lecture 15 device management

8/6/2019 Lecture 15 Device Management

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Device Management


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So far

We have c o vered CPU and mem o rymanagement

Co mputing is n o t interesting with o ut I/OsDevice management: the O S co mp o nentthat manages hardware devices

Pr o vides a unif o rm interface t o access deviceswith different physical characteristicsOptimize the perf o rmance o f individual devices


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I/O hardware

I/O device hardwareMany varieties

Device c o ntr o ller Co nversi o n a bl o ck o f bytes and I/O o perati o nsPerf o rms err o r c o rrecti o n if necessaryExp o se hardware registers t o co ntr o l the deviceTypically have f o ur registers:

Data-in register t o be read t o get inputData- o ut register t o be written t o send o utputS tatus register (all o ws the status o f the device t o bechecked)Co ntr o l register (c o ntr o ls the c o mmand the deviceperf o rms)


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Device Addressing

Ho w the CPU addresses the deviceregisters?Tw o appr o aches

Dedicated range o f device addresses in thephysical mem o ry

Requires special hardware instructi o ns ass o ciated withindividual devices

M emory-mapped I/O: makes no

distinctio

nbetween device addresses and mem o ry addressesDevices can be accessed the same way as n o rmalmem o ry, with the same set o f hardware instructi o ns


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Device Addressing Illustrated

Primarymem o ry

Device 1

Device 2

Mem o ryaddresses

Mem o ry-mapped I/Os

Primarymem o ry

Deviceaddresses

S eparate deviceaddresses


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Ways t o Access a Device

Ho w to input and o utput data t o and fr o man I/O device?

P olling: a CPU repeatedly checks thestatus o f a device f o r exchanging data+ S imple- Busy-waiting


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Interrupt-driven I/Os: A device c o ntr o ller no tifies the c o rresp o nding device driver

when the device is available+ Mo re efficient use o f CPU cycles- Data c o pying and m o vements- S lo w f o r character devices (i.e., o ne interrupt

per keyb o ard input)


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Direct memory access (D MA): uses anadditi o nal c o ntr o ller t o perf o rm datamo vements+ CPU is n o t inv o lved in c o pying data- I/O device is much m o re c o mplicated (need t o

have the ability t o access mem o ry).

- A pr o cess cann o t access in-transit data


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Double buffering: uses tw o buffers.While o ne is being used, the o ther is beingfilled.

Anal o gy: pipeliningExtensively used f o r graphics and animati o n

So a viewer d o es n o t see the line-by-line scanning


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Device Driver

An O S co mp o nent that is resp o nsible f o r hiding the c o mplexity o f an I/O device

So that the O S can access vari o usdevices in a unif o rm manner


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Types o f IO devices

Tw o categ o ries A block device st o res inf o rmati o n in fixed-size bl o cks, eacho ne with its o wn address

e.g., disks

A character device deliverso

r accepts a streamo

f characters, and individual characters are n o t addressablee.g., keyb o ards, printers, netw o rk cards

Device driver pr o vides interface f o r these tw o types o f devices

Other O S co mp o nents see bl o ck devices and character devices, but n o t the details o f the devices.Ho w to effectively utilize the device is the resp o nsibility o f thedevice driver


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Device Driver Illustrated

User applicati o nsVari o us O S co mp o nents

Device drivers

Device c o ntr o llers

I/O devices

User level

OS level

Hardware level


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Disk as An Example Device

30-year- o ld st o rage techn o lo gyIncredibly c o mplicated

A m o dern drive250,000 lines o f micr o co de


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Disk Characteristics

Disk platters: co ated with magneticmaterials f o r rec o rding

Diskplatters


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Disk arm: mo ves a c o mb o f disk headsOnly o ne disk head is active f o r reading/writing

Diskplatters

Disk arm

Disk heads


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Hard Disk Trivia

Aer o dynamically designed t o fly As cl o se t o the surface as p o ssibleNo r oo m f o r air m o lecules

Theref o re, hard drives are filled withspecial inert gasIf head t o uches the surface

Head crashS crapes o ff magnetic inf o rmati o n


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Each disk platter is divided int o co ncentrictracks

Diskplatters

Disk arm

Disk heads

Track


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A track is further divided int o sectors. Asect o r is the smallest unit o f disk st o rage

Diskplatters

Disk arm

Disk heads

Track

S ect o r


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A cylinder co nsists o f all tracks with agiven disk arm p o sitio n

Diskplatters

Disk arm

Disk heads

Track

S ect o r

Cylinder


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Cylinders are further divided int o z ones

