Content 5: Input/Output - SJTUcse.sjtu.edu.cn/OS/courseware/IO.pdf · 1 1 5: Input/Output 2 Content Principles of I/O hardware Principles of I/O Software I/O software layers Disks

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5: Input/Output

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Content

►Principles of I/O hardware►Principles of I/O Software►I/O software layers►Disks►Clocks►Power management

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Principles of I/O hardware

►People look at hardware in different ways►EE people look at hardware in terms of:

– Chips, wires, power suppliers, motors, etc.– i.e. physical components that make up it

►Programmers look at the interface– Commands, functions, and errors reported back

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I/O Devices

►Roughly classified into two classes:– Block devices and character devices

►One stores data in fixed-sized blocks– Blocks are addressable

►The other in/outputs a stream of characters– Characters are not addressable

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I/O Devices

►Example of block devices:– Disks, tapes (?)

►Example of character devices:– Mouse, keyboard, printers, NIS

►Some devices do not fit in any class– Clocks (can it be considered a IO device?)

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I/O Devices

►IO devices cover a wide range in speeds– Puts pressure on software to perform well

►Need to hid the discrepancies in IO speed– So that user operation is independent of it

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Data Rate of Typical Devices

400 KBScanner

100 KBLaser printer

16 KBDual ISDN lines

8 KBTelephone channel

7 KB56K modem

100 bytesMouse

10 bytesKeyboard

Data Rate (per second)Device

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1.25 MBClassic Ethernet

1.5 MBUSB

16.7 MBEIDE (ATA-2) disk

16.7 MBISA bus

12.5 MBFast Ethernet

6 MB40x CD-ROM

5 MBIDE disk

4 MBDigital camcorder

Data RateDevice

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50 MBFireWire

60 MBXGA monitor

78 MBSONET OC-12

20 GBSun Gigaplane XB backplane

528 MBPCI bus

320 MBUltrium tape

125 MBGigabit Ethernet

80 MBSCSI Ultra 2 disk

Data RateDevice

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Device Controllers

►I/O devices have components:– mechanical component – electronic component

►Electronic component is device controller– may be able to handle multiple devices– Also called adapter– Often takes the form of a printed circuit card

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Device Controllers

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Device Controllers

►Controller's tasks– Control the physical operation of the device– convert serial bit stream to block of bytes– perform error correction as necessary

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Device Controllers

►Exchange data with CPU via registers►By writing into these registers

– OS can command it to deliver or accept data– Or switch the device on or off

►By reading from the registers– OS can learn the status of the device

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Device Controllers

►In addition, controllers often have buffers– Which the OS can read or write

►Ex: a video controller may have Video RAM– A data buffer programs can write or read

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I/O Approaches

►3 ways for CPU to communicates with the control registers and its device buffers– Dedicated I/O– Memory-mapped I/O– Hybrid

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Dedicated I/O

►Each control register assigned an IO port– i.e. an 8 or 16 bit integer

►IO port and memory are separate►OS uses special instructions to read/write

– IN REG, PORT for reading from the device– OUT PORT, REG for writing to the device

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Dedicated I/O

►Computers use dedicated I/O include– Early computers, mainframes (IBM 360/370)

►Problems with this approach?– Controls are at low-level– No high level languages has IN/OUT operations– Protections can be problematic

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Memory-Mapped I/O

►An alternative to Dedicated I/O►Each control register is assigned a unique

memory address– Input/output done by reading/writing to the

designated memory addresses– Just like a ordinary memory access

►Introduced in PDP-11 computer

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Hybrid Approach

►Memory-mapped data buffer►Dedicated I/O ports►For example, Pentium use address:

– 640K to 1M for device data buffers– 0 to 64K for I/O ports

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Memory-Mapped I/O

memory

I/O ports

0xFFFF…

0

2 address 1 address space 2 address space

(a) (b) (c)

Dedicated I/O Memory-mapped I/O Hybrid

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Memory-Mapped I/O

►Problems for memory-mapped I/O?►Cache cause problems►For 1-bus system, both memory and device

have to examine addresses on the bus►For system with multiple memory buses:

– Devices may not see the addresses on the bus

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Memory-Mapped I/O

A dual-bus memory architecture

(a) A single-bus architecture

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Memory-Mapped I/O

►Many ways to solve the multiple buses issue►Fail and try

– If memory fails to respond, try other buses

►Snooping– A snooping device on the bus passes to devices

►Address filtering

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Address Filtering in Pentium

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Direct Memory Access

►Memory-mapped or not, CPU needs to address controllers to exchange data

►Can request data one byte at a time►Or request data in bulk

– This is accomplished via DMA

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►Can use only if there is DMA controller►DMA controller can be built in device

controller– requires one DMA controller for each device

►More commonly, a single DMA controller– One the parentboard

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Operation of a DMA transfer28


►DMA controller vary in sophistication►Simplest one handles 1 transfer at a time►Complex one have multiple channels

– With each channel deals with one device

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Issues with DMA

►How to access the bus?– Cycle stealing or burst mode?

