the marshalled parameters, and performs the procedure on the server
116
Ordinary pipes
Acts as a conduit allowing two processes to communicate
– cannot be accessed from outside the process that created it. Typically, a parent process creates a pipe and uses it to communicate with a child process that it created.
Ordinary Pipes allow communication in standard producer-consumer style
Producer writes to one end (the write-end of the pipe)
Consumer reads from the other end (the read-end of the pipe)
Ordinary pipes are therefore unidirectional
Require parent-child relationship between communicating processes
Windows calls these anonymous pipes
See Unix and Windows code samples in textbook
117
Named pipes
– can be accessed without a parent-child relationship.
Named Pipes are more powerful than ordinary pipes
Communication is bidirectional
No parent-child relationship is necessary between the communicating processes
Several processes can use the named pipe for communication
Provided on both UNIX and Windows systems
