Chapter 4 Device Management and Disk Scheduling DEVICE MANAGEMENT Content I/O device overview I/O device overview I/O organization and architecture I/O.

Chapter 4Chapter 4

Device Management and Disk Device Management and Disk SchedulingScheduling

DEVICE MANAGEMENTDEVICE MANAGEMENT

ContentContent

I/O device overviewI/O device overview

I/O organization and architectureI/O organization and architecture

I/O bufferingI/O buffering

Disk schedulingDisk scheduling

RAIDRAID

NotesNotes: Difficult to develop a general, consistent : Difficult to develop a general, consistent

solution. Just a generally discuss.solution. Just a generally discuss.


4.1 I/O Devices (P452)4.1 I/O Devices (P452)

Three categories:Three categories:

Human Readable DevicesHuman Readable Devices

٭ Such as Keyboard, Mouse, Display, Printer and so Such as Keyboard, Mouse, Display, Printer and so

on;on;

Machine Readable DevicesMachine Readable Devices

٭ Such as Actuator, Controller, Sensor, Tape Drive, Such as Actuator, Controller, Sensor, Tape Drive,

Disk Drive and so on;Disk Drive and so on;

Communication DevicesCommunication Devices

٭ Such as Modem, Network Adapter and so on.Such as Modem, Network Adapter and so on.


ClassifyClassify

There are many sorting standard. There are many sorting standard. E.g.E.g.

data ratesdata rates


Data Rates of DevicesData Rates of Devices


Differences in I/O DevicesDifferences in I/O Devices

ApplicationApplication٭ Disk used to store files requires Disk used to store files requires file-management file-management

softwaresoftware٭ Disk used to store virtual memory pages needs Disk used to store virtual memory pages needs

special hardware and softwarespecial hardware and software to support it to support it٭ Terminal used by system administrator may have Terminal used by system administrator may have

a higher priority than an ordinary usera higher priority than an ordinary user


Differences in I/O DevicesDifferences in I/O Devices

Complexity of controlComplexity of control

Unit of transferUnit of transfer

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

Data representationData representation

٭ Encoding schemesEncoding schemes

Error conditionsError conditions

٭ Devices respond to errors differentlyDevices respond to errors differently


4.2 Techniques for Performing I/O 4.2 Techniques for Performing I/O (p453 11.2)(p453 11.2)

Cyclic Test /Programmed Cyclic Test /Programmed

Interrupt-DrivenInterrupt-Driven

DMA ControlDMA Control


Programmed I/O --P29.Programmed I/O --P29.

I/O module performs the action, not I/O module performs the action, not the processorthe processor

Sets appropriate bits in the I/O Sets appropriate bits in the I/O status registerstatus register

No interrupts occurNo interrupts occurProcessor checks status until Processor checks status until

operation is completeoperation is completeBusy-waitsBusy-waits


Interrupt-Driven I/OInterrupt-Driven I/O

Processor is interrupted when I/O Processor is interrupted when I/O module ready to exchange datamodule ready to exchange data

Processor is free to do other workProcessor is free to do other work

No needless waitingNo needless waiting

Consumes a lot of processor time Consumes a lot of processor time because because every wordevery word read or written read or written passes through the processorpasses through the processor


Direct Memory AccessDirect Memory Access

Transfers a block of data directly Transfers a block of data directly to or from memoryto or from memory

An interrupt is sent when the An interrupt is sent when the task is completetask is complete

The processor is only involved at The processor is only involved at the beginning and end of the the beginning and end of the transfertransfer


Evolution of the I/O FunctionEvolution of the I/O Function

Processor directly controls a peripheral deviceProcessor directly controls a peripheral device

Controller or I/O module is addedController or I/O module is added

٭ Processor uses programmed I/O without Processor uses programmed I/O without

interruptsinterrupts

٭ Processor does not need to handle details of Processor does not need to handle details of

external devicesexternal devices



Controller or I/O module with interruptsController or I/O module with interrupts

٭ Processor does not spend time waiting for an I/O Processor does not spend time waiting for an I/O

operation to be performedoperation to be performed


٭ Blocks of data are moved into memory without Blocks of data are moved into memory without

involving the processorinvolving the processor

٭ Processor involved at beginning and end onlyProcessor involved at beginning and end only



I/O channelI/O channel٭ I/O module is a separate processor. Its with a I/O module is a separate processor. Its with a

specialized instruction set tailored(specialized instruction set tailored( 适合适合 ) for ) for I/O.I/O.