►Where to store the data?– Directly into memory or in DMA buffer?

►How to address the memory?– Virtual or physical address?

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Pros and Cons of DMA

►+ free up CPU►- Use CPU instead of DMA if:

– Device speed is fast– if CPU has nothing else to do– Save money (by getting rid of DMA)

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Principles of I/O Software

►Device independence is a key concept OS►Program should access all I/O devices in

same or similar ways– i.e. without specify the device in advance

►Ex: when reading a file, do not care if– The file is in floppy, disk, or CD-ROM

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Principles of I/O Software

►This is the responsibility of I/O software

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Goals of I/O Software

►Device independence– programs can access any I/O device – without specifying device in advance – (floppy, hard drive, or CD-ROM)

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►Uniform naming– name of a file or device a string or an integer– not depending on which machine

►Error handling– handle as close to the hardware as possible

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►Synchronous vs. asynchronous transfers– blocked transfers vs. interrupt-driven

►Buffering– data coming off a device cannot be stored in

final destination right away

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►Sharable vs. dedicated devices►Make dedicated devices appear sharable►Some devices are not sharable►Disks are sharable►Printer can be made sharable►Tape drives would not be sharable

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Programmed I/O

►Three different ways to perform I/O►Programmer I/O is the simplest►Interrupt-driven is the most common►DMA I/O used to improve efficiency

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Programmed I/O

►CPU wait for the I/O to complete►This is called Polling or Busy waiting

– Very bad– But simple to program

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Programmed I/OCopy_from_user(buffer,p,count) /*p is kernel buffer */For (i=0;i<count;i++) { /*loop on every character*/

while(*printer_status_reg!=READY); /*loop until ready*/*printer_data_register=p[i]; /*output one character*/

}Return_to_user();

Writing a string to printer using programmed I/O

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Programmed I/O

Steps in printing a string

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Interrupt-Driven I/O

►CPU setups and start I/O operation►CPU goes off to do other things►When I/O is done, CPU is interrupted►CPU handles the interrupt►CPU resumes interrupted operation

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►Code executed when print system call is madeCopy_from_user(buffer,p,count)Enable_interrupts();while(*printer_status_reg!=READY);

*printer_data_register=p[0];Scheduler();

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Interrupt-Driven I/O►Interrupt service procedure

If(count==0){unblock_user();

} else {*printer_data_register=p[i];count-count-1; i=i+1;

}acknowledge interrupt(); return_from_intertupt();

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I/O Using DMA

►An obvious con for interrupt-driven I/O is the frequent interrupts

►The solution is to use DMA

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I/O Using DMA

►Idea is to let DMA controller handle the I/O one character at a time

►In other words, busy wait with DMA

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I/O Using DMA

►Printing a string using DMA

Copy_from_user(buffer,p,count); acknowledge_interrupt();Set_up_DMA_controller(); unblock_user();Scheduler(); return_from_interrupt);

(a) (b)

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I/O Software Layers

►IO software are organized into layers►Each layer has a:

– Well-defined function to perform– Well-defined interface to the adjacent layer

►Exact division differs by systems

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I/O Software Layers

Hardware

Interrupt handlers

Device drivers

Device independent operating system software

User-level I/O software

I/OSoftwareLayer

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Interrupt Handlers

►Most I/O are interrupt-driven– Only rarely programmed I/O is used

►Therefore interrupt handlers is an integral part of the I/O software

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Interrupt Handlers

►Interrupt handlers are best hidden– So as little of OS know about it as possible

►Best way to hid is to:– have the driver starting an I/O operation block

until interrupt notifies of completion

►A drive can block itself by performing– a down ops on a semaphore or other methods

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Interrupt Handlers

►When notified of an interrupt:– The interrupt procedure does its task– then unblocks the driver that started the I/O

►Interrupt handler unblocks the driver by:– Sending a signal– Perform an up operation on a semaphore…

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Interrupt Steps

►Save registers that are not already saved by interrupt hardware

►Set up context for interrupt handler►Set up stack for interrupt handler►Ack interrupt controller►Reenable interrupts

