ProcessesandThreads

Prof. Bracy and0Van0RenesseCS04410

Cornell0University

based&on&slides&originally&created&by&Prof.&Sirer 1

Fun0Starts0Here!

What0is0involved0in0“starting0a0program”0or0“running0a0program”?! which0are0misnomers…How0can0I0run0multiple0processes0on0one0computer?

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What0is0a0program?Program0is0a0file0containing:! executable0code0(machine0instructions)! data0(information0manipulated0by0these0instructions)0that0together0describe0a0computation0Resides0on0diskObtained0through0compilation0and0linking

3

Preparing0a0Program

Source

files

compiler/

assembler

Object

files

Linker

PROGRAM

An&executable&file

in&a&standard& format,

such&as&ELF&on&Linux,&

Microsoft&PE&on&Windows

Header

Code

Initialized&data

BSS

Symbol&table

Line&numbers

Ext.&refs

static&

libraries

(libc)

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Process0!=0ProgramHeader

Code

Initialized&data

BSS

Symbol&table

Line&numbers

Ext.&refs

Code

Initialized&data

BSS

Heap

Stack

DLL’s

mapped& segments

Executable

Process.address.space

Program&is&passive

• Code&+&data

Process&is&running&program

• stack,&regs,&program&counter

Example:

We&both&run&IE:

T Same&program&

T Separate&processes

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What0is0a0Process?

A0process0is0an0abstraction of0a0computerRecap:0A0good0abstraction! is0portable0and0hides0implementation0details! has0an0intuitive0and0easyWtoWuse0interface! can0be0instantiated0many0times! is0efficient0and0reasonably0easy0to0implement

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Basic0Process0Abstraction

ENVIRONMENT

(files,&etc.)ADDRESS

SPACE

REGISTERS

CPU

STATE

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Typical0Virtual Address0Space0Layout0(of0a0single0process)

CODE

HEAP

STACK

0

(same0for0kernel0except0kernel0is0mapped0in0disjoint0range)

DATA

allocated

free

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Process0Interface

CreateProcess(initial0state)0" processIDGetStatus(processID)0" runningStatusTerminate(processID)WaitFor(processID)0" completionStatusListProcesses()0" [0pid1,0pid2,0…0]

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Kernel0implements0processes!

P1

OS&KERNEL

P2 P3

Supervisor&Mode

User&Mode

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Emulation…

One0option0is0for0the0hardware0to0simulate0multiple0instances0of0the0same0or0other0hardware+ Useful0for0debugging,0emulation0of0ancient0hardware,0etc.– Too0inefficient0for0modernWday0daily0use

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CPU0runs0each0process0directly

But0somehow each0process0has0its0own! Registers! Memory! I/O0resources! “thread0of0control”

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Physical0RAM0Layout

0x0KERNEL

P2

P1

P3

allocated

free

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Computer0Architecture0preWMMU

CPU

address&bus

data&bus

MEM

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Computer0Architecture0with0MMU

CPU

address&bus

data&bus

MEMMMU

15MMU&=&Memory&Management&Unit

virtual physical

For0now:0Very0Simple0MMUTwo0registers:0Base0and0BoundOperation:! supervisor0mode:

# addr_out :=0addr_in &00x7FFFFFFF

! user0mode:# if0addr_in

Back0to0Software:00Process0Control0Block (10per0process)Process0IdentifierProcess0arguments0(for0identification0with0ps etc.)Process0status0(runnable,0waiting,0zombie,0…)User0Identifier0(for0security)! beware:0superuser ≠0supervisor

RegistersBase0/0BoundScheduling0/0accounting0info…

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Abstract0life0of0a0process

New

Runnable

Zombie

interrupt&TTT

descheduling

dispatchRunning

Waiting

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Invariants

Supervisor0mode0" PC0points0to0kernel0codeEquivalently:0PC0points0to0user0code0" user0modeUser0code0runs0with0interrupts0enabled

For0simplicity0(for0now):! Kernel0code0runs0with0interrupts0disabled! Single0core

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createProcess(initial0state)

Allocate0memory0for0address0spaceInitialize0address0space! program0vs fork! program0≠0processAllocate0ProcessIDAllocate0process0control0blockPut0process0control0block0on0the0run0queue

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Running0a0programEvery0OS0provides0a0“loader”0that0is0capable0of0converting0a0given0program0into0an0executing0instance,0a0process! A0program0in0execution0is0called0a0process

The0loader:! reads0and0interprets0the0executable0file! Allocates0memory0for0the0new0process0and0sets0process’s0memory0to0contain0code0&0data0from0executable

! pushes0“argc”,0“argv”,0 “envp”0on0the0stack! sets0the0CPU0registers0properly0&0jumps0to0the0entry0point

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How0does0a0process0terminate?

