University*of*Washington*

Transcription

1 Roadmap C: car c = malloc(sizeof(car)); c->miles = 100; c->gals = 17; float mpg = get_mpg(c); free(c); Assembly language: Machine code: Computer system: get_mpg: pushq movq... popq ret %rbp %rsp, %rbp %rbp Java: Car c = new Car(); c.setmiles(100); c.setgals(17); float mpg = c.getmpg(); OS: &data Integers&floats Machinecode&C x86assembly Procedures&stacks Arrays&structs &caches Processes allocajon Javavs.C Again:Processes! DefiniJon:Aprocessisaninstanceofarunningprogram! Oneofthemostimportantideasincomputerscience! Notthesameas program or processor! ProcessprovideseachprogramwithtwokeyabstracJons:! Logicalcontrolflow! EachprocessseemstohaveexclusiveuseoftheCPU! Privatevirtualaddressspace! Eachprocessseemstohaveexclusiveuseofmainmemory! Howaretheseillusionsmaintained?! ProcessexecuAonsinterleaved(mulACtasking) done! Addressspacesmanagedbyvirtualmemorysystem now! 2 (VM) asweknowitsofar isvirtual!! OverviewandmoJvaJon! VMastoolforcaching! AddresstranslaJon! VMastoolformemorymanagement! VMastoolformemoryprotecJon! Programsrefertovirtualmemoryaddresses! movl (%ecx),%eax!! Conceptuallymemoryisjustaverylargearrayofbytes! Eachbytehasitsownaddress FF\\\\\\F! SystemprovidesaddressspaceprivatetoparAcular process! AllocaJon:CompilerandrunXJmesystem! Wheredifferentprogramobjectsshouldbestored! AllallocaAonwithinsinglevirtualaddressspace! But! Weprobablydon thaveexactly2 w bytesofphysicalmemory.! Wecertainlydon thave2 w bytesofphysicalmemory foreveryprocess.! WehavemulApleprocessesthatusuallyshouldnot 00\\\\\\ interferewitheachother,butsomeamesshouldsharecodeordata. 3 4

2 Problem1:HowDoesEverythingFit? 5 64Xbitaddressescanaddress severalexabytes (18,446,744,073,709,551,616bytes) Physicalmainmemoryoffers afewgigabytes (e.g.8,589,934,592bytes)? virtualaddressspaceperprocess, withmanyprocesses (Actually,it ssmallerthanthat dotcomparedtovirtualmemory.) Problem2:Management 6 Physicalmainmemory What goes where? stack heap.text.data Process Process2 Process3 Processn x Problem3:HowToProtect 7 Physicalmainmemory Processi Processj Problem4:HowToShare? Physicalmainmemory Processi Processj Howcanwesolvetheseproblems? 8

3 IndirecJon! Anyproblemincomputersciencecanbesolvedbyaddinganotherlevel ofindirecjon. - David-Wheeler,-inventor-of-the-subrou7ne-(a.k.a.-procedure)- IndirecJon! Indirec7on:theabilitytoreferencesomethingusinganame,reference,or containerinsteadthevalueitself.aflexiblemappingbetweenanameanda thingallowschangingthethingwithoutnoafyingholdersofthename.! WithoutIndirecJon Name Thing! WithoutIndirecJon Name Thing! WithIndirecJon Name Thing! WithIndirecJon Name Thing WhatifIwanttomoveThing? Thing! ExamplesofindirecJon:! DomainNameService(DNS):translaAonfromnametoIPaddress! phonesystem:cellphonenumberportability! snailmail:mailforwarding! 911:routedtolocaloffice! DynamicHostConfiguraAonProtocol(DHCP):localnetworkaddressassignment! callcenters:routecallstoavailableoperators,etc. Thing 9 1 IndirecJonin Process Processn mapping AddressSpaces! Virtualaddressspace:SetofN=2 n virtualaddresses {0,1,2,3,,NC1}! Physicaladdressspace:SetofM=2 m physicaladdresses(n>=m) {0,1,2,3,,MC1}! Everybyteinmainmemoryhas:! onephysicaladdress! zero,one,or+morevirtualaddresses! Eachprocessgetsitsownprivatevirtualaddressspace! Solvesthepreviousproblems 1 12