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Interrupt Steps

►Copy registers from where saved►Run service procedure►Setup MMU for process to run next►Load new process' registers►Start running the new process

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Device Drivers

►The software that talks to the controller►Device specific►Tailored to individual device characteristics►Written by device manufacturers►Part of the OS Kernel

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Device Drivers

►Know about the details of the devices►Disk driver knows about sectors, tracks,

cylinders, heads, arm motions, motor drives►Mouse driver knows about button pressed

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Device Drivers

►Device drivers’ tasks:►Accept abstract read/write requests

– from device-independent software above it

►See that read/write requests carried out►Initialize device, turn on/off device►Power management for device

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Device Drivers

Communications between drivers and device controllers goes over the bus

Logical position of device drivers

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Device Drivers

►Most OS define a standard interfaces for– Block devices– Character devices

►The interface consists of a number of procedures that the rest of OS can call

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Device Drivers

►On UNIX, OS is a single binary with device drivers compiled into it

►Adding new drivers or modifying existing ones means recompile the OS

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Device Drivers

►In Windows, drivers are dynamically loaded

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Device Driver Operation

►Checking input parameters to see if valid– If not, report error

►Translate abstract request to concrete– convert a linear block # to head, track, sector...

►Check device is busy– If yes, request queued for later process

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►Check hardware status to see if the request can be handled now– May need to turn on the device– Or start a motor

►Control the operation by issuing a sequence of commands to the device registers

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►Block itself and wait for interrupt►Upon waking up, check for I/O errors►Pass data to the layer above it►Return status to its caller

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Device-Independent I/O Software

►Some I/O software is device specific►But some are not!►Basic function is to:

– Perform IO ops that are common to all devices– Provide uniform interface to user-level software

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Functions of the device-independent I/O software

Providing a deice-independent block size

Allocating and releasing dedicate devices

Error reporting

Buffering

Uniform interfacing for device drivers


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►Exact boundary between drivers & device independent software is device dependent

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Uniform Interfacing

►Make all devices look the same or similar– By standardizing driver interface

►Specify the functions a driver provides►Specify kernel function a driver can call

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Uniform Interfacing

Without a standard driver interface With a standard driver interface

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Buffering

►Buffering is indispensable in I/O►It speed up the data transfer►It synchronize between CPU and devices►It prevents overflow

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Buffering

Unbuffered Buffer in user space Buffer in kernel Double bufferfollowed by in the kernel

copying to user space72

Drawback of Buffering

►Slows down performance

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Drawback of Buffering

Networking may involve many copies74

Error Reporting

►Errors far more common in I/O– Due to interfacing with outside world

►Errors inside the computer case are rare►Error reporting depends on types of error

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Error Reporting

►Programming errors– Process ask for something impossible– Read from an output device

►Actual I/O errors– Accessing a damaged part of disk

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Error Reporting

►For programming errors, just report error back to the caller

►For actual I/O errors, – May take corrective measures– May ask the caller what to do– May simply fail and return an error code

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User-Space I/O Software

►It is often convenient for some part of I/O software to run in the user space

►Most OS do put part of IO in user space!

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►Ex: Count = write(fd, buffer, nbytes)►Library routine write will be linked with

the program and contained in the binary program present in memory at run time

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►Other I/O functions in user space include:►Format:

– Scanf and printf

►Spooling– Spooling daemon runs as a user process

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I/O system Layers and their main functions


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Disks

►Come in a variety of types►Magnetic disks:

– hard disk and floppy disks

►Optical disks:– CD-ROM, CD-RW, DVD

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Magnetic Disks

►Integral part of a computer system►Some contains a microcontroller

– IDE Disks

►Some have little electronics and just deliver a simple serial bit of stream– Let the device controller do the work!

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Magnetic Disks

►Some disks offer parallel processing►Doing seek on two ore more drives

– Overlapped seeks

►Read/write while seeking on another drive►Floppy disk cannot offer parallelism

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Disk Drives

►Disks are usually organized into drives►A drive may have up to 16 disks

– With each disk having its own read/write head

►Read data from one disk at a time

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Tracks and Cylinders

►The surface of a disk is divided in tracks– A track is one cycle

►All tracks of a disk drive forms a cylinder– A disk drive can have up to 16 disks