I/O processorI/O processor٭ I/O module has its own local memoryI/O module has its own local memory٭ Its a computer in its own rightIts a computer in its own right



Takes control of the system from the CPU to Takes control of the system from the CPU to transfer data to and from memory over the system transfer data to and from memory over the system busbus

Cycle stealingCycle stealing is used to transfer data on the is used to transfer data on the system bussystem bus

The instruction cycle is suspended so data can be The instruction cycle is suspended so data can be transferredtransferred

The CPU pauses one bus cycleThe CPU pauses one bus cycleNo interrupts occur while cycle stealingNo interrupts occur while cycle stealing

٭ Do not save contextDo not save context


Direct Memory Access Direct Memory Access


DMADMA

The Starting location in memory to read from or The Starting location in memory to read from or

write to, communicated on the data lines and stored write to, communicated on the data lines and stored

by the DMA module in its by the DMA module in its address registeraddress register..

The number of words to be read or written, again The number of words to be read or written, again

communicated via the data lines and stored in the communicated via the data lines and stored in the

data count registerdata count register..


DMA and Interrupt BreakpointDMA and Interrupt Breakpoint

The processor is suspended The processor is suspended just before it needs to just before it needs to use the bususe the bus. The DMA unit then transfers one word . The DMA unit then transfers one word and returns control to the processor.and returns control to the processor.

Cycle stealing causes the CPU to execute more Cycle stealing causes the CPU to execute more slowly.slowly.


DMA and Interrupt BreakpointDMA and Interrupt Breakpoint


DMADMA

Number of required bus cycles can be cut by Number of required bus cycles can be cut by

integrating the DMA and I/O functionsintegrating the DMA and I/O functions

Path between DMA module and I/O module that Path between DMA module and I/O module that

does not include the system busdoes not include the system bus


DMADMA


DMADMA


DMADMA


4.3 Operating System Design Issues4.3 Operating System Design Issues

EfficiencyEfficiency٭ Most I/O devices extremely slow compared to Most I/O devices extremely slow compared to

main memorymain memory٭ Use of Use of multiprogrammingmultiprogramming allows for some allows for some

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

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

processes which is an I/O operationprocesses which is an I/O operation٭ Thus, a major effort in I/O design has been Thus, a major effort in I/O design has been

schemes for improving the efficiency of I/O. schemes for improving the efficiency of I/O. e.g. disk I/Oe.g. disk I/O


Operating System Design IssuesOperating System Design Issues

GeneralityGenerality٭ Desirable to handle all I/O devices in a Desirable to handle all I/O devices in a uniform uniform

mannermanner٭ Hide most of the details of device I/O in lower-Hide most of the details of device I/O in lower-

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

Struct general_op{Struct general_op{

int (*read)(…)int (*read)(…)

int (*write)(…)int (*write)(…)

};};

driver1:driver1:Struct general_op dev_op={ dev1_read,Struct general_op dev_op={ dev1_read,

dev1_writedev1_write};};

driver2:driver2:Struct general_op dev_op={ dev2_read,Struct general_op dev_op={ dev2_read,

dev2_writedev2_write};};

Gen_read(fd,…)Gen_read(fd,…){{ dev_op=dev_op=mapmap(fd);(fd);

dev_op->read(…);dev_op->read(…);}}


Logical Structure of the I/O Function (e.g. Logical Structure of the I/O Function (e.g. Hierarchical p458)Hierarchical p458)

Logical I/OLogical I/O٭ Is concerned with general I/O function, and is not Is concerned with general I/O function, and is not

concerned with the detail of actually controlling. concerned with the detail of actually controlling. E.g. read, write…E.g. read, write…

Device I/ODevice I/O

٭ Implement I/O instructions, channel commands, Implement I/O instructions, channel commands, and controller orders. May use buffering and controller orders. May use buffering techniques.techniques.


Logical Structure of the I/O FunctionLogical Structure of the I/O Function

Scheduling and ControlScheduling and Control٭ This is the level of software that actually interacts This is the level of software that actually interacts

with the I/O module and the device hardware.with the I/O module and the device hardware.



