Introduc)ontoI/O

CSE421/521:Opera)ngSystemsKarthikDantu

SlidesadoptedfromCS162classatBerkeley,CSE451atU-WashingtonandCSE421byProfKosaratUB

Assignment#1-Clarifica)ons•  Grading–  5%-Design–  10%-Implementa)on/tests

•  Git– Workingwithgithubtosetuprepos–  Shouldbeupbyearlynextweek

•  Detailsofscheduling–  Incomingclassesandrecita)ons–  Assignmentgivenearly

•  Deadlines–  9/26–Designdocument–  10/3–Implementa)onviaAutograder

Recall:UNIXSystemStructure

User Mode

Kernel Mode

Hardware

Applications

Standard Libs

HowDoestheKernelProvideServices?

•  Yousaidthatapplica)onsrequestservicesfromtheopera)ngsystemviasyscall,but…

•  I’vebeenwri)ngallsortofusefulapplica)onsandInevereversawa“syscall”!!!

•  That’sright.•  Itwasburiedintheprogramminglanguagerun)melibrary(e.g.,libc.a)

•  …Layering

OSRun-TimeLibrary

OS

Proc 1

Proc 2

Proc n…

OS

Appln login Window Manager

…OS library OS library OS library

AKindofNarrowWaist

Compilers

Web Servers

Web Browsers

Databases

Email

Word Processing

Portable OS Library

System Call Interface

Portable OS Kernel

Platform support, Device Drivers

x86 ARMPowerPC

Ethernet (1Gbs/10Gbs) 802.11 a/g/n/ac SCSI ThunderboltGraphics

PCIHardware

Software

System

UserOS

Application / Service

KeyUnixI/ODesignConcepts•  Uniformity–  fileopera)ons,deviceI/O,andinterprocesscommunica)onthroughopen,read/write,close

–  Allowssimplecomposi)onofprograms•  find|grep|wc…

•  Openbeforeuse–  Providesopportunityforaccesscontrolandarbitra)on–  Setsuptheunderlyingmachinery,i.e.,datastructures

•  Byte-oriented–  Evenifblocksaretransferred,addressingisinbytes

•  Kernelbufferedreads–  Streamingandblockdeviceslooksthesame–  readblocksprocess,yieldingprocessortoothertask

Puhngittogether:webserver

Server

Kernel

Hardware

requestbuffer

replybuffer

11. kernel copy from user buffer to network buffer

Network interface Disk interface

12. format outgoing packet and DMA

6. disk request

10. network socket write

1. network socket read

2. copy arriving packet (DMA)

syscall

wait

interrupt

3. kernel copy

RTU

5. file readsyscall

8. kernel copy

RTU

7. disk data (DMA)

interrupt

4. parse request 9. format reply

Request Reply

Kernel buffer reads

KeyUnixI/ODesignConcepts•  Uniformity

–  fileopera)ons,deviceI/O,andinterprocesscommunica)onthroughopen,read/write,close

–  Allowssimplecomposi)onofprograms•  find|grep|wc…

•  Openbeforeuse–  Providesopportunityforaccesscontrolandarbitra)on–  Setsuptheunderlyingmachinery,i.e.,datastructures

•  Byte-oriented–  Evenifblocksaretransferred,addressingisinbytes

•  Kernelbufferedreads–  Streamingandblockdeviceslooksthesame–  readblocksprocess,yieldingprocessortoothertask

•  Kernelbufferedwrites–  Comple)onofout-goingtransferdecoupledfromtheapplica)on,allowingittocon)nue

Puhngittogether:webserver

Server

Kernel

Hardware

requestbuffer

replybuffer

11. kernel copy from user buffer to network buffer

Network interface Disk interface

12. format outgoing packet and DMA

6. disk request

10. network socket write

1. network socket read

2. copy arriving packet (DMA)

syscall

wait

interrupt

3. kernel copy

RTU

5. file readsyscall

8. kernel copy

RTU

7. disk data (DMA)

interrupt

4. parse request 9. format reply

Request Reply

Kernel buffer writes

KeyUnixI/ODesignConcepts•  Uniformity

–  fileopera)ons,deviceI/O,andinterprocesscommunica)onthroughopen,read/write,close

–  Allowssimplecomposi)onofprograms•  find|grep|wc…

•  Openbeforeuse–  Providesopportunityforaccesscontrolandarbitra)on–  Setsuptheunderlyingmachinery,i.e.,datastructures

