CS61C:GreatIdeasinComputerArchitecture(MachineStructures)

MoreI/O:DMA

VladimirStojanovic&NicholasWeaverhttp://inst.eecs.berkeley.edu/~cs61c/

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Review:I/O• “MemorymappedI/O”:Devicecontrol/dataregistersmappedtoCPUaddressspace

• CPUsynchronizeswithI/Odevice:– Polling– Interrupts

• “ProgrammedI/O”:– CPUexecslw/sw instructionsforalldatamovementto/fromdevices

– CPUspendstimedoing2things:1. Gettingdatafromdevicetomainmemory2. Usingdatatocompute

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Workingwithrealdevices• “MemorymappedI/O”:Devicecontrol/dataregistersmappedtoCPUaddressspace

• CPUsynchronizeswithI/Odevice:– Polling– Interrupts

• “ProgrammedI/O”:DMA– CPUexecslw/sw instructionsforalldatamovementto/fromdevices

– CPUspendstimedoing2 things:1. Gettingdatafromdevicetomainmemory2. Usingdatatocompute

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Agenda

• DirectMemoryAccess(DMA)• Disks

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What’swrongwithProgrammedI/O?

• Notidealbecause…1. CPUhastoexecutealltransfers,couldbedoing

otherwork2. Devicespeedsdon’talignwellwithCPUspeeds3. Energycostofusingbeefygeneral-purposeCPU

wheresimplerhardwarewouldsuffice• UntilnowCPUhassolecontrolofmainmemory

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PIOvs.DMA

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DirectMemoryAccess(DMA)

• AllowsI/Odevicestodirectlyread/writemainmemory

• NewHardware:theDMAEngine• DMAenginecontainsregisterswrittenbyCPU:– Memoryaddresstoplacedata– #ofbytes– I/Odevice#,directionoftransfer– unitoftransfer,amounttotransferperburst

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OperationofaDMATransfer

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[FromSection5.1.4DirectMemoryAccessinModernOperatingSystemsbyAndrewS.Tanenbaum,HerbertBos,2014]

DMA:IncomingData

1. Receiveinterruptfromdevice2. CPUtakesinterrupt,beginstransfer– InstructsDMAengine/devicetoplacedata@certainaddress

3. Device/DMAenginehandlethetransfer– CPUisfreetoexecuteotherthings

4. Uponcompletion,Device/DMAengineinterrupttheCPUagain

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DMA:OutgoingData

1. CPUdecidestoinitiatetransfer,confirmsthatexternaldeviceisready

2. CPUbeginstransfer– InstructsDMAengine/devicethatdataisavailable@certainaddress

3. Device/DMAenginehandlethetransfer– CPUisfreetoexecuteotherthings

4. Device/DMAengineinterrupttheCPUagaintosignalcompletion

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DMA:Somenewproblems

• WhereinthememoryhierarchydoweplugintheDMAengine?Twoextremes:– BetweenL1andCPU:• Pro:Freecoherency• Con:TrashtheCPU’sworkingsetwithtransferreddata

– BetweenLast-levelcacheandmainmemory:• Pro:Don’tmesswithcaches• Con:Needtoexplicitlymanagecoherency

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DMA:Somenewproblems

• HowdowearbitratebetweenCPUandDMAEngine/Deviceaccesstomemory?Threeoptions:– BurstMode• Starttransferofdatablock,CPUcannotaccessmemoryinthemeantime

– CycleStealingMode• DMAenginetransfersabyte,releasescontrol,thenrepeats- interleavesprocessor/DMAengineaccesses

– TransparentMode• DMAtransferonlyoccurswhenCPUisnotusingthesystembus

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Agenda

• DirectMemoryAccess(DMA)• Disks

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ComputerMemoryHierarchy

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MagneticDisk– commonI/Odevice• Akindofcomputermemory

– Informationstoredbymagnetizingferritematerialonsurfaceofrotatingdisk• Similartotaperecorderexceptdigitalratherthananalogdata

• Atypeofnon-volatilestorage– Retainsitsvaluewithoutapplyingpowertodisk.

• TwoTypesofMagneticDisk1. HardDiskDrives(HDD)– faster,moredense,non-removable.2. Floppydisks– slower,lessdense,removable(nowreplacedbyUSB

“flashdrive”).