4 Mapping ASystemUsingPhysicalAddressing Avirtualaddresscanbe mappedtoeitherphysical memoryordisk. P1 svirtualaddressspace Physical Physicaladdress (PA) 4 Mainmemory 0: 1: 2: 3: 4: 5: 6: 7: 8: MX1: Disk Dataword P2 svirtualaddressspace! Usedin simple systemswith(usually)justoneprocess:! embeddedmicrocontrollersindeviceslikecars,elevators,anddigital pictureframes ASystemUsingVirtualAddressing! Physicaladdressesarecompletely-invisible-to-programs.! Usedinallmoderndesktops,laptops,servers,smartphones! Oneofthegreatideasincomputerscience CPU-Chip- Virtualaddress (VA) 4100 MMU ManagementUnit Dataword Physicaladdress (PA) 4 Mainmemory 0: 1: 2: 3: 4: 5: 6: 7: 8: MX1: 15 Why(VM)?! Efficientuseoflimitedmainmemory(RAM)! UseRAMasacacheforthepartsofavirtualaddressspace! somenonccachedpartsstoredondisk! some(unallocated)nonccachedpartsstorednowhere! KeeponlyacAveareasofvirtualaddressspaceinmemory! transferdatabackandforthasneeded! Simplifiesmemorymanagementforprogrammers! Eachprocessgetsthesamefull,privatelinearaddressspace! Isolatesaddressspaces! Oneprocesscan tinterferewithanother smemory! becausetheyoperateindifferentaddressspaces! UserprocesscannotaccessprivilegedinformaAon! differentsecaonsofaddressspaceshavedifferentpermissions 16

5 VMandtheHierarchy! Thinkof-virtualmemory-asarrayofN=2 n conjguousbytes.-! Pagesofvirtualmemoryareusuallystoredinphysical memory,butsomejmesspilltodisk.-! Pagesareanotherunitofalignedmemory(sizeisP=2 p bytes)! Eachvirtualpagecanbestoredinanyphysicalpage. or:asdramcachefordisk! Thenphysicalmainmemoryisusedasacacheforthe virtualmemoryarray! Thecacheblocksarecalledpages+(sizeisP=2 p bytes) Disk VP VP Unallocated Cached Uncached Unallocated Cached Uncached Cached VP2 nxp X Uncached 2 n X Virtualpages(VP's) storedondisk 2 m X Empty Empty Empty PP PP PP2 mxp X Physicalpages(PP's) cachedindram 17 VP VP Unallocated Cached Uncached Unallocated Cached Uncached Cached VP2 nxp X Uncached NX Virtualpages(VPs) storedondisk MX Empty Empty Empty PP PP PP2 mxp X Physicalpages(PPs) cachedindram 18 Hierarchy:Core2Duo Reg Throughput: Latency: SRAM StaAcRandomAccessMemory 16B/cycle 3cycles L IXcache 32KB L DXcache 8B/cycle 14cycles ~4MB L2 unified cache Not-drawn-to-scale- Miss-penalty-(latency):-33x- DRAM DynamicRandomAccessMemory 2B/cycle 10cycles ~4GB Main B/3cycles millions! Thinkof-virtualmemory-asanarrayofN=2 n conjguous bytesstoredon-a-disk.- ~50GB Disk DesignConsequences! Largepage-size:typically4X8KB,someJmesupto4MB! FullyassociaJve! Anyvirtualpagecanbeplacedinanyphysicalpage! Requiresa large mappingfuncaon differentfromcpucaches! HighlysophisJcated,expensivereplacementalgorithmsinOS! ToocomplicatedandopenCendedtobeimplementedinhardware! WriteXbackratherthanwriteXthrough Miss-penalty-(latency):-10,000x- 19 2

6 AddressTranslaJon 2 How-do-we-perform-the--virtual-L>-physical-address- transla7on?- 0: 1: MX1: Mainmemory MMU 2: 3: 4: 5: 6: 7: Physicaladdress (PA) Dataword 8: Virtualaddress (VA) CPU-Chip AddressTranslaJon:PageTables! Apage-table-isanarrayofpage-table-entries(PTEs)that mapsvirtualpagestophysicalpages. 22 null null resident pagetable (disk) Physical-page- number-or-- disk-address- PTE PTE7 PP VP PP3 VP VP6 How-many-page-tables-are-in-the-system? Oneperprocessstoredinphysicalmemory managedbyhw(mmu),os AddressTranslaJonWithaPageTable 23 Virtual page number (VPN) Virtual page offset (VPO) Physicalpagenumber(PPN) Physical page offset (PPO) Virtual-address-(VA)- Physical-address-(PA)- Valid Physicalpagenumber(PPN) Pagetable baseregister (PTBR) Page-table-- Pagetableaddress forprocess Validbit=0: pagenotinmemory (pagefault) Inmostcases,thehardware (themmu)canperformthis translajononitsown, withoutsoswareassistance Thisfeelsfamiliar PageHit! Page-hit:-referencetoVMbytethatisinphysicalmemory null null resident pagetable (disk) Physical-page- number-or-- disk-address- PTE PTE7 PP VP PP3 VP VP6 Virtual address 24