4.44.4 I/O Buffering(p460)I/O Buffering(p460)

Reasons for bufferingReasons for buffering٭ Processes must wait for I/O to complete before Processes must wait for I/O to complete before

next proceeding. E.g: write to device (next proceeding. E.g: write to device (busy busy waiting or suspension on an interruptwaiting or suspension on an interrupt))

٭ Certain pages of the I/O request process must Certain pages of the I/O request process must remain in main memory during I/O.remain in main memory during I/O.

٭ Single-process deadlockSingle-process deadlock


I/O BufferingI/O Buffering

Block-orientedBlock-oriented٭ Information is stored in fixed sized blocksInformation is stored in fixed sized blocks٭ Transfers are made a block at a timeTransfers are made a block at a time٭ Used for disks and tapesUsed for disks and tapes

Stream-orientedStream-oriented

٭ Transfer information as a stream of bytesTransfer information as a stream of bytes٭ Used for terminals, printers, communication Used for terminals, printers, communication

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


Single BufferSingle Buffer

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

Block-orientedBlock-oriented٭ Input transfers made to bufferInput transfers made to buffer٭ Block moved to user space when neededBlock moved to user space when needed٭ Another block is moved into the bufferAnother block is moved into the buffer

▪ Read aheadRead ahead




Block-orientedBlock-oriented

٭ User process can process one block of data while User process can process one block of data while next block is read innext block is read in

٭ Swapping can occur since input is taking place in Swapping can occur since input is taking place in system memory, not user memorysystem memory, not user memory

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



Stream-orientedStream-oriented٭ Used a line at time.Used a line at time.٭ User input from a terminal is one line at a time wiUser input from a terminal is one line at a time wi

th carriage return signaling(th carriage return signaling( 回车信号回车信号 ) the end o) the end of the line.f the line.

٭ Output to the terminal is one line at a time.Output to the terminal is one line at a time.



Performance comparison with no bufferingPerformance comparison with no buffering

٭ T: the time to input a blockT: the time to input a block

٭ C: the time to computationC: the time to computation

٭ M: the time to move dataM: the time to move data

٭ Max [C,T]+M and T+CMax [C,T]+M and T+C


Double BufferDouble Buffer

Use two system buffers instead of one.Use two system buffers instead of one.

A process can transfer data to or from one buffer A process can transfer data to or from one buffer while the operating system empties or fills the othwhile the operating system empties or fills the other buffer.er buffer.

Performance: max(C,T).Performance: max(C,T).


Circular BufferCircular Buffer

More than two buffers are used.More than two buffers are used.

Each individual buffer is one unit in a circular Each individual buffer is one unit in a circular buffer.buffer.

Used when I/O operation must keep up with Used when I/O operation must keep up with process.process.



Disk and Disk Scheduling(p463)Disk and Disk Scheduling(p463)

The speed of processors and main memory has The speed of processors and main memory has far outstripped that for disk access.far outstripped that for disk access.

The performance of disk storage subsystem is of The performance of disk storage subsystem is of vital concern.vital concern.

How to improve the performance of disk?How to improve the performance of disk?

٭ General methodsGeneral methods٭ Special methodsSpecial methods


Disk StructureDisk Structure

HeadersHeaders

CylindersCylinders

TracksTracks

SectorsSectors

Capability=headers*cylinders*sectors*byte per Capability=headers*cylinders*sectors*byte per sectorsector


Disk Performance ParametersDisk Performance Parameters

To read or write, the disk head must be positioned To read or write, the disk head must be positioned at the desired track and at the beginning of the at the desired track and at the beginning of the desired sectordesired sector

Seek time Seek time ٭ movable-head system or fixed-head systemmovable-head system or fixed-head system٭ Time it takes to position the head at the desired Time it takes to position the head at the desired

tracktrackRotational delay or rotational latencyRotational delay or rotational latency

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


Disk Performance ParametersDisk Performance Parameters

Data transfer timeData transfer timeWait for device or channelWait for device or channel


RPS (Rotational Positional Sensing)RPS (Rotational Positional Sensing)

When seek command has been issued, the When seek command has been issued, the channel is released to handle other I/O channel is released to handle other I/O operations.operations.

RPS miss. Must rotate again and reconnect RPS miss. Must rotate again and reconnect again.again.


