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I/O Management and Disk Scheduling

Chapter 8 Advanced Operating System

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Test

In your opinion, what multimedia applications or tasks, if any, might be candidates for inclusion in a near-future operating system

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Categories of I/O Devices

Human readable Used to communicate with the user Printers Video display terminals

Display Keyboard Mouse

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Categories of I/O Devices

Machine readable Used to communicate with electronic

equipment Disk and tape drives Sensors Controllers Actuators

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Categories of I/O Devices

Communication Used to communicate with remote

devices Digital line drivers Modems

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Differences in I/O Devices

Data rate May be differences of several orders

of magnitude between the data transfer rates

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Differences in I/O Devices

Application Disk used to store files requires file

management software Disk used to store virtual memory

pages needs special hardware and software to support it

Terminal used by system administrator may have a higher priority

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Differences in I/O Devices

Complexity of control Unit of transfer

Data may be transferred as a stream of bytes for a terminal or in larger blocks for a disk

Data representation Encoding schemes

Error conditions Devices respond to errors differently

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Performing I/O

Programmed I/O Process is busy-waiting for the operation

to complete Interrupt-driven I/O

I/O command is issued Processor continues executing

instructions I/O module sends an interrupt when

done

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Performing I/O

Direct Memory Access (DMA) DMA module controls exchange of

data between main memory and the I/O device

Processor interrupted only after entire block has been transferred

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Direct Memory Access

Processor delegates I/O operation to the DMA module

DMA module transfers data directly to or form memory

When complete DMA module sends an interrupt signal to the processor

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DMA

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DMA Configurations

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DMA Configurations

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Operating System Design Issues

Efficiency Most I/O devices extremely slow compared

to main memory Use of multiprogramming allows for some

processes to be waiting on I/O while another process executes

I/O cannot keep up with processor speed Swapping is used to bring in additional

Ready processes which is an I/O operation

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Operating System Design Issues

Generality Desirable to handle all I/O devices in

a uniform manner Hide most of the details of device I/O

in lower-level routines so that processes and upper levels see devices in general terms such as read, write, open, close, lock, unlock

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I/O Buffering

Reasons for buffering Processes must wait for I/O to

complete before proceeding Certain pages must remain in main

memory during I/O

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I/O Buffering Block-oriented

Information is stored in fixed sized blocks Transfers are made a block at a time Used for disks and tapes

Stream-oriented Transfer information as a stream of bytes Used for terminals, printers,

communication ports, mouse and other pointing devices, and most other devices that are not secondary storage

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Single Buffer

Operating system assigns a buffer in main memory for an I/O request

Block-oriented Input transfers made to buffer Block moved to user space when

needed Another block is moved into the buffer

Read ahead

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Single Buffer

Block-oriented User process can process one block of

data while next block is read in Swapping can occur since input is

taking place in system memory, not user memory

Operating system keeps track of assignment of system buffers to user processes

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Single Buffer

Stream-oriented Used a line at time User input from a terminal is one line

at a time with carriage return signaling the end of the line

Output to the terminal is one line at a time

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I/O Buffering

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Double Buffer Use two system buffers instead of one A process can transfer data to or from

one buffer while the operating system empties or fills the other buffer

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Circular Buffer More than two buffers are used Each individual buffer is one unit in a circular

buffer Used when I/O operation must keep up with

process

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Disk Performance Parameters To read or write, the disk head must

be positioned at the desired track and at the beginning of the desired sector

Seek time Time it takes to position the head at the

desired track Rotational delay or rotational latency

Time its takes for the beginning of the sector to reach the head

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Disk Performance Parameters

Access time Sum of seek time and rotational delay The time it takes to get in position to

read or write Data transfer occurs as the sector

moves under the head

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Disk Scheduling Policies

Seek time is the reason for differences in performance

For a single disk there will be a number of I/O requests

If requests are selected randomly, we will poor performance

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Process B reads tracks 3, 5

Process A reads tracks 2, 5

Process C reads tracks 8, 4

OS has to read these tracks:

2,3,4,5,8.

Disk Scheduling

At runtime, I/O requests for disk tracks come from the processes

OS has to choose an order to serve the requests

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Disk Scheduling The order that the read/write head is

moved to satisfy several I/O requests determines the total seek time affects performance the OS cannot change the rotational delay or

transfer time, but it can try to find a ‘good’ order that spends less time in seek time.