•  Byte-oriented–  Evenifblocksaretransferred,addressingisinbytes

•  Kernelbufferedreads–  Streamingandblockdeviceslooksthesame–  readblocksprocess,yieldingprocessortoothertask

•  Kernelbufferedwrites–  Comple)onofout-goingtransferdecoupledfromtheapplica)on,

allowingittocon)nue•  Explicitclose

I/O&StorageLayers

High Level I/O

Low Level I/O Syscall

File System

I/O Driver

Application / Servicestreams

handles

registers

descriptors

Commands and Data Transfers

Disks, Flash, Controllers, DMA

TheFileSystemAbstrac)on•  High-levelidea

–  Filesliveinhierarchicalnamespaceoffilenames•  File

–  Namedcollec)onofdatainafilesystem–  Filedata

•  Text,binary,linearizedobjects–  FileMetadata:informa)onaboutthefile

•  Size,Modifica)onTime,Owner,Securityinfo•  Basisforaccesscontrol

•  Directory–  “Folder”containingfiles&Directories–  Hierachical(graphical)naming

•  Paththroughthedirectorygraph•  Uniquelyiden)fiesafileordirectory

–  /home/ff/cs162/public_html/fa17/index.html–  LinksandVolumes(later)

CHigh-LevelFileAPI–Streams(review)

•  Operateon“streams”-sequenceofbytes,whethertextordata,withaposi)on

#include<stdio.h>FILE*fopen(constchar*filename,constchar*mode);intfclose(FILE*fp);

Mode Text Binary Descriptions

r rb Open existing file for readingw wb Open for writing; created if does not exista ab Open for appending; created if does not existr+ rb+ Open existing file for reading & writing.w+ wb+ Open for reading & writing; truncated to zero if exists, create otherwisea+ ab+ Open for reading & writing. Created if does not exist. Read from beginning,

write as append

Connec)ngProcesses,Filesystem,andUsers

•  Processhasa‘currentworkingdirectory’

•  AbsolutePaths–  /home/kdantu/cs421

•  Rela)vepaths–  index.html,./index.html-currentWD–  ../index.html-parentofcurrentWD–  ~,~kdantu-homedirectory

CAPIStandardStreams•  Threepredefinedstreamsareopenedimplicitlywhenaprogramisexecuted–  FILE*stdin–normalsourceofinput,canberedirected–  FILE*stdout–normalsourceofoutput,canberedirected–  FILE*stderr–diagnos)csanderrors,canberedirected

•  STDIN/STDOUTenablecomposi)oninUnix–  Recall:UseofpipesymbolsconnectsSTDOUTandSTDIN

•  find|grep|wc…

ChighlevelFileAPI–StreamOps#include<stdio.h>//characterorientedintfputc(intc,FILE*fp);//rtncorEOFonerrintfputs(constchar*s,FILE*fp);//rtn>0orEOFintfgetc(FILE*fp);char*fgets(char*buf,intn,FILE*fp);

#include<stdio.h>//characterorientedintfputc(intc,FILE*fp);//rtncorEOFonerrintfputs(constchar*s,FILE*fp);//rtn>0orEOFintfgetc(FILE*fp);char*fgets(char*buf,intn,FILE*fp);//blockorientedsize_tfread(void*ptr,size_tsize_of_elements,size_tnumber_of_elements,FILE*a_file);size_tfwrite(constvoid*ptr,size_tsize_of_elements,size_tnumber_of_elements,FILE*a_file);

ChighlevelFileAPI–StreamOps

ChighlevelFileAPI–StreamOps#include<stdio.h>//characterorientedintfputc(intc,FILE*fp); //rtncorEOFonerrintfputs(constchar*s,FILE*fp); //rtn>0orEOFintfgetc(FILE*fp);char*fgets(char*buf,intn,FILE*fp);//blockorientedsize_tfread(void*ptr,size_tsize_of_elements,size_tnumber_of_elements,FILE*a_file);size_tfwrite(constvoid*ptr,size_tsize_of_elements,size_tnumber_of_elements,FILE*a_file);//formattedintfprintf(FILE*restrictstream,constchar*restrictformat,

...);intfscanf(FILE*restrictstream,constchar*restrictformat,

...);

ExampleCode#include<stdio.h>

#defineBUFLEN256FILE*outfile;charmybuf[BUFLEN];

intstoretofile(){char*instring;outfile=fopen("/usr/homes/testing/tokens","w+");if(!outfile)return(-1);//Error!while(1){instring=fgets(mybuf,BUFLEN,stdin);//catchesoverrun!//Checkforerrororendoffile(^D)if(!instring||strlen(instring)==0)break;//Writestringtooutputfile,exitonerrorif(fputs(instring,outfile)<0)break;}fclose(outfile);//Flushesfromuserspace}

