Operating Systems Ch 1
OS is
"Everything a vendor ships when you order an operating system"is a good approximation
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)
Virtualization
Allows operating systems to run applications within other OSes Vast and growing industry 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 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
Peer-to-peer
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
Process needs resources to accomplish its task
CPU, memory, I/O, files Initialization data
Cache smaller than storage being cached
Cache management important design problem Cache size and replacement policy
Cloud Computing 2
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
OS is a control program
Controls execution of programs to prevent errors and improper use of the computer
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
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 it 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
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
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
OS activities
Free-space management Storage allocation Disk scheduling
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
I/O devices
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
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
Caching 3
Information in use copied from slower to faster storage temporarily
OS is a resource allocator
Manages all resources Decides between conflicting requests for efficient and fair resource use
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
Main memory - only large storage media that the CPU can access directly
Random access Typically volatile
Timesharing (multitasking) 2
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
Distributed environment situation even more complex
Several copies of a datum can exist
Storage systems organized in hierarchy
Speed Cost Volatility
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
Some storage need not be fast
Tertiary storage includes optical storage, magnetic tape Still must be managed - by OS or applications Varies between WORM (write-once, read-many-times) and RW (read-write)
Interrupt Handling
The operating system preserves the state of the CPU by storing registers and the program counter Determines which type of interrupt has occurred: polling vectored interrupt system Separate segments of code determine what action should be taken for each type of interrupt
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
Virtualization 2
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)
Direct Memory Access Structure
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
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
Everything else is either
a system program (ships with the operating system) , or an application program.
Kernel
"The one program running at all times on the computer"
Two types of Multiprocessor
1. Asymmetric Multiprocessing - each processor is assigned a specific task. 2. Symmetric Multiprocessing - each processor performs all tasks
Multiprocessor advantages
1. Increased throughput 2. Economy of scale 3. Increased reliability - graceful degradation or fault tolerance
What is Operating System
A program that acts as an intermediary between a user of a computer and the computer hardware
trap or exception
A trap or exception is a software-generated interrupt caused either by an error or a user request
Multiprocessor system
Also known as parallel systems, tightly-coupled systems
OS is
An operating system is interrupt driven
Distributed computing
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 - user need not know where resource resides
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
data migration
HDD -> main memory -> cache -> hardware register
4 components in Computer System
Hardware, Op System, Application Programs, Users
Caching 2
Important principle, performed at many levels in a computer (in hardware, operating system, software)
Interrupt architiecture
Interrupt architecture must save the address of the interrupted instructio
interrupt vecto
Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines
Main Frame or Minicomputers
Keep users happy designed to maximize resource utilization
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
Real-Time Embedded Systems
Real-time embedded systems most prevalent form of computers Vary considerable, 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
Transition from User to kernel mode
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
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 allows user to change to effective ID with more rights
process
a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity.
Dual-mode
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 Increasingly CPUs support multi-mode operations i.e. virtual machine manager (VMM) mode for guest VMs
Protection
any mechanism for controlling access of processes or users to resources defined by the OS
Multi-threaded process
as one program counter per thread
Single-threaded process
as one program counter specifying location of next instruction to execute Process executes instructions sequentially, one at a time, until completion
Caching
copying information into faster storage system; main memory can be viewed as a cache for secondary storage
Usually disks used to store
data that does not fit in main memory or data that must be kept for a "long"period of time
dedicated systems have what
dedicated resources but frequently use shared resources
Security
defense of the system against internal and external attacks Huge range, including denial-of-service, worms, viruses, identity theft, theft of service
User Desires of OP System
ease of use, good performance, convenience
secondary storage
extension of main memory that provides large nonvolatile storage capacity
SS disk
faster than hard disks, nonvolatile Various technologies Becoming more popular
Device Driver
for each device controller to manage I/O Provides uniform interface between controller and kerne
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
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
embedded
little to no user interface
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
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
Process termination
requires reclaim of any reusable resources
Handhelds
resource poor, optimized for usability and battery life
Hard Disk
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