Seek TimeSeek Time

Ts Ts ＝＝ m×nm×n ＋＋ ss

n=number of tracks traversedn=number of tracks traversed

m=constant that depends on the disk drivem=constant that depends on the disk drive

s=startup times=startup time


Rotational DelayRotational Delay

Tr Tr ＝＝ 1/(2r)1/(2r)

r=rotation speed, in revolutions per secondr=rotation speed, in revolutions per second

Example:Example:

r=3600rpm, Tr=1/120=8.3msr=3600rpm, Tr=1/120=8.3ms

r=300rpm, Tr=1/10=100msr=300rpm, Tr=1/10=100ms


Transfer TimeTransfer Time

T T ＝＝ bb ／／ (r×N)(r×N)

b=number of bytes to be transferredb=number of bytes to be transferred

N=number of bytes on a trackN=number of bytes on a track

r=rotation speed,in revolutions per secondr=rotation speed,in revolutions per second


Total Average Access TimeTotal Average Access Time

Ta=Ts+Tr+TTa=Ts+Tr+T


A Timing ComparisonA Timing Comparison

Seek time: 20msSeek time: 20ms

Transfer rate: 1MB/sTransfer rate: 1MB/s

r=3600rpmr=3600rpm

512byte per sector512byte per sector

32 sector per track32 sector per track

Suppose to read 128k dataSuppose to read 128k data



60*16k=960k<1MB/s60*16k=960k<1MB/s

Sequential organizationSequential organization

(20(20 ＋＋ 8.38.3 ＋＋ 16.7)16.7) ＋＋ (8.3(8.3 ＋＋ 16.7)×716.7)×7 ＝＝ 220(ms)220(ms)

Randomly organizationRandomly organization

(20(20 ＋＋ 8.38.3 ＋＋ 0.5)×2560.5)×256 ＝＝ 7373(ms)7373(ms)



It is worthwhile to improve the I/O efficiency with It is worthwhile to improve the I/O efficiency with

purely random accesspurely random access to the disk to the disk


Disk Scheduling PoliciesDisk Scheduling Policies

Seek time is the reason for differences in Seek time is the reason for differences in performance.performance.

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

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



First-in, first-out (FIFO)First-in, first-out (FIFO)٭ Process request sequentially.Process request sequentially.٭ Fair to all processes.Fair to all processes.٭ Approaches random scheduling in performance Approaches random scheduling in performance

if there are many processes.if there are many processes.



PriorityPriority٭ Goal is not to optimize disk use but to meet other Goal is not to optimize disk use but to meet other

objectives.objectives.٭ Short batch jobs may have higher priority.Short batch jobs may have higher priority.٭ Provide good interactive response time.Provide good interactive response time.


FIFO (Starting at Track 100)FIFO (Starting at Track 100)

Next track accessed number of tracks traversed

55 45

58 3

39 19

18 21

90 72

160 70

150 10

38 112

184 146

Total seek length 498

Average seek length 55.3

5555 、、 5858 、、 3939 、、 1818 、、 9090 、、 160160 、、 150150 、、 3838 、、 184184



Last-in, first-outLast-in, first-out٭ The device is given to the most recent user so The device is given to the most recent user so

there should be little arm movementthere should be little arm movement٭ Possibility of starvation since a job may never Possibility of starvation since a job may never

regain the head of the lineregain the head of the line



Shortest Service Time FirstShortest Service Time First٭ Select the disk I/O request that requires the least Select the disk I/O request that requires the least

movement of the disk arm from its current movement of the disk arm from its current positionposition

٭ Always choose the minimum seek timeAlways choose the minimum seek time


Next track Number of track traversed 90 10 58 32 55 3 39 16 38 1 18 20 150 132 160 10 184 24

Total seek length 248

Average seek length 27.6

5555、、 5858、、 3939、、 1818、、 9090、、 160160、、 150150、、 3838、、 118484

SSTFSSTF



SCANSCAN٭ Arm moves in one direction only, satisfying all Arm moves in one direction only, satisfying all

outstanding requests until it reaches the last outstanding requests until it reaches the last track in that directiontrack in that direction