CStreamAPIposi)oning

•  Preserveshighlevelabstrac)onofuniformstreamofobjects•  Addsbufferingforperformance

intfseek(FILE*stream,longintoffset,intwhence);longintftell(FILE*stream)voidrewind(FILE*stream) High%Level%I/O%%

Low%Level%I/O%%Syscall%

File%System%

Upper%I/O%Driver%

Lower%I/O%Driver%

offset (SEEK_CUR)

offset (SEEK_SET) offset (SEEK_END)

What’sbelowthesurface??Application / Service

High Level I/O

Low Level I/O Syscall

File System

I/O Driver

streamshandlesregisters

descriptors

Commands and Data Transfers

Disks, Flash, Controllers, DMA

CLowlevelI/O•  Opera)onsonFileDescriptors–asOSobjectrepresen)ngthestateofafile–  Userhasa“handle”onthedescriptor

#include<fcntl.h>#include<unistd.h>#include<sys/types.h>intopen(constchar*filename,intflags[,mode_tmode])intcreat(constchar*filename,mode_tmode)intclose(intfiledes)

Bit vector of:•  Access modes (Rd, Wr, …)•  Open Flags (Create, …)•  Operating modes (Appends, …)

Bit vector of Permission Bits:•  User|Group|Other X R|W|X

http://www.gnu.org/software/libc/manual/html_node/Opening-and-Closing-Files.html

CLowLevel:standarddescriptors

•  Crossinglevels:Filedescriptorsvs.streams•  Don’tmixthem!

#include<unistd.h>STDIN_FILENO-macrohasvalue0STDOUT_FILENO-macrohasvalue1STDERR_FILENO-macrohasvalue2intfileno(FILE*stream)FILE*fdopen(intfiledes,constchar*opentype)

CLowLevelOpera)ons

•  Whenwritereturns,dataisonitswaytodiskandcanberead,butitmaynotactuallybepermanent!

ssize_tread(intfiledes,void*buffer,size_tmaxsize)-returnsbytesread,0=>EOF,-1=>errorssize_twrite(intfiledes,constvoid*buffer,size_tsize)-returnsbyteswrittenoff_tlseek(intfiledes,off_toffset,intwhence)intfsync(intfildes)–waitfori/otofinishvoidsync(void)–waitforALLtofinish

Andlotsmore!•  TTYsversusfiles•  Memorymappedfiles•  FileLocking•  AsynchronousI/O•  GenericI/OControlOpera)ons•  Duplica)ngdescriptors

intdup2(intold,intnew)intdup(intold)

Anotherexample:lowio-std.c#include<stdlib.h>#include<stdio.h>#include<string.h>#include<unistd.h>#include<sys/types.h>#defineBUFSIZE1024intmain(intargc,char*argv[]){charbuf[BUFSIZE];ssize_twritelen=write(STDOUT_FILENO,"Iamaprocess.\n",16);ssize_treadlen=read(STDIN_FILENO,buf,BUFSIZE);ssize_tstrlen=snprintf(buf,BUFSIZE,"Got%zdchars\n",readlen);writelen=strlen<BUFSIZE?strlen:BUFSIZE;write(STDOUT_FILENO,buf,writelen);exit(0);}

What’sbelowthesurface??Application / Service

High Level I/O

Low Level I/O Syscall

File System

I/O Driver

streamshandlesregisters

descriptors

Commands and Data Transfers

Disks, Flash, Controllers, DMA

Recall:SYSCALL

•  Lowlevellibparametersaresetupinregistersandsyscallinstruc)onisissued–  Atypeofsynchronousexcep)onthatenterswell-definedentrypointsintokernel

What’sbelowthesurface??