7 PageFault! Page-fault:-referencetoVMbytethatisNOTinphysical memory Virtual address Whathappenswhenapage faultoccurs? resident pagetable VP (disk) VP VP6 PP PP3 FaultExample:PageFault! UserwritestomemorylocaAon! ThatporAon(page)ofuser smemory iscurrentlyondisk! Pagehandlermustloadpageintophysicalmemory! ReturnstofaulAnginstrucAon:movisexecutedagain!! Successfulonsecondtry int a[1000]; main () { a[500] = 13; } 80483b7: c d d movl 0xd,0x8049d10 User-Process- movl OSexcep1on:+page+fault+ returns Physical-page- number-or-- disk-address- PTE null null PTE7 Create+page+and++ load+into+memory HandlingPageFault! Pagemisscausespagefault(anexcepAon) HandlingPageFault! Pagemisscausespagefault(anexcepAon)! Pagefaulthandlerselectsavic1mtobeevicted(hereVP4) Virtual address resident pagetable VP (disk) VP PP PP3 Virtual address Physical-page- number-or-- disk-address- PTE null null PTE7 Physical-page- number-or-- disk-address- PTE null null PTE7 resident pagetable VP (disk) VP PP PP3 VP6 VP

8 HandlingPageFault! Pagemisscausespagefault(anexcepAon)! Pagefaulthandlerselectsavic1mtobeevicted(hereVP4) Virtual address resident pagetable VP (disk) VP VP6 PP PP3 HandlingPageFault! Pagemisscausespagefault(anexcepAon)! Pagefaulthandlerselectsavic1mtobeevicted(hereVP4)! OffendinginstrucAonisrestarted:pagehit! Virtual address Physical-page- number-or-- disk-address- PTE null null PTE7 Physical-page- number-or-- disk-address- PTE null null PTE7 resident pagetable VP (disk) VP VP6 PP PP Whydoesitwork? WhydoesVMworkonRAM/disk?Locality.! workswellforavoidingdiskaccessesbecause oflocality! SamereasonthatL1/L2/L3cacheswork! Thesetofvirtualpagesthataprogramis acjvely accessing atanypointinjmeiscalleditsworking-set-! Programswithbejertemporallocalitywillhavesmallerworkingsets! If(workingsetsizeofoneprocess<mainmemorysize):! Goodperformanceforoneprocessalercompulsorymisses! Butif SUM(workingsetsizesofallprocesses)>mainmemorysize:! Thrashing:+Performancemeltdown+wherepagesareswapped(copied) betweenmemoryanddiskconanuously.cpualwayswaiangorpaging.! Fullquote: EveryproblemincomputersciencecanbesolvedbyaddinganotherlevelofindirecAon, but+that+usually+will+create+another+problem. 3 32

9 SimplifyingLinkingandLoading! Linking! Eachprogramhassimilarvirtual addressspace! Code,stack,andsharedlibraries alwaysstartatthesameaddress! Loading! execve() allocatesvirtualpages for.textand.datasecaons =createsptesmarkedasinvalid! The.text and.data secaons arecopied,pagebypage,on demandbythevirtualmemory system 0xc x x Kernelvirtualmemory Userstack (createdatrunjme) MemoryXmappedregionfor sharedlibraries RunXJmeheap (createdbymalloc) Read/writesegment (.data,.bss) ReadXonlysegment (.init,.text,.rodata) Unused invisibleto usercode %esp (stack pointer) brk Loaded from the executable file 33 VMforManagingMulJpleProcesses! KeyabstracJon:eachprocesshasitsownvirtualaddressspace! Itcanviewmemoryasa+simple+linear+array+! Withvirtualmemory,thissimplelinearvirtualaddressspace neednotbeconjguousinphysicalmemory! ProcessneedstostoredatainanotherVP?JustmapittoanyPP! Virtual- Address- Space-for- Process-1:- Physical-- Address-- Space- (DRAM)- Virtual- Address- Space-for- Process-2:- NX NX VP 1 VP 2 VP 1 VP 2 Address-- transla7on MX PP 2 PP 6 PP 8 (e.g.,readxonly librarycode) 34 VMforProtecJonandSharing! ThemappingofVPstoPPsprovidesasimplemechanismto protectmemoryandtosharememorybetweenprocesses.! Sharing:justmapvirtualpagesinseparateaddressspacestothesame physicalpage(here:pp6)! ProtecJon:processsimplycan taccessphysicalpagestowhichnoneof itsvirtualpagesaremapped(here:process2can taccesspp2). Virtual- Address- Space-for- Process-1:- VP 1 VP 2 Address-- transla7on PP 2 Physical-- Address-- Space- (DRAM)- ProtecJonWithinaSingleProcess! Canweusevirtualmemorytocontrolread/write/execute permissions?how? Virtual- Address- Space-for- Process-2:- NX VP 1 VP 2 PP 6 PP 8 (e.g.,readxonly librarycode) NX MX 35 36