►Each track is further divided into sectors– A sector is the basic unit of I/O

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Disk Drives

Sector

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Sectors

►Numbered from 0 to some maximum►Sectors can either be continuous

– i.e. logically adjacent = physical adjacent

►Or interleave– i.e. logically adjacent != physical adjacent

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Sectors

►Interleaving factor: – Physical distance between logically adjacent

sectors on a track

►Gaps are maintained between sectors– This allows continuous read

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Sectors

Non-interleaving Interleaving

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Disk Read/Write Process

►First move head to the correct cylinder– seek

►Wait for the sector to pass by the head– rotation

►Actually read/write data– transfer

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Terms

►Locality of Reference: When record is read from disk, next request is likely to come from near the same place in the file

►Cluster: Smallest unit of file allocation, usually several sectors

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Terms

►Extent: A group of physically contiguous clusters

►Internal fragmentation: Wasted space within sector if record size does not match sector size; wasted space within cluster if file size is not a multiple of cluster size

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Seek Time

►Time for I/O head to reach desired track. – Largely determined by distance between I/O

head and desired track

►Track-to-track time: – time to move from track to an adjacent track

►Average Seek time: Average time to reach a track for random access

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Other Factors

►Rotational Delay or Latency: – Time for data to rotate under I/O head

►One half of a rotation on average►At 7200 rpm, this is 8.3/2 = 4.2ms

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Transfer time

►Time for data to move under the I/O head►At 7200 rpm: Number of sectors

read/Number of sectors per track * 8.3ms

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Disk Spec Example

►16.8 GB disk with 10 platters– 1.68GB/platter

►13,085 tracks/platter►256 sectors/track►512 bytes/sector

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Disk Spec Example

►Track-to-track seek time: 2.2 ms►Average seek time: 9.5ms►4KB clusters, 32 clusters/track►Interleaving factor of 3►5400RPM

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Disk Access Cost Example► Read a 1MB file divided into 2048 records of 512

bytes (1 sector) each► Assume all records are on 8 contiguous tracks► First track: 9.5 + 11.1/2 + 3 x 11.1 = 48.4 ms► Remaining 7 tracks: 2.2 + 11.1/2 + 3 x 11.1 = 41.1

ms► Total: 48.4 + 7 * 41.1 = 335.7ms

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Disk Access Cost Example► Read a 1MB file divided into 2048 records of 512

bytes (1 sector) each► Assume all file clusters are randomly spread

across the disk► 256 clusters. Cluster read time is (3 x 8)/256 of

a rotation for about 1 ms► 256(9.5 + 11.1/2 + (3 x 8)/256) is about 3877 ms.

or nearly 4 seconds

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How Much to Read

►Read time for one track:►9.5 + 11.1/2 + 3 x 11.1 = 48.4ms►Read time for one sector►9.5 + 11.1/2 + (1/256)11.1 = 15.1ms

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How Much to Read

►Read time for one byte :

►9.5 + 11.1/2 = 15.05 ms►Nearly all disk drives read/write one

sector at every I/O access►Also referred to as a page

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Disk Specification

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Physical Geometry

►Different tracks have different lengths– Outer tracks are longer than inner tracks– thus tracks may have differing # of sectors

►But this cause difficult to OS– Solution is to provide a virtual geometry

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Physical Geometry of a Disk

A possible virtual geometry for this disk

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IBM 75GXP Specification

►Size: 4”×6”×1”, ►Capacity: 76GB►RPM: 7200►Platters: 5►Sectors per track: 370-792►Data rate: 174-374Mbps

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Virtual Specification

►Number of tracks: 34327►Sectors per track: 512►Average seek time: 9.2ms►Track density: 28350/in

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RAID

►Redundant Array of Inexpensive Disks►A type of disk organization►Goal: improve performance and reliability►Idea: use multiple disks (drives) as 1 unit►Key technique: stripe

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RAID

►Depending on how stripe is accomplished►RAID can be classified into 6 levels►RAID 0: stripe in blocks, no redundancy►RAID 1:

– full redundancy, may or may not stripe

►RAID 2: stripe in bits, 1 bit dedicated ECC

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RAID

►RAID 3: stripe in words, more ECC►RAID 4:

– stripe in blocks with dedicated ECC

►RAID 5: – stripe in blocks with round-robin ECC

►RADI 2 and 3 require disk synchronization

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Raid levels 0 through 2

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Raid levels 3 through 5

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Disk Formatting

►Disk must be formatted to be useful►Format involves writing basic information

into the disk:– Sector information, boot information– Partition tables

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Disk Formatting

►A disk sector

The preamble contains some pattern that allows the hardware to recognize the start of a sector

ECC contains error correction code for data recovery

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Cylinder Skew

►To allow continuous read of data across cylinders, the numbering of sectors in neighboring cylinders are gapped