! Exit(processStatus)! Terminate(ProcessID)! Executing0an0illegal0instruction! Using0up0quota! Accessing0illegal0memory0addresses

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TimeWSharing

Illusion:0multiple0processes0run0at0same0timeReality:0only0one0process0runs0at0a0time! For0no0more0than0a0few010s0of0millisecondsBut0this0can0happen0in0parallel0to0another0process0waiting0for0I/O!

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Context0Switching

Kernel0runsSwitch0to0process01Trap0to0kernelSwitch0to0another0(or0same)0processTrap0to0kerneletc. Context3switches

P1 K P2 K P2 KK P1

trap dispatch

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Kernel0Operation0(conceptual)

Initialize0devicesInitialize0“First0Process”For0ever! while0device0interrupts0pending

# handle0device0interrupts! while0system0calls0pending

# handle0system0calls! if0run0queue0is0nonWempty

# select0a0runnable0process0and0switch0to0it

! otherwise# wait0for0device0interrupt 25

Dispatch:0kernel0" process

CurProc :=0&PCB0of0current0processSet0user0base/bound0registerRestore0process0registersExecute0ReturnFromInterrupt instruction! Sets0user0mode! Enables0interrupts! Restores0program0counter

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Trap:0process0" kernel

Hardware:! Disables0interrupts! Sets0supervisor0mode! Saves0user0PC0and0SP0on0kernel0stack

# why0not0on0process0stack?

! Sets0kernel0stack0pointer! Sets0PC0to0trap0handlerSoftware:! Save0process0registers0in0PCB0of0CurProc! Handle0trap0(syscall,0device0interrupt,0etc.)

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Causes0for0traps

Clock0interruptDevice0interruptSystem0callPrivileged0instructionDivide0by0zeroBad0memory0access…

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Questions0about0system0calls

How0does0a0process0specify0what0system0call0to0invoke0and0what0parameters0to0use?How0does0the0kernel0protect0itself0and0other0processes?How0does0the0kernel0return0a0result0to0the0process?How0does0the0kernel0prevent0accidentally0returning0privacy0sensitive0data?

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Class0Project

Implement0sleep(delay)0system0call

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How0Much0To0AbstractUnix0and0Windows0provide0processes0that0look0like0idealized0machines,0with0nice0looking0file0abstractions,0network0abstractions,0graphical0windows,0etc.Xen,0KVM,0etc.0provide0processes0that0look0just0like0real0hardware! virtualization

Requires0different0kinds0of0things0from0kernels! Unix/Windows:0 implement0files,0network0protocols,0window0management

! Xen/KVM/…:0emulate0hardware

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Virtual0Machine0Abstraction(“type01”)

Virtual&Machine&Monitor&kernel

Unix&Kernel Windows&NT&Kernel

P1 P2 P3 P4 P5

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Things0to0emulate

Supervisor0modeMMU0registersDevice0registers…

Hardware0can0help! MultiWlevel0supervisor! MultiWlevel0MMU! …

FLASH&/&ROM

BLOCK&OF&RAM

BLOCK&OF&RAM

DEVICE&REGISTERS

BITMAP& /&SCREEN

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Processes0Under0Unix/Linuxfork()0system0call0to0create0a0new0process! Old0process0called0parent,0new0process0called0child

int fork()0clones the0invoking0process:! Allocates0a0new0PCB0and0process0ID! Allocates0a0new0address0space! copies0the0parent’s0address0space0into0the0child’s! in0parent,0fork()0 returns0PID0of0child! in0child,0fork()0 returns0a0zero.int fork()0returns0TWICE!

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Example

int main(int argc, char **argv)

{ int parentPid = getpid();

int pid = fork();

if (pid == 0) {printf(“The child of %d is %d\n”, parentPid, getpid());

exit(0);

} else {

printf(“My child is %d\n”, pid);

exit(0);

}

}What does this program print?

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Bizarre…(both0if0and0else0are0executed)$ cc a.c

$ ./a.out

The child of 23873 is 23874

My child is 23874

Parent

Child

Kernel

fork()

retsys

v0=0v0=23874

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execve()0fork()0gets0us0a0new0address0space

int execve(char0*programName,0…)0completes0the0picture! throws0away0the0contents0of0the0calling0address0space! replaces0it0with0the0program0in0file0named0by0programName! starts0executing0at0header.startPC! PCB0remains0the0same0otherwise0(same0PID)

Pros:0Clean,0simpleCon:0duplicate0operations

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Process0Termination,0part01Process0executes0last0statement0and0calls0exit())syscall! Process’0resources0are0deallocated by0operating0system

Parent0may0terminate0execution0of0child0process0(kill)! Task0assigned0to0child0is0no0longer0required! If0parent0is0exiting

# Some0OSes don’t0allow0child0to0continue0if0parent0terminates! All0children0terminated0W cascading0termination

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Process0Termination,0part02

Process0first0goes0into0“zombie0state”Parent0can0wait0for0zombie0children! Syscall:0wait()0" (pid,0exit0status)After0wait()0returns,0PCB0of0child0is0garbage0collected