10 ProtecJonWithinaSingleProcess! Extendpagetableentrieswithpermissionbits Process-j:- VP0: VP1: VP2: Valid Yes Yes Yes READ No Yes No WRITE Yes No Yes EXEC PhysicalPageNum No No No PP9 PP6 PP1 PP 8 PP9 PP 11 Terminology! contextswitch! SwitchbetweenprocessesonthesameCPU! pagein! Movepagesofvirtualmemoryfromdisktophysicalmemory! pageout! Movepagesofvirtualmemoryfromphysicalmemorytodisk! thrash! Totalworkingsetsizeofprocessesislargerthanphysicalmemory! MostAmeisspentpaginginandoutinsteadofdoingusefulcomputaAon AddressTranslaJon:PageHit CPU-Chip- VA Data MMU 1)ProcessorsendsvirtualaddresstoMMU(memory+management+unit) 2C3)MMUfetchesPTEfrompagetableincache/memory 4)MMUsendsphysicaladdresstocache/memory 5)Cache/memorysendsdatawordtoprocessor 5 2 PTEA PTE 3 PA 4 Cache/ AddressTranslaJon:PageFault! MMUchecksthesepermissionbitsoneverymemoryaccess! Ifviolated,raisesexcepAonandOSsendsSIGSEGVsignaltoprocess (segmentaaonfault) Physical-- Process-i:- Valid READ WRITE EXEC PhysicalPageNum Address-Space- VP0: Yes Yes No No PP6 VP1: Yes No No Yes PP4 VP2: Yes Yes Yes No PP2 PP 2 PP4 PP 6 CPU-Chip- VA 7 MMU ExcepJon 1)ProcessorsendsvirtualaddresstoMMU Cache/ 2C3)MMUfetchesPTEfrompagetableincache/memory 4)Validbitiszero,soMMUtriggerspagefaultexcepAon 5)HandleridenAfiesvicAm(and,ifdirty,pagesitouttodisk) 6)HandlerpagesinnewpageandupdatesPTEinmemory 7)Handlerreturnstooriginalprocess,restarAngfaulAnginstrucAon 4 2 PTEA PTE 3 Pagefaulthandler VicJmpage 5 Newpage 6 Disk 39 4

11 Hmm TranslaJonSoundsSlow!! TheMMUaccessesmemorytwice:oncetofirstgetthePTE fortranslajon,andthenagainfortheactualmemoryrequest fromthe! ThePTEsmaybecachedinL1likeanyothermemoryword! Buttheymaybeevictedbyotherdatareferences! AndahitintheL1cachesAllrequires1C3cycles! What+can+we+do+to+make+this+faster?+ SpeedingupTranslaJonwithaTLB! SoluJon:addanothercache!! Transla7on-Lookaside-Buffer(TLB):! SmallhardwarecacheinMMU! Mapsvirtualpagenumberstophysicalpagenumbers! Containscompletepage+table+entriesforsmallnumberofpages! ModernIntelprocessors:128or256entriesinTLB! Muchfasterthanapagetablelookupincache/memory 4 42 TLBHit TLBMiss TLB 2 PTE CPU-Chip- CPU-Chip- TLB 2 4 PTE VPN 3 VPN 3 VA MMU PA 4 Cache/ VA MMU PTEA PA Cache/ 5 Data Data 5 6 ATLBhiteliminatesamemoryaccess ATLBmissincursanaddiJonalmemoryaccess(thePTE) Fortunately,TLBmissesarerare.DoesaTLBmissrequirediskaccess? 43 44