►To allow continuous read of data in the same track, sector may also be skewed

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Cylinder Skew

Anillustration of cylinder

skew

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Sector Interleaving

No interleaving Single interleaving Double interleaving

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Disk Arm Scheduling

►Time required to read or write a disk block determined by 3 factors– Seek time– Rotational delay– Actual transfer time

►Seek time dominates

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Disk Arm Scheduling

►Therefore scheduling goal is to minimize seek time

►Hence: Shortest Seek First (SSF)►Improved: Elevator Scheduling

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Disk Arm Scheduling

►Shortest Seek First scheduling algorithm

Initialposition

Pendingrequests

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Disk Arm Scheduling

►Elevator algorithm for scheduling requests

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Error Handling►Disk defect is inevitable

– Defect in uniform substrate – Defect in fine oxide coating

►Motion defect increase as time goes by– Read/write head moves into the wrong place

►Environment pollution– Dust and speck cause temporary RW errors

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Error Handling

►Disk defect cause bad blocks►If the defect is small, let the ECC correct►Otherwise, let entire sector go bad and

deal with bad sector in controller or OS

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Error Handling

►2 approaches to handle bad sectors:►Deal with them in controller

– Disk tested before shipping – bad sector marked and substituted with spare

►Deal with them in operating system– OS check for bad sectors and record them

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Error Handling

(a) Found a bad sector in a disk track (b) Find a replace sector(c) Shift all sectors to bypass the bad sector

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Stable Storage

►Property of stable storage:– Either write correctly – Or leave the old data intact

►Need both stable read and stable write►Idea is to use mirrors

– Many storage systems adapt this approach

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Stable Storage

Analysis of the influence of crashes on stable writes

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Optical Disks

►Optical disks are becoming popular►1980, Philips developed CD format

– CD are said to last for 100 years

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The Making of CD

► (with laser) Burn a 0.8 micron diameter holes in a coated glass master disk

►A mold is made from the master disk– With bumps where the laser holes are

►A polycarbonate disk is made from mold

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The Making of CD

►A very thin layer of reflective aluminum is deposited on the polycarbonate

►Place a protective lacquer on the disk►The depression in the polycarbonate

substrate are called pits►The unburn areas between pits called lands

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The Making of CD

►The pits and lands are written in a single continuous spiral starting near the hole – working out a distance of 32mm toward edge

►The spiral makes 22,188 revolutions around the disk

►If unwound, it would be 5.6km long

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Structure of a CD or CD-ROM

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Logical data layout on CD-ROM

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CD-ROM

►1984, Philips & Sony developed the CD-ROM standard for storing computer data

►1986, graphics and multimedia is added►High Sierra file standard developed

– It becomes IS 9660

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CD-Recordable

►Nick name CD-R►Can be used to recording data once

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Making of CD-R

►Starts with polycarbonate blanks►But they contains a 0.6mm groove

– To guide the laser for writing

►The groove has a sinusoidal excursion of 0.3mm at a frequency of exactly 22.05kHz– To provide continuous feedback so that

rotation speed can be accurately monitored

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Making of CD-R

►CD-Rs look like regular CD-ROMs►But they are gold coated instead of

aluminum for the reflective layer►Two kinds of dyes are used:

– Cyanine, which is green– Pthalocyanine, which is a yellowish orange

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Making of CD-R

►Initially, the dye are transparent and lets laser lights pass through and reflect off the gold layers

►To write, laser is turned into high power (8-16mW), and hits a spot of dye

►The dye heats up and break chemical bond

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Making of CD-R

►This creates a dark spot►When read back (at 0.5mW), the

photodetector sees a difference between the dark spots and the transparent areas

►This is interpreted as pits and lands

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CD-R

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CD-R

►Cross section of a CD-R disk and laser– not to scale

►Silver CD-ROM has similar structure– without dye layer– with pitted aluminum layer instead of gold

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CD-Rewritable

►Same as CD-R, except it uses►Alloy of silver, indium, antimony, and

tellurium for the recording layer►Instead of cyanine or ptholcyanine

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CD-Rewritable

►CD-RW drive use laser of 3 differing power►High power for recording►Lower power for read back►Middle power for reforming its state

– To become a land again

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DVD

►Digital Video (Versatile) Disk►Developed by cooperating industries:

– Movie, consumer electronics, computer

►DVD uses same general design as CDs►But with the following differences:

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DVD

►Smaller pits (0.4 micron vs. 0.8 micron)►Tighter spiral

– 0.74 micron between tracks– vs. 1.6 microns for CDs

►Red laser (at 0.65 micro vs. 0.78 microns)

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DVD

►Together, the changes raise the capacity sevenfold to 4.7GB

►But the switch to red laser means that DVD player will need a second laser or fancy conversion to read existing CDs

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DVD

►There are four DVD formats:►Single-sided, single-layer (4.7GB)►Single-sided, double-layer (8.5GB)►Double-sided, single-layer (9.4GB)►Double-sided, dual-layer (17GB)

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A double sided, dual layer DVD disk

DVD

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Clocks

►Also called Timers►Essential for OS to work properly►Synchronizing various circuits in computer►Regulates the OS interrupts

– Prevents one process from monopolizing CPU

►Keeps time of day (real-time)

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Clock Hardware

►Two types of clocks are commonly used►Power-line clocks

– Cause interrupts on every voltage cycle– At either 50Hz (220V) or 60Hz (110V)– Used to dominate, now very rare

►Crystal Oscillator Clocks– Now commonly used in computers

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Crystal Oscillator Clocks

►Consists of a crystal oscillator, a counter, and a holding register

►Using electronics, output signal frequency can reach 1000MHz or even more

►The signal is fed to counter to count down to zero, and causes a CPU interrupt

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Crystal Oscillator Clocks

A programmable clock

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Clock Software

►Clock hardware only generates interrupts at known intervals

►Everything else involved is done by software►Also called the clock driver

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Tasks of Clock Drivers►Maintaining time of day

– Explain shortly

►Preventing process from running longer than they are allowed to– by recording clock ticks a process has run– Call scheduler when quantum exhausts

►Accounting for CPU usage

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Tasks of Clock Drivers

►Handling the ALARM system call by users►Providing watchdog timers

– Do something after a timer expires– i.e. floppy disk needs to get to speed before read

►Profiling, monitoring, and data gathering– Providing statistical information about system

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Time of Day

►Just requires to incrementing a counter at each clock ticks

►Need to watch the length of the counter►A 32 bit counter will overflow in 2 years

with clock rate of 60Hz►3 approaches proposed to solve the problem

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Time of Day

Three ways to maintain the time of day

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Handling Alarm

►A process can request OS to give warning when certain time interval

►Each such process requires a clock– Thus multiple clocks may be needed

►If the # of physical clocks are not enough– We simulate multiple clocks in software

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Handling Alarm

►If many signals are expected, it is more efficient to chain requests

►Sorted on time in a linked list►Each entry tells how many clock ticks

following the previous one to wait before causing a signal

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Handling Alarm

Simulating multiple timers with a single clock

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Soft Timers

►A 2nd clock available for timer interrupts– specified by applications– no problems if interrupt frequency is low

►Avoid interrupts by kernel checking for soft timer before it exits to user mode

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Soft Timers

►How well does soft timers work depends on rate of kernel entries

►If kernel entry is rare►Application deadline could miss►Average entry rate is 2 to 18 micro seconds

– Thus, a soft timer goes off every 12 is doable

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Character Oriented Terminals

►A rather frequent used device in old times►Can only pass ASCII characters, not pixels►RS-232 Terminal Hardware

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RS-232 Terminal Hardware

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►Communicate with computer 1 bit at a time►Called a serial line –►bits go out in series, 1 bit at a time

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►Windows uses COM1 and COM2 ports, first to serial lines

►Computer and terminal are completely independent

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Central buffer pool

Input Software

Dedicated buffer for each terminal

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Input Software

Linefeed (unchangeable)NLCTRL-J

Carriage return (unchangeable)CRCTRL-M

End of fileEOFCTRL-D

Force core dump (SIGQUIT)QUITCTRL-\

Interrupt process (SIGINT)INTRDEL

Start outputSTARTCTRL-Q

Stop outputSTOPCTRL-S

Interpret next character literallyLNEXTCTRL-V

Erase entire line being typedKILLCTRL-U

Backspace one characterERASECTRL-H

CommentPOSIX nameCharacter

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Input Software

►Characters handled specially in canonical mode

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►accepted by terminal driver on output►ESC is ASCII character (0x1B)►n,m, and s are optional numeric parameters

The ANSI escape sequences

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The ANSI escape sequences

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Parallel port

Display Hardware

Memory-mapped displays► driver writes directly into display's video RAM

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Display Hardware

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Display Hardware

►A video RAM image – simple monochrome display– character mode

►Corresponding screen– the xs are attribute bytes

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Input Software

►Keyboard driver delivers a number– driver converts to characters– uses a ASCII table

