WEEK TWO

PARALLEL SYSTEMS Parallel systems are also called tightly coupled systems Parallel systems are those multi-processing systems which

share, the bus, the clock, memory and peripheral devices Communication usually takes place through the shared

memory Provide increased throughput because of n processors Save money due to sharing of peripherals, storage devices,

files etc. Increase reliability as failure of one processor will not badly

affect the system- graceful degradation (fault-tolerant systems) and fail soft systems

Require hardware duplication for continued operation using primary and backup processors to detect, diagnose and correct failures

Processes can share certain data structures to avoid idle/overloading of processing

Additional front-end /back-end processors (slave) relieve the load of main CPU

PARALLEL SYSTEMS Symmetric multiprocessing (SMP)

Each processor runs an identical copy of the operating system

Many processes can run at once without performance deterioration

SMP means that all processors are peer –no master-slave relationship exists between them

Asymmetric multiprocessing (AMP) Master-slave relationship exists with

processors Each processor is assigned a specific task;

master processors schedules and allocates work to slave processors

More common in extremely large systems

DISTRIBUTED SYSTEMS A collection of components that execute on different

computers Distribute the computation among several physical processors

using networks (LAN/MAN(Metropolitan Area Network)/WAN) Termed as loosely-coupled systems as processors do not share

memory or clock Distributed processors may vary in size and functions in

accordance with their utilization at various sites Processors/sites/nodes/computers communicate through high

speed communication lines such as high-speed buses, telephone lines, microwave dishes, radio links etc.

Characteristics of distributed systems are: Multiple independent components Heterogeneous systems Components are not shared by all users Concurrent processing at different processors Multiple points of control Multiple points of failure (but more fault tolerant)

DISTRIBUTED SYSTEMS Implementation Challenges

Heterogeneity Resource sharing Openness Concurrency Scalability Fault tolerance Transparency Security Coherency

Advantages of distributed systems Increased reliability and availability Local processing Fast response Lower communication costs Data and load sharing Modular growth Protection and security Resource sharing Enhanced output

DISTRIBUTED SYSTEMS Client-Server Systems (Architecture)

Services are provided by the servers and used by clients

Clients of servers but servers need not to know about clients

Clients and servers are logical processes Mapping of processes to processors is not necessarily

1:1 Centralized systems today act as server systems to satisfy

requests generated by client systems using PCs Servers can be categorized as:

Computer - Server Systems provide an interface to which clients can send requests to perform an action, in response to which they perform an action and send back results to the respective client

File – Server Systems provide a file-system interface where clients can create, update, read and delete files

DISTRIBUTED SYSTEMS Peer to peer systems

End users share resources via exchange between computers so information is distributed among member nodes instead of concentrated at a single server (decentralized computing)

Examples: messenger in communication, audio-video in remote collaboration, distributed computing and file sharing

Advantages : increases extensibility, higher system availability, sharing files and resources, exchange of messages, optimized loading of processors, concurrent processing for enhanced output, higher throughput and improved resilience

Structural Characteristics : Inherently scalability, self –organization, congestion minimization and fault-tolerance

CLUSTERED SYSTEMS Clustered computers share resources and are

very closely linked via LAN networking Clustering provides high reliability. In an asymmetric clustering, one machine is in

hot standby mode while the other is running the applications (the active server). It takes over when the active server machine fails.

In symmetric clustering, all N hosts (two or more) are running the applications and they are monitoring each other. It is more efficient.

Cluster directions include global clusters and a lot of R&D is going on in this direction.

Storage Area Networks (SAN) allow easy attachment of multiple hosts to multiple storage units.

BASIC SYSTEM

Group of independent servers which: Function as a single system Appear to users as single system Are managed as a single system

OTHER TYPES OF CLUSTERS Parallel Clusters – It allows multiple hosts to access the same data on a shared

storage (e.g. Oracle parallel server) Clustering over WAN

Do not allow share access to data on a disk Distributed Lock Manager (DLM) Distributed file systems must provide

access control and locking to the files to ensure no conflicting operations occur. DLM facility is therefore included

High Availability Clusters High-Availability clusters exist to keep the overall services of the clusters

available as much as possible in order to take into account the fallibility of computer hardware and software

Load Balancing Clusters Load balancing clusters provide a more practical system for business

needs. As the name implies, that systems entails sharing the processing load as evenly as possible across a cluster of computers running the same set of applications

Load balancing clusters distribute the networks or compute load across multiple nodes