12 SimpleSystemExample(small)! Addressing! 14Cbitvirtualaddresses! 12Cbitphysicaladdress! Pagesize=64bytes VPN VirtualPageNumber VPO VirtualPageOffset SimpleSystemPageTable! Onlyshowingfirst16entries(outof256=2 8 ) VPN- PPN- VPN- PPN A B 04 0C D 2D 06 0E F 0D PPN PhysicalPageNumber PPO PhysicalPageOffset! Whataboutarealaddressspace?Readmoreinthebook SimpleSystemTLB! 16entries! 4XwayassociaJve TLBignorespageoffset.Why? TLBtag TLBindex SimpleSystemCache! 16lines,4Xbyteblocksize! Physicallyaddressed! Directmapped cachetag cacheindex cacheoffset virtualpagenumber virtualpageoffset physicalpagenumber physicalpageoffset Tag- 24 2D 2D 0B Idx- 8 9 A B C D E F B0-3A B B2-5 DA 34 1B B3-89 3B 15 D3 PPN- 02 Tag- 07 0A PPN- 34 Tag A PPN- 0D 0D Tag PPN- 2D Tag Set- 2 3 B D 03 B F F DF B D 72 C2 B Tag B D 3 16 Idx

13 TLB Currentstateofcaches/tables VPN- PPN- VPN- PPN Set- Tag- PPN- Tag- PPN- Tag- PPN- Tag- PPN D A D A B C D 0A D 2D 06 0E F 0D Pagetable Cache Idx- Tag- B0- B1- B2- B3- Idx- Tag- B0- B1- B2- B A D 2 1B A 2D DA 3B 3 36 B 0B D 8F 09 C D F 1D D E B D C2 DF 03 F 14 AddressTranslaJonExample# VirtualAddress:0x03D4 VPN 0x0F TLBI 3 TLBT 0x03 TLBHit? Y PageFault? PPN: N 0x0D PhysicalAddress TLBT TLBI VPN CT CO CI 0x5 CT 0x0D Hit? Byte: Y 0x36 VPO PPN CI PPO CO 49 5 AddressTranslaJonExample#2 VirtualAddress:0x0B8F VPN 0x2E TLBI 2 TLBT 0x0B TLBHit? N PageFault? PPN: Y TBD PhysicalAddress TLBT TLBI VPN CT CO CI CT Hit? Byte: VPO PPN PPO 5 CI CO AddressTranslaJonExample#3 VirtualAddress:0x0020 VPN 0x0 TLBI TLBT 0x0 TLBHit? N PageFault? PPN: N 0x28 PhysicalAddress TLBT TLBI VPN CT VPO PPN PPO CO CI 0x8 CT 0x28 Hit? Byte: N Mem 52 CI CO

14 ServicingaPageFault! (1)Processorsignalsdiskcontroller! ReadblockoflengthP starangatdiskaddressx! StorestarAngatmemoryaddressY! (2)Readoccurs! DirectMemoryAccess(DMA)! UndercontrolofI/Ocontroller! (3)ControllersignalscompleJon! Interruptsprocessor! OSresumessuspendedprocess Processor Reg Cache (1)IniJateBlockRead (3)Read Done MemoryXI/Obus I/O controller Summary! Programmer sviewofvirtualmemory! Eachprocesshasitsownprivatelinearaddressspace! Cannotbecorruptedbyotherprocesses! Systemviewofvirtualmemory! Usesmemoryefficientlybycachingvirtualmemorypages! Efficientonlybecauseoflocality! Simplifiesmemorymanagementandsharing! SimplifiesprotecAonbyprovidingaconvenientinterposiAoningpointto checkpermissions (2)Direct Address Transfer Disk Disk SystemSummary! L1/L2Cache! PurelyaspeedCuptechnique! BehaviorinvisibletoapplicaAonprogrammerand(mostly)OS! Implementedtotallyinhardware!! SupportsmanyOSCrelatedfuncAons! ProcesscreaAon,taskswitching,protecAon! OperaAngSystem(solware)! Allocates/sharesphysicalmemoryamongprocesses! MaintainshighCleveltablestrackingmemorytype,source,sharing! HandlesexcepAons,fillsinhardwareCdefinedmappingtables! Hardware! Translatesvirtualaddressesviamappingtables,enforcingpermissions! AcceleratesmappingviatranslaAoncache(TLB) 55 SystemSummary! L1/L2Cache! Controlledbyhardware! Programmercannotcontrolit! Programmercanwritecodeinawaythattakesadvantageofit!! ControlledbyOSandhardware! Programmercannotcontrolmappingtophysicalmemory! ProgrammercancontrolsharingandsomeprotecAon! viaosfuncaons(notin351) 56