Diskplatters

Disk arm

Disk heads

Track

S ect o r

Cylinder

Zo nes


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Z one-bit recording : z o nes near the edgeo f a disk st o re m o re inf o rmati o n (higher bandwidth)

Diskplatters

Disk arm

Disk heads

Track

S ect o r

Cylinder

Zo nes


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Mo re Ab o ut Hard Drives Than Y o u Ever Want t o Kno w

T rack skew : starting p o sitio n o f each track isslightly skewed

Minimize r o tati o nal delay when sequentially transferring

bytes acr o ss tracksT hermo-calibrations: peri o dically perf o rmed t o acc o unt f o r changes o f disk radius due t o temperature changesTypically 100 t o 1,000 bits are inserted betweensect o rs t o acc o unt f o r min o r inaccuracies


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Disk Access Time

S eek time: the time t o po sitio n diskheads (~8 msec o n average)R

otational latency: the time to

r o

tate thetarget sect o r to underneath the head Assume 7,200 r o tati o ns per minute (RPM)7,200 / 60 = 120 r o tati o ns per sec o nd1/120 = ~8 msec per r o tati o n

Average r o tati o nal delay is ~4 msec


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Disk Access Time

T ransfer time: the time t o transfer bytes Assumpti o ns:

58 Mbytes/sec4-Kbyte disk bl o cks

Time t o transfer a bl o ck takes 0.07 msec

Disk access timeS eek time + r o tati o nal delay + transfer time


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Disk Perf o rmance Metrics

L atency S eek time + r o tati o nal delay

BandwidthBytes transferred / disk access time


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Examples o f Disk Access Times

If disk bl o cks are rand o mly accessed Average disk access time = ~12 msec Assume 4-Kbyte bl o cks4 Kbyte / 12 msec = ~340 Kbyte/sec

If disk bl o cks o f the same cylinder arerand o mly accessed with o ut disk seeks

Average disk access time = ~4 msec4 Kbyte / 4 msec = ~ 1 Mbyte/sec


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Examples o f Disk Access Times

If disk bl o cks are accessed sequentiallyWith o ut seeks and r o tati o nal delaysBandwidth: 58 Mbytes/sec

Key t o goo d disk perf o rmanceMinimize seek time and r o tati o nal latency


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Disk Trade o ffs

Larger sect o r size better bandwidth

Wasteful if o nly 1 byte o ut o f 1 Mbyte isneeded

S ect o r size S pace utilizati o n Transfer rate

1 byte 8 bits/1008 bits (0.8%) 80 bytes/sec (1 byte / 12 msec)

4 Kbytes 4096 bytes/4221 bytes (97%) 340 Kbytes/sec (4 Kbytes / 12 msec)1 Mbyte (~100%) 58 Mbytes/sec (peak bandwidth)


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Disk C o ntr o ller

Few p o pular standardsIDE (integrated device electr o nics)

ATA (AT attachment interface)S CS I (small c o mputer systems interface)

DifferencesPerf o rmanceParallelism


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Disk Device Driver

Majo r g o al: reduce seek time f o r diskaccesses

S chedule disk request t o minimize disk armmo vements


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Disk Arm S cheduling P o licies

F irst come, first serve ( FCFS ): requestsare served in the o rder o f arrival+ Fair am o ng requesters- P oo r f o r accesses t o rand o m disk bl o cks

S hortest seek time first ( SSTF ): picksthe request that is cl o sest t o the current

disk arm po

sitio

n+ G oo d at reducing seeks- May result in starvati o n


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SC AN : takes the cl o sest request in thedirecti o n o f travel (an example o f elevat o r alg o rithm)+ n o starvati o n- a new request can wait f o r alm o st tw o fullscans o f the disk


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C ircular SC AN ( C - SC AN) : disk armalways serves requests by scanning ino ne directi o n.

Once the arm finishes scanning f o r o nedirecti o nReturns t o the 0 th track f o r the next r o und o f

scanning


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First C o me, First S erve

Request queue: 3, 6, 1, 0, 7Head start p o sitio n: 2

To tal seek distance: 1 + 3 + 5 + 1 + 7 =17

Tracks

0

5

43

2

1

6

7

Time


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S ho rtest S eek Distance First



Tracks

0

5

43

2

1

6

7

Time


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S CAN


To tal seek distance: 1 + 1 + 3 + 3 + 1 = 9

Time

Tracks

0

5

43

2

1

6

7


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C- S CAN



Time

Tracks

0

5

43

2

1

6

7


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Loo k and C-L oo k

S imilar t o S CAN and C- S CANthe arm g o es o nly as far as the final request in

each directi o n, then turns ar o undLoo k f o r a request bef o re c o ntinuing t o m o ve in a

given directi o n.

lecture 15 device management

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