Such a system is perfectly suited for large number of users MOSIX

MOSIX uses a modified kernel to create a process load balanced cluster Servers and networks can join or leave the clusters increasing or

decreasing of the clusters

WHY ARE CLUSTERS IMPORTANT? Clusters improve systems availability Clusters enable application scaling Clusters simplify system management Clusters are a superior server solution

CLUSTERS IMPROVE SYSTEM AVAILABILITY

When a network server fails, the service it provides is down

When a cluster server fails, the service it provides is “failover” and downtime is avoided

CLUSTERS ENABLE APPLICATION SCALING

With networked SMP servers, application scaling is limited to one server

With clusters, applications scale across multiple SMP servers (typically up to 16 servers)

CLUSTERS SIMPLIFY SYSTEM MANAGEMENT

Clusters present a Single System Image i.e. the cluster looks like a single server to management applications

Hence, reduce management costs

REAL-TIME SYSTEMS Used as control devices in dedicated application Sensors gathering information, computer analyzing it and

adjusting appropriate controls to modify the sensor output Hard Real – Time System

Guarantees that critical tasks are completed in time Used with controls and robotics requiring precision

movements Conflicts with time-sharing systems not supported by

general purpose operating systems Secondary storage limited or absent, data stored in short

term memory or read only memory (ROM) Soft Real – Time System

A critical task gets priority over the tasks and retains that priority until its execution is completed

Due to lack of deadline support, these are risky to use for industrial controls and robotics

Useful in applications (multimedia, virtual reality) requiring advanced operating system features

REAL-TIME SYSTEMS Such systems are used in controlled scientific

experiments, medical imaging systems and industrial controlled systems

Processing must be done with the definite constraints or the system will fail (quick response)

More RAM required Advanced features of OS such as virtual reality

are not needed

HANDHELD SYSTEMS PDAs such as Palm-Pilots or cellular phones

with connectivity to networks like internet Small size, weigh less Issues:

Limited memory Slow processors Small display screens (Web clipping is used

to display contents in web pages) Some handheld devices also use wireless

technology (WAP) allowing remote access to e-mail and web browsing

Major benefits of handheld systems are convenience and portability

COMPUTING ENVIRONMENT Traditional Computing

PCS, terminals, laptops, etc. attached to networks Portals provide web access to servers Handheld devices are used to get necessary information Firewalls are used in some applications for security purposes

Web-based computing Workstations, handheld PDAs and cellular phones provide access to

web-based computing It has increased the emphasis on networking (wired or wireless

access) It provides faster network connectivity Load balancers distribute network connections among a pool of

similar servers Embedded Computing

Computers run embedded real-time systems These devices are found every where (car engines, robots, ovens,

controllers etc.) These have low or little user-interface Can be used to computerize houses (central heating and lighting,

alarm system etc.)

COMPUTER – SYSTEM STRUCTURESCHAPTER 2

Computer System Operations I/O Structure Storage Structure Storage Hierarchy Hardware Protection General System Architecture

COMPUTER-SYSTEM OPERATIONS A modern computer consists of a CPU, memory,

system bus and a number of device controllers I/O Devices and the system can execute

concurrently Each device controller is in charge of a particular

device type A device controller for each device which contains

local buffer storage and a special purpose registers A bootstrap program is required to initialize the

computer system 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

COMPUTER-SYSTEM ARCHITECTURE

COMMON FUNCTIONS OF INTERRUPTS The occurrence of an event is usually signaled by an

interrupt either the hardware or software System call or monitor call is executed to trigger an

interrupt 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

Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt

A trap is a software-generated interrupt caused either by an error or a user request

An operating system is interrupt driven and priority interrupts have been introduced in modern systems

INTERRUPT HANDLING When the CPU is interrupted, it stops what it is doing and

immediately transfers execution to a fix location to execute the interrupt the routine through a table of pointers which is stored in LMA

On completion of execution of service routine, the CPU resumes the interrupted computation

LMA locations hold the addresses of the interrupt service routines (Interrupt Vector i.e. Memory Address of an Interrupt handler) for the various devices

Separate segments of code determine what action should be taken for each type of interrupt

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 systems

Interrupts are important part of a modern computer system and should be handled immediately

System call is a method used by a process to request action by the operating system