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Input Software

►Exceptions, adaptations needed for other languages– many OS provide for loadable keymaps or code

pages

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Output Software for Windows

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►Sample window located at (200,100) on XGA display

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Skeleton of a Windows Main#include <windows.h>Int WINAPI WinMain(HINSTANCE h, HINSTANCE, hprev, char *szCmd, int

iCmdShow){ WNDCLASS wndclass; /*class object for this window*/

MSG msg; /* incoming messages are stored here*/HWND hwnd; /* handle (pointer) to the window object *//* initialize wndclass */wndclass.lpfnWndProc = WndProc; /*tells which procedure to call */wndclass.lpszClassName =“Program name”; /* Text for title bar */wndclass.hlcon = Loadlcon(NULL,IDI_APPLICATION); /*load program icon*/wndclass.hCursor=LoadCursor(NULL,IDC_ARROW); /*load mouse cursor*/

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Skeleton of a Windows MainRegisterClass(&wndclass); /*tell Windows about wndclass */hwnd=CreateWindow(…); /*allocate storage for the window*/ShowWindow(hwnd,iCmdShow); /*display the window on screen*/UpdateWindow(hwnd); /*tell the window to paint itself*/While(GetMessage(&msg,NULL,0,0)){ /*get message from queue*/

TranslateMessasge(&msg); /*translate the message*/DispatchMessage(&msg) /*send msg to appropriate procedure*/

}Return(msg.wParam);

}

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Skeleton of a Windows MainLong CALLBACK WndProc(HWND hwnd, UINT messasge, UINT wParam, long

IParam){

/* declarations go here */

switch(message) {case WM_CREATE: …; return …; /* create window*/case WM_PAINT: …; return …; /* repaint contents of window*/case WM_DESTROY: …; return …; /* destroy window */

}Return(DefWindowProc(hwnd, messasge, wParam, IParam)); /*default*/

}

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An example rectangle drawn using Rectangle182


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Output Software for Windows►Copying bitmaps using BitBlt.

– before– after

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►Examples of character outlines at different point sizes

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X Windows Network Terminals

Clients and servers X Window System

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Skeleton of an X Windows application program

#include <X11/Xlib.h>#include <X11/Xutil.h>Main(int argc, char *argv[]){

display disp; /*server identifier*/Window win; /*window identifier*/GC gc; /*graphic context identifier*/XEvent event; /*storage for one event*/int running=1;

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disp=XOpenDisplay(“display_name”); /*connect to the X server*/win=XCreateSimpleWindow(disp, …); /*allocate memory for new window*/XsetStandardProperties(disp, …); /*announces window to window mgr*/gc=XCreateGC(disp, win, 0, 0); /*create graphic context*/XSelectInput(disp, win, ButtonPressMask | KeyPressMAsk | ExposureMask);XMapRaised(disp, win); /*display window; send Expose event*/

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While(running) {XNextEvent(disp, &event); /*get next event*/switch(event.type) {

case Expose: …; break; /*repaint window*/case ButtonPress: …; break; /*process mouse click*/case Keypress: …; break; /*process keyboard input */

}}XFreeGC(disp,gc); /*release graphic conext */XDestroyWindow(disp, win); /*deallocate window’s memory space */XCloseDisplay(disp); /*tear down network connection*/

}

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The SLIM Network Terminal

The architecture of the SLIM terminal system

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The SLIM Network Terminal

Messages used in the SLIM protocolfrom the server to the terminals

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Power Management

►A PC usually has a 200W power supply– With 85% efficiency

►If 100 million of PC turn on at once– they use 20,000 megawatts of electricity

►This equals to the output of 20 average-sized nuclear power plants!

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Power Management

►For battery-powered computers, i.e. laptops, notebooks, power is also a big issue

►Reducing power consumption could mean longer battery life

►Operating system plays a major role here

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Power Management

►Two approaches can be taken by the OS:►Turn off parts of computer

– Turned off component do not consume power

►Restrict application’s power usage– Saving power by degrading quality

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Power Management

Power consumption of various parts of a laptop computer

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Power Management

►The biggest power consuming parts are:►Display, disk, and CPU►Cutting display power usage by lighting up

part of the screen that is active►Cutting disk power by spinning it down►Cutting CPU power by slowing it down

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Lighting Part of Screen

The use of zones for backlighting the display

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Cutting CPU Speed

► (a) Running at full clock speed► (b) Cutting voltage by two

– cuts clock speed by two, – cuts power by four

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Issues in Turning Off Devices

►Who to turn off?►When to turn off?►Any state saving in turning off?►Will turning on require more power?►Need any context switch?►How much power is saved?