٭ Direction is Direction is reversedreversed٭ Max delay:2T. T: a scan time from inner track to Max delay:2T. T: a scan time from inner track to

outer track outer track


SCANSCAN

Next track Number of tracks traversed 150 50 160 10 184 24 90 94 58 32 55 3 39 16 38 1 18 20

total seek length 250

average seek length 27.8

5555、、 5858、、 3939、、 1818、、 9090、、 160160、、 150150、、 3838、、 118484



C-SCANC-SCAN٭ Restricts scanning to one direction onlyRestricts scanning to one direction only٭ When the last track has been visited in one directiWhen the last track has been visited in one directi

on, the arm is returned to the opposite end of the on, the arm is returned to the opposite end of the disk and the scan begins againdisk and the scan begins again

٭ Max delay: T+Smax. Smax is the maximum seek Max delay: T+Smax. Smax is the maximum seek time time


CSCANCSCAN

Next track number of tracks traversed 150 50 160 10 184 24 18 166 38 20 39 1 55 16 58 3 90 32

total seek length 322

average seek length 35.8

5555、、 5858、、 3939、、 1818、、 9090、、 160160、、 150150、、 3838、、 118484



N-step-SCANN-step-SCAN٭ Segments the disk request queue into subqueues oSegments the disk request queue into subqueues o

f length N.f length N.٭ Subqueues are process one at a time, using SCAN.Subqueues are process one at a time, using SCAN.

٭ New requests added to other queue when queue is New requests added to other queue when queue is processed.processed.

FSCANFSCAN

٭ Two queuesTwo queues٭ One queue is empty for new requestOne queue is empty for new request



Figure 11.7 P469Figure 11.7 P469


RAID(p471)RAID(p471)

Redundant Array of Inexpensive DisksRedundant Array of Inexpensive Disks

Redundant Array of Independent DisksRedundant Array of Independent Disks


RAID RAID

Using multiple disks can:Using multiple disks can:٭ I/O requests can be handled in I/O requests can be handled in parallelparallel..٭ The The reliabilityreliability of disk can be improved. of disk can be improved.


RAID CharacteristicsRAID Characteristics

RAID is a set of physical disk drives viewed by the RAID is a set of physical disk drives viewed by the operating system as a single logical drive. operating system as a single logical drive. ((transparenttransparent))

Data are distributed across the physical drives of an Data are distributed across the physical drives of an array.(array.(distributeddistributed))

Redundant disk capacity is used to store parity Redundant disk capacity is used to store parity information, which guarantees data recoverability information, which guarantees data recoverability in case of disk failure.(in case of disk failure.(reliabilityreliability))


Strip Unit(Strip Unit( 条化单元条化单元 ))

Data are striped across the available disks. (fig Data are striped across the available disks. (fig 11.9)The strips are mapped round robin to 11.9)The strips are mapped round robin to consecutive array members.consecutive array members.

Small strip: Small strip:

٭ Parallel Access TechniqueParallel Access Technique

Large strip: Large strip:

٭ Independent Access TechniqueIndependent Access Technique


Verify MethodsVerify Methods

Mirror storageMirror storage

Hamming code(distance =n)Hamming code(distance =n)

Parity verify (distance =2)Parity verify (distance =2)

٭ X4(i)X4(i) ＝＝ X3(i)+X2(i)+X1(i)+X0(i)X3(i)+X2(i)+X1(i)+X0(i)٭ X1(i)X1(i) ＝＝ X4(i)+X3(i)+X2(i)+X0(i)X4(i)+X3(i)+X2(i)+X0(i)٭ Distribute or centralize storageDistribute or centralize storage


RAID CharacteristicsRAID Characteristics

Strip unit and storage methodStrip unit and storage method

Verify method and storage methodVerify method and storage method


RAID 0 (non-redundant)RAID 0 (non-redundant)


RAID 0RAID 0


RAID 0RAID 0

non-redundantnon-redundantnon-verifynon-verifydistributed storagedistributed storageLower reliabilityLower reliabilityLower costLower costParallel I/O accessParallel I/O access


RAID 1 (Mirrored)RAID 1 (Mirrored)

DistributedDistributedMirrored/redundancyMirrored/redundancyNo verifyNo verify


RAID 1RAID 1

Read performance is better than RAID 0 (choose the Read performance is better than RAID 0 (choose the seek time fewer disk to access)seek time fewer disk to access)

Write performance is worse than RAID 0Write performance is worse than RAID 0

Higher costHigher cost

Recovery is simpleRecovery is simple

It can have higher parallel performance than It can have higher parallel performance than RAID0RAID0


RAID 2 (Redundancy Through Hamming Code)RAID 2 (Redundancy Through Hamming Code)


RAID 2RAID 2

Strips are very small, as a single byte or word.Strips are very small, as a single byte or word.