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Class0Project

Write0a0simple0Linux0command0line0interpreter

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Multiple0Cores

Modern0computers0often0have0several0if0not0dozens0of0coresEach0core0has0its0own0registers,0but0cores0share0memory0and0devicesCores0can0run0user0processes0in0parallelCores0need0to0synchronize0access0to0PCBs0and0devices! we’ll0learn0later0how0to0do0synchronizing

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MultiWCore0Architecture

BUS

CORE&1 CORE&2 CORE&3

RAM FLASH/&

ROM

SCREEN

BUFFER

DISK

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Abstraction…multiWthreaded0process

THREAD&1 THREAD&2

ENVIRONMENT(files,&etc.)

ADDRESS&SPACE

(MEMORY)

CPU&+&registers

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THREAD&3

Why0MultiWthreading?Simpler,0more0efficient0for0a0process0to0take0advantage0of0multicore0machines! Instead0of0starting0multiple0processes,0each0with0its0own0address0space0and0a0single0thread0running0on0a0single0core

Not0just0for0CPU0parallelism:0I/O0parallelism0can0be0achieved0even0if0I/O0operations0are0“blocking”Program0structuring:0for0example,0servers0dealing0with0concurrent0incoming0eventsMight0well0have0more0threads0than0cores!!

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Processes0and0Address0Spaces• What0happens0when0Apache0wants0to0run0multiple0concurrent0computations0?

• Two0heavyweight0address0spaces0for0two0concurrent0computations0?

Kernel

0x0

kernel

PCBs

EMACS

Apache

Mail

physical&memory

Kernel

0x0

kernel

PCBs

EMACS

Apache

Mail

physical&memory

Apache?

46

Processes0and0Address0SpacesEliminate0duplicate0address0spaces0and0place0concurrent0computations0in0the0same0address0space

Kernel

0x0

kernel

PCBs

EMACS

Apache

Mail

physical&memory

Kernel

0x0

kernel

PCBs

EMACS

Apache

Mail

physical&memory

$

Architecture

Process0consists0of! One0address0space0containing0chunks0of0memory! Shared0I/O0resources! Multiple0threads

# Each0with0its0own0registers,0in0particular0PC0and0SP# Each0has0its0own0stack0 in0the0address0space# Code0and0data0is0shared

Other)terms)for)threads! Lightweight)Process! Thread)of)Control! Task

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Memory0Layout

CODE

DATA

STACK&1

STACK&3

STACK&2

SP

PC

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Threads

LighterWweight0than0processes! process0is0abstract0computer (CPU,0mem,0devices,0…)! thread0is0abstract0core

Threads0need0to0be0mutually0trusting! Why?

Ideal0for0concurrent0programs0where0lots0of0code0and0data0are0shared! Servers,0GUI0code,0…

49 50

Sharing

What’s0shared0between0threads?! code0and0data0(address0space)! Privileges! Almost0everything0in0the0process

What0don’t0they0share?! PC,0registers,0stack0pointer,0and0stack

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Separation0of0Thread0and0Process0conceptsConcurrency0(multiWthreading)00is0useful0for:! improving0program0structure! handling0concurrent0events0(e.g.,0web0requests)! building0parallel0programs

" multiWthreading0is0useful0even0on0a0uniprocessor

To0be0useful,0thread0operations0have0to0be0fast

Option0#1:0Kernel0threadsKernel0threads! Threads0share0a0single0location0in0memory! Separate0PCBs0for0each0thread! Each0PCB0has0same:0base0&0bound0register0values! Each0PCB0has0different:0 PC,0SP,0registers

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Kernel

0x0

kernel

PCBs

EMACS

Apache

Mail

physical&memory

PCB

PCB

Option0#2:0User0ThreadsBuild0a0miniature0O.S.0in0user0space! OS0doesn’t0know0about0multiple0 threads! Single0PCBGenerally0more0efficient0than0kernel0threads! Why?

But…0kernel0threads0simplify0system0call0handling0and0scheduling! Why?

Kernel

0x0

kernel

PCBs

EMACS

Apache

Mail

physical&memory

User0Thread0Implementation0(P1!)User0process0supports:! Thread0Control0Block0(TCB)0table

# one0entry0per0thread! “context0switch”0operations

# save/restore0thread0state0in0TCB#much0like0kernelWlevel0context0switches

! yield()0operation:# thread0releases0core,0allows0another0thread0to0use0it# Automatic0preWemption0not0always0supported

! Thread0scheduler54

System0calls

With0user0threads,0a0process0may0have0multiple0systems0calls0outstanding0simultaneously0(one0per0thread)Kernel0PCB0must0support0this! why?

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Things0to0Think0about

Coordination! How0do0cores0/0threads0synchronize0access0to0shared0memory0and0devices?Scheduling! Which0runnable0process0/0thread0runs0when?

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