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Save Power from Applications

►Telling the programs to use less energy– may mean poorer user experience

►Examples– change from color output to black and white– speech recognition reduces vocabulary– less resolution or detail in an image

Questions?

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Comparisons

0.0010.010.03Tape0.250.360.50Floppy0.010.100.25Disk1.507.00$45.00RAM

Early 2000Mid 1997Early 1996Medium

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Comparisons

►RAM is usually volatile►RAM is about 1/4 million times faster than

disk

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Golden Rule of File Processing

►Minimize the number of disk accesses!►Arrange information so that you get what

you want with few disk accesses►Arrange information to minimize future

disk accesses

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Golden Rule of File Processing

►An organization for data on disk is often called a file structure

►Disk-based space/time tradeoff: Compress information to save processing time by reducing disk accesses

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Device Independence

►Problem: – lots of different brands of disks, disk

interfaces, – And disk controllers– each with their own custom control interface

►Same is true of any I/O device– network card, video card, keyboard, mouse, etc.

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Device Independence

►Add a “device driver” layer– to hide this diversity/complexity

►An abstraction makes things “better”– for things that use the abstraction

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Device Independence

Millions of hardware devices

Rest of operating systems

Device drivers

Physical interface

Virtual interface

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Example Abstractions

►Threads: – user doesn’t have to worry about sharing CPU

►Addresses spaces: – user doesn’t have to worry about sharing

physical memory– or physical memory size

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Example Abstractions

►TCP: – user doesn’t have to worry about the unreliable

network

►Device drivers: – frees the user (the rest of the OS) from

worrying about differences in devices – e.g. the device registers used to control the

device

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Byte Oriented Block Oriented

►Disks are accessed in terms of blocks (sectors)– e.g. 512 bytes

►But programs deal in bytes►Questions

– How to read less than a block– How to write less than a block

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Disk Geometry

►Disk is made of a stack of spinning platters

►The top and bottom surface of each platter has concentric circles of data (called tracks)

►The set of tracks at the same radial distance is called a cylinder

►Each track is made of a number of sectors

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Disk Geometry

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Accessing a Disk

►Queueing time (wait for disk to be free): – 0-infinity

►Position disk arm and head (seek and rotate): – 0-12 ms

►Access disk data: – size/transfer rate, e.g. disk transfer rate of

49 MB/s on Seagate Barracuda 1181677LW)

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Optimizing Disk Performance

►Disk is slow!►Best option is to eliminate the disk I/O

– e.g. via file caching

►If you do have to go to disk– try to keep positioning time small

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►If you do have to re-position– try get lots of data to amortize the positioning

overhead

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►efficiency = – transfer time / (positioning time + transfer

time)

►e.g. on Seagate Barracuda 1181677LW, – transferring 300 KB takes 6 ms (50%

efficiency)

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Reducing Positioning Time

►Optimize when writing data to disk►Place items that will be accessed together

near each other on disk– How to know in advance that two items will be

accessed together?

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Reducing Positioning Time

►Optimize when reading data from disk– minimize positioning time at time of access– rather than at time of creation

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Disk Scheduling

►Re-order a set of disk accesses to decrease the total positioning time– can be implemented in the OS– or in the disk hardware

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Disk Scheduling

►FCFS (first come, first served)►SSTF (shortest seek time first)►SCAN►LOOK►C-SCAN

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FCFS

►e.g. (start at track 53): – 98, 183, 37, 122, 14, 124, 65, 67

►total head movement: – 640 tracks (average 80 per seek)

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SSTF

►e.g. 53->65->67->37->14->98->122->124->183


►Any problems with SSTF?

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SCAN

►Sweep the disk from one end to the other►Then back again►Sometimes called “Elevator algorithms”

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LOOK

►A variant of SCAN►when sweeping toward the end

– stop if there are no requests beyond the current point

– e.g. 53->37->14->65->67->98->122->124->183


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C-SCAN

►Always sweep the disk from beginning to end

►Then seek all the way back to the beginning

►And repeat

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Disk Scheduling

►Disk scheduling only improves positioning time with multiple outstanding requests– otherwise service the sole request

Documents

Content 5: Input/Output - SJTUcse.sjtu.edu.cn/OS/courseware/IO.pdf · 1 1 5: Input/Output 2 Content Principles of I/O hardware Principles of I/O Software I/O software layers Disks