Hamming code in special disksHamming code in special disks

Lower cost than RAID1 but higher than RAID3, 4,5Lower cost than RAID1 but higher than RAID3, 4,5


RAID 3 (Bit-Interleaved Parity)RAID 3 (Bit-Interleaved Parity)

Same as RAID2 but use parity verify,so it only need Same as RAID2 but use parity verify,so it only need a single redundant diska single redundant disk


RAID 3—PerformanceRAID 3—Performance

Small strip bringSmall strip bring Parallel Access Technique but onlParallel Access Technique but only one I/O request can be executed at a time.y one I/O request can be executed at a time.

Lower cost than RAID2.Lower cost than RAID2.


RAID 4 (Block-Level Parity)RAID 4 (Block-Level Parity)


RAID 4RAID 4

Large stripLarge strip

Parity verify and storage into a special diskParity verify and storage into a special disk

Parity disk become a bottleneckParity disk become a bottleneck


RAID 5 (Block-Level Distributed Parity)RAID 5 (Block-Level Distributed Parity)


RAID 5RAID 5

Same as RAID4 but parity information distributedSame as RAID4 but parity information distributed

Avoid bottleneck found in RAID Level 4Avoid bottleneck found in RAID Level 4


Disk Cache(p479)Disk Cache(p479)

Buffer in main memory for disk sectors.Buffer in main memory for disk sectors.Contains a copy of some of the sectors on the disk.Contains a copy of some of the sectors on the disk.According to locality principle to cache .According to locality principle to cache .


Using MethodsUsing Methods

From disk-cache From disk-cache copycopy to user process space. to user process space.

Using a shared memory capability and passing a Using a shared memory capability and passing a pointer to the appropriate slot in the disk cache.pointer to the appropriate slot in the disk cache.


Least Recently Used (I)Least Recently Used (I)

The block that has been in the cache the longest The block that has been in the cache the longest with no reference to it is replaced.with no reference to it is replaced.

The cache consists of a stack of blocks.The cache consists of a stack of blocks.

Most recently referenced block is on the top of the Most recently referenced block is on the top of the stack.stack.

When a block is referenced or brought into the When a block is referenced or brought into the cache, it is placed on the top of the stack.cache, it is placed on the top of the stack.


Least Recently Used (I)Least Recently Used (I)

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

Blocks don’t actually move around in main Blocks don’t actually move around in main memory.memory.

A stack of pointers is used.A stack of pointers is used.


Least Frequently Used (II)Least Frequently Used (II)

The block that has experienced the fewest references The block that has experienced the fewest references is replaced.is replaced.

A counter is associated with each block.A counter is associated with each block.

Counter is incremented each time block accessed.Counter is incremented each time block accessed.

Block with smallest count is selected for Block with smallest count is selected for replacement.replacement.

Some blocks may be referenced many times in a Some blocks may be referenced many times in a short period of time and then not needed any more.short period of time and then not needed any more.


Integrate Methods(1)Integrate Methods(1)

New Section Old Section

Re-reference;

count unchangedRe-reference;

count: =count+1

Mi ss(new bl ock brought i n)

Count: =1

(a) FIFO



A area in new section , the count not add.A area in new section , the count not add.A area whose count is the least in old section will be A area whose count is the least in old section will be

replaced.replaced.The count remains at 1 as long as the block remains The count remains at 1 as long as the block remains

in the new sectionin the new section



(b) Use of Three Sections

New Secti on Mi ddl e Secti on Ol d Sect i on



The count in middle section can add , but not The count in middle section can add , but not replace the area in this sectionreplace the area in this section


Appendix 11a p492Appendix 11a p492

Disk storage devicesDisk storage devices

Chapter 4 Device Management and Disk Scheduling DEVICE MANAGEMENT Content I/O device overview I/O device overview I/O organization and architecture I/O.

Documents

processor processor

data processor

processor slide

io function processor

io operation

io p453

io module ready

io devices p452