INTERRUPT TIME LINE FOR A SINGLE PROCESS DOING OUTPUT

I/O STRUCTURE The computer system has a number of device controllers

connected through a common bus A device controller contains local buffer storage and a set of

special purpose registers The device driver is responsible for moving the data between

the peripheral devices and it controls its local buffer storage I/O Interrupts are used by the device controllers for transfer

of data I/O methods: Synchronous and Asynchronous In synchronous method, after I/O starts, control returns to

user program only upon I/O completion Waiting for I/O may be accomplished by either wait

instruction or wait loop Wait instruction idles the CPU until the next interrupt Wait loop continues until an interrupt occurs At most one I/O request is outstanding at a time, no

simultaneous I/O processing

I/O STRUCTURE In asynchronous method, after I/O starts, control returns to

user program without waiting for I/O completion It requires:- System call – request to the operating system 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. Operating system indexes into I/O device table to

determine device status and to modify table entry to include interrupt

Operating System will maintain a queue for each I/O device

An I/O device interrupts when it needs service, OS determines I/O device and updates its table entry

An interrupt signals completion of an I/O request, control then returns from I/O interrupt to another request or user program

Interrupt schemes vary from system to system This method increases system efficiency

TWO I/O METHODS

Synchronous Asynchronous

DEVICE STATUS TABLE

DIRECT MEMORY ACCESS STRUCTURE Involvement of CPU in data transfer is a time

consuming process. It the CPU needs two microseconds to respond to each interrupt and interrupts arrive every four microseconds then left time is less for process execution

DMA is 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

DIRECT MEMORY ACCESS STRUCTURE DMA controller has its own registers for source

and destination addresses A device driver sets the DMA controller register

to use the appropriate source and destination addresses, transfer length and it is then instructed to start I/O operation

While the DMA controller is performing the data transfer, the CPU is free to perform other tasks

As the DMA controller steals memory cycles from the CPU so it slows down CPU execution during DMA operation

DMA interrupts the CPU when the transfer has been completed

STORAGE STRUCTURE Registers Cache Main Memory Electronic Disk Magnetic Disk Optical Disk Hard Disk Magnetic Tape

REGISTERS Registers are available in the CPU and are

accessible within one cycle of the CPU clock Faster operations are carried out on contents of

CPU registers due to faster accesses Processor does not stall while performing

operations on registers Size of the registers is very small Registers are volatile

CACHE Cache needs to stall as RAM is slower than CPU speed

for providing data required to complete the instruction Cache is a faster memory between the CPU and main

memory and is a remedial measure to reduce idling time of CPU

Cache is a memory buffer which stores information required by the CPU using register-allocation and register-replacement algorithms

Instruction cache holds the next instruction to be executed whereas data cache keeps required data for the instruction. They are known as hardware caches

Cache has limited size so cache management is a problem for designers

Careful selection of the cache size and of a replacement algorithm can provide 80 – 99 % of all accesses within the cache – maximizing system performance

Caches are unstable

MAIN MEMORY Main memory can be viewed as a fast cache for secondary storage Program must be loaded in the RAM for execution and main memory

is a large media that CPU can access directly Main memory is implemented in a semiconductor technology (DRAM

– Dynamic Access Random Memory Stores each bit of data in a separate capacitor)

Load and store instructions specify memory addresses for interaction A typical instruction is executed using fetch-decode-execute cycle All programs and data can not be stored in RAM due to its size and

volatility Special I/O instructions allow data transfers between the device

controller registers and main memory In memory-mapped I/O, ranges of memory addresses are set aside

and are mapped to the device registers for providing more convenient access to I/O devices

In programmed I/O, CPU uses polling to watch the bit in the control register to see whether the device is ready for transfer of data between device and main memory

In an interrupt-driven I/O, CPU receives an interrupt when the device is ready for the data transfer

MAGNETIC DISKS Magnetic Disks provide a large space for storing

programs and data on permanent basis Disks are relatively simple and consist of Disk speed depends upon transfer rate and positioning

time (seek time and rotational latency) Head crash damages the magnetic surface and the

whole disk is replaced for safety of data and programs The storage size of a Hard Disk in in GBs FDD rotates slowly than HDD which reduces wear on

the disk surface. Its storage capacity is very small compare to HD or CD

Buses attached to a disk drive or EIDE (Enhancements to Integrated Drive Electronics), ATA and CSI

Data transfer through a bus is carried out between the host controller and disk controller

Magnetic disks are non-volatile

MOVING-HEAD DISK MECHANISM

MAGNETIC TAPES Magnetic tapes are used to backup the data

and programs in order to protect any loss due to HD failure

Magnetic tapes can hold large quantities of data/programs

Access time is slow compared to HD,CD, Main Memory etc.