If requests are selected randomly, we will get the worst possible performance...

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Disk Scheduling Policy

FIFO: fair, but near random scheduling

SSTF: possible starvation SCAN: favor requests for tracks near

the ends C-SCAN FSCAN: avoid “arm stickiness” in

SSTF, SCAN and C-SCAN

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First-in-first-out, FIFO process requests in the order that the

requests are made fair to all processes approaches random scheduling in

performance if there are many processes

1 2 7 108

Read write head starting at track 8, in the direction of increasing track number Pending

read/write request for a track

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Shortest Service Time First, SSTF

select the disk I/O request that requires the least movement of the disk arm from its current position

always choose the minimum seek time new requests may be chosen before an

existing request1 2 7 108

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SCAN arm moves in one direction only,

satisfying all outstanding requests until there is no more requests in that direction. The service direction is then reversed.

favor requests for tracks near the ends

1 2 7 108

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C-SCAN restrict scanning to one direction only when the last track has been visited in

one direction, the arm is returned to the opposite end of the disk and the scan begins again.

1 2 7 108

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FSCAN “Arm stickiness” in SSTF, SCAN, C-SCAN in

case of repetitive requests to one track FSCAN uses two queues. When a scan

begins, all of the requests are in one of the queues, with the other empty. During the scan, all new requests are put into the other queue.

Service of new requests is deferred until all of the old requests have been processed.

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Disk Cache

Buffer in main memory for disk sectors

Contains a copy of some of the sectors on the disk

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Disk Cache, Hit and Miss

When an I/O request is made for a particular sector, the OS checks whether the sector is in the disk cache. If so, (cache hit), the request is

satisfied via the cache. If not (cache miss), the requested

sector is read into the disk cache from the disk.

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Least Recently Used The block that has been in the cache

the longest with no reference to it is replaced

The cache consists of a stack of blocks Most recently referenced block is on

the top of the stack When a block is referenced or brought

into the cache, it is placed on the top of the stack

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Least Recently Used

The block on the bottom of the stack is removed when a new block is brought in

Blocks don’t actually move around in main memory

A stack of pointers is used

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Least Frequently Used The block that has experienced the

fewest references is replaced A counter is associated with each block Counter is incremented each time block

accessed Block with smallest count is selected for

replacement Some blocks may be referenced many

times in a short period of time and the reference count is misleading

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Hybrid and

Open source

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Windows Supports a wide range of computer

hardware Large library software Unstable: blue screen of death (Except

windows XP) Requires a system reboot when facing

errors Secure operating system Target of Viruses and spyware

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Windows

Microsoft’ s reaction Windows TCO and Security Windows Genuine Advantage

Notifications Internet Explorer has lost market share

to Firefox and Opera Internet Explorer: Windows Firefox: Windows, Linux Opera: Windows, Linux

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Windows Total cost of ownership (TCO)

Cost of computer and software Maintenance Training Technical support Hardware and software upgrade

Windows has a lower TCO: “Get the Facts”

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Windows

Security Closed-source is invisible for crackers

Windows Vista (Longhorn) High security Easy to management Detecting hardware feature problem Faster start up time

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Linux

Cost less More secure: Open source OS for networking Not viruses target

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Technical Comparison Kernel space: The central module of an operating

system. It is the part of the operating system that loads first, and it remains in main memory. Because it stays in memory, it is important for the kernel to be as small as possible while still providing all the essential services required by other parts of the operating system and applications. Typically, the kernel is responsible for memory management, process and task management, and disk management.

Windows: file system, Internet explorer, windows media player, GUI

Linux: only file system Memory management

Windows: Swap file Linux: avoiding swapping and allocating memory

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Technical Comparison Swapping

To replace pages or segments of data in memory.

Swapping is a useful technique that enables a computer to execute programs and manipulate data files larger than main memory.

The operating system copies as much data as possible into main memory, and leaves the rest on the disk.

When the operating system needs data from the disk, it exchanges a portion of data (called a page or segment ) in main memory with a portion of data on the disk.

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Technical Comparison Stability

Windows: blue screen of death (instability)

Linux: More stability Common source of instability is due

to bugs in various device drivers Device Driver

Windows: run in kernel space Linux: run in user space