High Level I/O

Low Level I/O Syscall

File System

I/O Driver

Application / Servicestreams

handlesregisters

descriptors

Commands and Data Transfers

Disks, Flash, Controllers, DMA

File descriptor number - an int

File Descriptors - a struct with all the info about the files

InternalOSFileDescriptor•  InternalDataStructuredescribingeverythingaboutthe

file–  Whereitresides–  Itsstatus–  Howtoaccessit

•  Pointer:structfile*file

FileSystem:fromsyscalltodriver

ssize_tvfs_read(structfile*file,char__user*buf,size_tcount,loff_t*pos){ssize_tret;if(!(file->f_mode&FMODE_READ))return-EBADF;if(!file->f_op||(!file->f_op->read&&!file->f_op->aio_read))return-EINVAL;if(unlikely(!access_ok(VERIFY_WRITE,buf,count)))return-EFAULT;ret=rw_verify_area(READ,file,pos,count);if(ret>=0){count=ret;if(file->f_op->read)ret=file->f_op->read(file,buf,count,pos);elseret=do_sync_read(file,buf,count,pos);if(ret>0){fsnotify_access(file->f_path.dentry);add_rchar(current,ret);}inc_syscr(current);}returnret;}

In fs/read_write.c

LowerLevelDriver•  Associatedwithpar)cularhardwaredevice•  Registers/Unregistersitselfwiththekernel•  Handlerfunc)onsforeachofthefileopera)ons

Recall:DeviceDrivers•  DeviceDriver:Device-specificcodeinthekernelthatinteractsdirectlywiththedevicehardware–  Supportsastandard,internalinterface–  SamekernelI/Osystemcaninteracteasilywithdifferentdevicedrivers–  Specialdevice-specificconfigura)onsupportedwiththeioctl()system

call

•  DeviceDriverstypicallydividedintotwopieces:–  Tophalf:accessedincallpathfromsystemcalls

•  implementsasetofstandard,cross-devicecallslikeopen(), close(), read(), write(), ioctl(), strategy()

•  Thisisthekernel’sinterfacetothedevicedriver•  TophalfwillstartI/Otodevice,mayputthreadtosleepun)lfinished

–  Boqomhalf:runasinterruptrou)ne•  Getsinputortransfersnextblockofoutput•  MaywakesleepingthreadsifI/Onowcomplete

LifeCycleofAnI/ORequest

Device DriverTop Half

Device DriverBottom Half

DeviceHardware

Kernel I/OSubsystem

UserProgram

Sowhathappenswhenyoufgetc?

High Level I/O

Low Level I/O Syscall

File System

I/O Driver

Application / Service

streamshandlesregisters

descriptors

Commands and Data Transfers

Disks, Flash, Controllers, DMA

Communica)onbetweenprocesses•  Canweviewfilesascommunica)onchannels?

•  ProducerandConsumerofafilemaybedis)nctprocesses– Maybeseparatedin)me(ornot)

•  However,whatifdatawriqenonceandconsumedonce?–  Don’twewantsomethingmorelikeaqueue?–  Cans)lllooklikeFileI/O!

write(wfd,wbuf,wlen);

n=read(rfd,rbuf,rmax);

Communica)onAcrosstheworldlookslikefileIO

•  ConnectedqueuesovertheInternet–  Butwhat’stheanalogofopen?– Whatisthenamespace?–  Howaretheyconnectedin)me?

write(wfd,wbuf,wlen);

n=read(rfd,rbuf,rmax);

RequestResponseProtocol

write(rqfd,rqbuf,buflen);

n=read(rfd,rbuf,rmax);

Client (issues requests) Server (performs operations)

requests

responses

write(wfd,respbuf,len);

n=read(resfd,resbuf,resmax);

service requestwait

RequestResponseProtocol

write(rqfd,rqbuf,buflen);

n=read(rfd,rbuf,rmax);

Client (issues requests) Server (performs operations)

write(wfd,respbuf,len);

n=read(resfd,resbuf,resmax);

service requestwait

requests

responses

Client-ServerModels

•  Fileservers,web,FTP,Databases,…•  Manyclientsaccessingacommonserver

Server

Client 1

Client 2

Client n

***

Conclusion(I)•  SystemCallInterfaceis“narrowwaist”betweenuserprogramsandkernel

•  StreamingIO:modeledasastreamofbytes– MoststreamingI/Ofunc)onsstartwith“f”(like“fread”)–  Databufferedautoma)callybyC-libraryfunc)ons

•  Low-levelI/O:–  Filedescriptorsareintegers–  Low-levelI/Osupporteddirectlyatsystemcalllevel

•  STDIN/STDOUTenablecomposi)oninUnix–  UseofpipesymbolsconnectsSTDOUTandSTDIN

•  find|grep|wc…

Conclusion(II)•  DeviceDriver:Device-specificcodeinthekernelthatinteractsdirectlywiththedevicehardware–  Supportsastandard,internalinterface–  SamekernelI/Osystemcaninteracteasilywithdifferentdevicedrivers

•  Fileabstrac)onworksforinter-processescommunica)on(localorInternet)