Magnetic tapes for non-volatile Storage/handling of information is very slow

due to winding/rewinding of tapes Random access is not available on tapes

STORAGE HIERARCHY Storage systems can be organized in a

hierarchy according to Speed Capacity Cost Volatility

Register, cache and memory are constructed using semiconductor memory and are volatile

Electronic disks can be volatile or non volatile All secondary storage devices (magnetic disk,

optical disk, floppy disk, magnetic drums) are non volatile

STORAGE-DEVICE HIERARCHY

COHERENCE AND CONSISTENCY The same data may appear in different levels of the

storage system. For example, value of variable (X) of file G may reside on magnetic disk, main memory, cache or CPU register

In a multi-tasking environment, each process wishing to use the value of variable (X) must obtain the most recently updated value

A copy of variable (X) may exist simultaneously in several caches having different value in multiprocessor environment. For cache coherency, the system hardware must make sure that an update to value of X in one cache is immediately reflected in all other caches where X resides for concurrent execution of file G

For cache consistency in a distributed environment, the various replicas of the file G may be accessed and updated so system must ensure that when a replica is updated in one computer, all other replicas are brought up-to-date quickly through client or server initiated approach

MIGRATION OF X FROM DISK TO REGISTER

HARDWARE PROTECTION Dual-Mode Operation I/O Protection Memory Protection CPU Protection

DUAL MODE OPERATION Sharing of system resources improved system utilization

but increased problems. Many jobs could be affected by a bug in a program

A good operating system must ensure that a faulty program can not cause other programs to execute incorrectly

If a user program fails, the hardware will trap to OS, the OS dumps the memory of the program for debugging and terminates it

The hardware-supported dual-mode operation protects the OS, all other programs and their data from any mal-functioning program

User-mode of operation (mode-bit is 1) Monitor/supervisor/system mode of operation (mode-bit

is 0) Whenever an interrupt or trap occurs, the hardware

switches from user-mode to monitor-mode. OS is in the monitor mode

DUAL-MODE OPERATION The dual-mode of operation provides us with

the means for protecting the OS from errant users and errant users from one another

The hardware allows privileged instructions (i.e. system calls) to be executed in only monitor mode

When an interrupt or fault occurs hardware switches to monitor – mode

Monitor User

Interrupt/Fault

Set user mode

I/O PROTECTION All I/O instructions are defined as privileged

instructions so users can not issue instructions from user-mode

Must ensure that a user program can never gain control of the computer in monitor-mode (i.e. a user program that, as part of its execution, stores a new address in the interrupt vector)

To do I/O, a user program executes a system call to request that the OS performs I/O on its behalf and returns the control to user after completion of I/O operation

MEMORY PROTECTION Must provide memory protection for the interrupt

vector, the interrupt service routines, and user programs from one another

In order to have memory protection, two registers are used to determine the range of legal addresses a program may access:

Base register – holds the smallest legal physical memory address

Limit Register – Contains the size of the range Memory outside the range is protected A trap is generated if any user’s program attempts to

access unauthorized memory area When executing in monitor- mode the OS has

unrestricted access to monitor and user’s memory The load instructions for the base and limit registers

are privileged instructions

CPU PROTECTION A program may be stuck:

In an infinite loop Fail to call system services Fail to return control to the OS

Timer - Interrupts computer after specified period to ensure that OS maintains control

Timer is decremented with every time click When timer reaches the value 0, an interrupt

occurs and control is automatically transferred to OS

Timer is also commonly used to implement time sharing mechanism

Timer can be used to compute the current time Load-timer is privileged instruction

NETWORK STRUCTURES LAN (Local Area Networks)

Were introduced in 1970 for economical use of a number of small computers and share resources

Cover a small geographical area and are generally used in an office environment

Communication links or LANs are high speed and lower error rates

High quality cables (TP, Fiber optic etc.) are used for establishing LANs

Common topologies are bus, ring and star Communications speed range from Mbps to

Gbps A typical LAN may consist of PCs/Laptops/ PDAs,

shared peripheral devices and or more gateways

LOCAL AREA NETWORK STRUCTURE

NETWORK STRUCTURES WAN (Wide Area Networks)

Emerged in the late1960s to provide efficient communication among sites

Are physically distributed over a large geographical area Hardware and software resources are shared conveniently and

economically by a wide community of users ARPANET grew from four sites to million of sites using internet The communication links (telephone lines, leased lines, microwave links,

satellite channels) are relatively slow and less reliable Communication processors control the communication links for

transferring information among the various sites The Internet WAN provide the ability for hosts at geographically

separated sites to communicate with one another The host computers differ from one an other in speed, type, operating

system etc. Connections between networks use a telephone-system service to

provide communication The router controls the path each message takes through the net.

Dynamic routing enhances communication efficiency whereas static routing reduces security risks

Modems convert digital data to analog signals and vice versa for communication

WANs are slower than LANs

WIDE AREA NETWORKS