• Purposeincomputersystems(HardDrive):1. Workingfilesystem+long-termbackupforfiles2. Secondary“backingstore”formain-memory.Large,inexpensive,

slowlevelinthememoryhierarchy(virtualmemory)

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PhotoofDiskHead,Arm,Actuator

Arm

Head

Spindle

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DiskDeviceTerminology

• Severalplatters,withinformationrecordedmagneticallyonbothsurfaces(usually)

• Bitsrecordedintracks,whichinturndividedintosectors (e.g.,512Bytes)

• Actuator moveshead (endofarm)overtrack(“seek”),waitforsector rotateunderhead,thenreadorwrite

OuterTrack

InnerTrackSector

Actuator

HeadArmPlatter

17Videoofharddiskinaction

HardDrivesareSealed.Why?• Theclosertheheadtothedisk,the

smallerthe“spotsize”andthusthedensertherecording.– MeasuredinGbit/in^2– ~900Gbit/in^2isstateoftheart– Startedoutat2Kbit/in^2– ~450,000,000ximprovementin~60

years• Disksaresealedtokeepthedust

out.– Headsaredesignedto“fly”ataround

3-20nmabovethesurfaceofthedisk.– 99.999%ofthehead/armweightis

supportedbytheairbearingforce(aircushion)developedbetweenthediskandthehead.

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3-20nm

DiskDevicePerformance(1/2)

• Disk Access Time = Seek Time + Rotation Time + Transfer Time + Controller Overhead– SeekTime=timetopositiontheheadassemblyatthepropercylinder– RotationTime=timeforthedisktorotatetothepointwherethefirst

sectorsoftheblocktoaccessreachthehead– TransferTime=timetakenbythesectorsoftheblockandanygaps

betweenthemtorotatepastthehead

Platter

Arm

Actuator

HeadSectorInnerTrack

OuterTrack

ControllerSpindle

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DiskDevicePerformance(2/2)

• Averagevaluestoplugintotheformula:• RotationTime:Averagedistanceofsectorfromhead?– 1/2timeofarotation

• 7200RevolutionsPerMinuteÞ 120Rev/sec• 1revolution=1/120secÞ 8.33milliseconds• 1/2rotation(revolution)Þ4.17ms

• Seektime:Averageno.trackstomovearm?– Numberoftracks/3(seeCS186forthemath)– Then,seektime=numberoftracksmoved× timetomoveacrossonetrack

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Butwait!

• Performanceestimatesaredifferentinpractice:

• Manydiskshaveon-diskcaches,whicharecompletelyhiddenfromtheoutsideworld– Previousformulacompletelyreplacedwithon-diskcacheaccesstime

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WheredoesFlashmemorycomein?• ~10yearsago:Microdrives andFlashmemory(e.g.,CompactFlash)wenthead-to-head– Bothnon-volatile(retainscontentswithoutpowersupply)

– Flashbenefits:lowerpower,nocrashes(nomovingparts,needtospinµdrivesup/down)

– Diskcost=fixedcostofmotor+armmechanics,butactualmagneticmediacostverylow

– Flashcost=mostcost/bitofflashchips– Overtime,cost/bitofflashcamedown,becamecostcompetitive

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FlashMemory/SSDTechnology

• NMOStransistorwithanadditionalconductorbetweengateandsource/drainwhich“traps”electrons.Thepresence/absenceisa1or0

• Memorycellscanwithstandalimitednumberofprogram-erasecycles.ControllersuseatechniquecalledwearlevelingtodistributewritesasevenlyaspossibleacrossalltheflashblocksintheSSD.

WhatdidAppleputinitsiPods?Samsung flash

16 GB

shuffle nano classic touch

Toshiba 1.8-inch HDD80, 120, 160 GB

Toshiba flash2 GB

Toshiba flash32, 64 GB

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FlashMemoryinSmartPhones

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iPhone6:upto128GB

FlashMemoryinLaptops– SolidStateDrive(SSD)

26capacitiesupto512GB

iClicker Question• Wehavethefollowingdisk:– 15000Cylinders,1ms tocross1000Cylinders– 15000RPM=4ms perrotation–Wanttocopy1MB,transferrateof1000MB/s– 1ms controllerprocessingtime

• Whatistheaccesstimeusingourmodel?

DiskAccessTime=SeekTime+RotationTime+TransferTime+ControllerProcessingTime

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A B C D E

10.5 ms 9 ms 8.5ms 11.4ms 12 ms

iClicker Question

• Wehavethefollowingdisk:– 15000Cylinders,1ms tocross1000Cylinders– 15000RPM=4ms perrotation– Wanttocopy1MB,transferrateof1000MB/s– 1ms controllerprocessing time

• Whatistheaccesstime?Seek=#cylinders/3*time=15000/3*1ms/1000cylinders=5msRotation=timefor½rotation=4ms /2=2msTransfer=Size/transferrate=1MB/(1000MB/s)=1msController=1msTotal=5+2+1+1=9ms

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“Andinconclusion…”

• I/Ogivescomputerstheir5senses• I/Ospeedrangeis100-milliontoone• Pollingvs.Interrupts• DMAtoavoidwastingCPUtimeondatatransfers• Disksforpersistentstorage,replacedbyflash

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